AN ABSTRACT OF THE THESIS OF

AN ABSTRACT OF THE THESIS OF Bongim Jun for the degree of Doctor of Philosophy in Physics presented on July 1, 2002. Title: Radiation Effects on Ill-V Heterostructure Devices Abstract approved: Redacted for Privacy S. Subramanian The neutron and electron radiation effects in IllV compound semiconductor heterostructure devices are studied in this thesis. Three types of devices investigated are A1GaAs/GaAs high electron mobility transistors (HEMTs), A1GaAs/InGaAs/GaAs heterostructure insulated gate field effect transistors (HIGFETs), and InP/InCaAs/InGaAs single heteroj unction bipolar transistors (SHBTs). HEMTs and HIGFETs are primarily investigated for neutron irradiation effects. Detailed optimized processing of HEMT devices is introduced. Numerical as well as analytical models that incorporate radiation induced degradation effects in HEMTs and HIGFETs are developed. The most prominent radiation effects appearing on both HEMT and HIGFET devices are increase of threshold voltage (gm) (VT) and decrease of transconductance as radiation dose increases. These effects are responsible for drain current degradation under given bias conditions after irradiation. From our experimental neutron irradiation study and our theoretical models, we concluded that threshold voltage increase is due to the radiationinduced acceptorlike (negatively charged) traps in the GaAs channel region removing carriers. The mobility degradation in the channel is responsible for g decrease. Series resistance increase is also related to carrier removal and mobility degradation. Traps introduced in the GaAs region affect the device performance more than the traps in the A1GaAs doped region. VT and g of HIGFET devices are less affected by neutron radiation than they are in HEMTs. This difference is attributed to different shapes of the quantum well in the two devices. The main effects of electron and neutron irradiation of SHBTs are decrease of collector current (Is), decrease of commonemitter DC gain, increase of the collector output conductance (Ic/VcE), and increase of collectorcollector offset voltage. The decrease of breakdown voltage of reverse biased baseemitter junction diode is responsible for increasing the output conductance after irradiation. Basecollector junction degradation also induces collectoremitter offset voltage increase. ©Copyright by Bongim Jun July 1, 2002 All Rights Reserved Radiation Effects on Ill-V Heterostructure Devices by Bongim Jun A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Completed July 1, 2002 Commencement June 2003 Doctor of Philosophy thesis of Bongim Jun presented on July 1, 2002 APPROVED: Redacted for Privacy Major Professor, representii'g Physics Redacted for Privacy Chair of the of Physics Redacted for Privacy Dean of the Gradu I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. Redacted for Privacy Bongim Jun, Author ACKNOWLEDGMENTS I am deeply grateful to my advisor, Prof. S. Subramanian for providing invaluable guidance, advice and education to complete this project. I also extend my gratitude to Prof. Henri Jansen not only for giving me an opportunity to be a teaching assistant but also for his encouragement and guidance throughout graduate school. My committee members, William Warren, Thomas Plant, and Kevin Gable assisted me with their services and efforts by reviewing my thesis, and I would like to acknowledge them as well. Without the continual technical support and cooperation of the staff in the Department of Nuclear Engineering at Oregon State University for radiation experiments, it would not have been possible for me to complete my projects. I also thank Dr. Andy Peczalski in Honeywell Sensor Laboratory for providing electronic devices to extend my project to a more advanced level quantitatively and qualitatively. The immeasurable support, encouragement, and love of my family in Korea, especially my mother, are deeply appreciated from the bottom of my heart. Moreover, I will remember for the rest of my life the consistent and invaluable friendship and love that I received from my best friend, MiHae Kim. I am also indebted to MyungJu Lee and YungHae Kim for their prayers, love, and help during my times of need. Most of all, I thank God for walking with me always through my life and allowing me to have this opportunity to complete my study, and I pray to Him to use this achievement for His ultimate plan for me. TABLE OF CONTENTS CHAPTER 1: .............................. .......................... INTRODUCTION 1.1 Motivation 1.2 Thesis Structure CHAPTER 2: Page ...................... ...................... ........... .................. ............... RADIATION EFFECTS FUNDAMENTALS 2.1 Radiation Fundamentals 2.2 Radiation Environments 2.3 Radiation Effects on Semiconductor Devices 1 3 4 6 7 9 13 2.3.1 Total Dose Effects and Induced Damage in Semiconductors 15 20 2.3.2 SingleEvent Phenomena 22 2.3.3 Displacement Damage Effects 26 2.3.4 NonIonizing Energy Loss and Displacement Kerma 2.5 ..... Basic Mechanisms of Radiation Effects in Bulk Devices ..... 2.6 Defect Characteristics 2.7 Thermal Annealing Effect of Bulk Damage 2.8 Basic Mechanisms of Radiation Effects in Heterostructure Devices 39 . 2.4 Radiation Effects on Ill-V Compound Semiconductors ....................... ............ 2.8.1 Degradation Effects in High Electron Mobility Transistors 2.8.2 Radiation Effects in Heterojunction Bipolar Transistors . CHAPTER 3: ....................... .......................... 29 32 34 36 40 45 DEVICE DESCRIPTION AND EXPERIMENT DETAILS 48 3.1 HEMT Layer Structure 49 3.2 Mask Set Design 53 TABLE OF CONTENTS (Continued) Page 3.2.1 Unpassivated Devices 3.3 53 59 . 3.2.2 Passivated Devices ..................... Processing Description ....................... 72 3.3.1 Lapping ........................... 3.3.2 Cleaning ........................... 3.3.3 Photolithography ...................... 3.3.4 Etch ............................. 3.3.5 Metallization and LiftOff ................. 3.3.6 Passivation .......................... 3.4 Measurement SetUp for Electrical Characterizations ...... 81 3.4.1 CurrentVoltage Measurement Setup Configuration 81 84 . . . 3.4.2 Hall Measurement Setup Configuration .......... 3.5 3.6 Electron Irradiation ........................ Neutron Irradiation ......................... 88 90 3.6.1 Irradiation Process on HFET Devices ........... 3.6.2 Irradiation Process on SHBT Devices ........... CHAPTER 4: 91 92 94 HEMT DEVICE MODELS 4.1 2Dimensional Electron Gas in A1GaAs/GaAs Interface ..... 4.2 Charge Control Model for HEMT Device Characterization 95 . 101 Charge Control Model ................... 102 104 106 Application to FET Devices .................... 108 4.2.1 4.2.2 4.2.3 4.3 72 73 74 74 75 77 . . Equilibrium ......................... Radiation Effects on Charge Control Model ........ effect ............................. CurrentVoltage Characteristics including series resistance effect ............................. 4.3.1 CurrentVoltage Characteristics without series resistance 4.3.2 108 111 TABLE OF CONTENTS (Continued) CHAPTER 5: 5.1 Page IRRADIATION EFFECTS IN HFETs 117 Neutron Irradiation Effects on HFETs .............. 118 5.1.1 Threshold Voltage Shift ................... 118 5.1.2 CurrentVoltage Characteristics of A1GaAs/GaAs HEMT Devices ............................ 122 5.2 Neutron Irradiation Effects on HIGFET Device ......... 129 Current-Voltage Characteristics .............. 129 5.2.2 Breakdown Voltage Degradation .............. 130 5.2.1 .......................... 5.3 Annealing Study 5.4 Modeling for Carrier and Mobility Degradation in HEMT . . 133 5.4.1 Theoretical Model for Carrier Removal .......... 5.4.2 Modeling for Mobility Degradation Effect on HEMT 5.4.3 Experimental Results and Analysis ............ 5.4.4 Modeling for Radiationinduced Threshold Voltage Shift 5.4.5 5.4.6 5.4.7 Discussion 136 143 144 146 147 149 150 . . . Results from Simulation .................. Results from Experiment .................. 5.5 131 .......................... Electron Irradiation Effects in HIGFETs ............. .................. .................. 5.5.1 'D 5.5.2 'D VGS VDS Characteristics characteristics 155 155 159 5.5.3 Transconductance ...................... 165 CHAPTER 6: IRRADIATION EFFECTS IN SHBTs 168 6.1 HBT Fundamentals ......................... 169 6.2 SHBT Device Structure ...................... 170 6.3 SHBT Electrical Characterization Setup Configuration ..... 173 6.4 Electron Irradiation Effects in SHBT Devices .......... 174 TABLE OF CONTENTS (Continued) Page 6.4.1 CurrentVoltage Characteristics .............. 175 6.5 6.4.2 Annealing Effect after Electron Irradiation ........ 185 Neutron Irradiation Effects in SHBT Devices ........... 188 6.5.1 CurrentVoltage Characteristics .............. 190 6.5.2 I 6.6 V Diode Characteristics ................ 196 Discussion .............................. 198 CHAPTER 7: CONCLUSION AND FUTURE WORK BIBLIOGRAPHY 200 208 Figure LIST OF FIGURES Domain boundaries for protons and electrons in earth's radiation Page 2.1 belts. (After reference [37]) ..................... 2.2 Photoelectric effect inducing total dose damage on electronic de- 2.3 Compton effect inducing total dose damage on electronic devices by x-ray beams ............................ 17 2.4 Pair production (a) and pair annihilation process (b) ....... 19 2.5 Induced ionization path due to a heavy ion impacted on (a) pn 2.6 vices by incident photons ...................... junction device and (b) MOS transistor .............. The crosssectional view of the formation of Frenkel defects and cascades as a function of initial primary knockon atom energy created by heavy particles ...................... 2.7 Electron energy band diagram associated with defect energy lev- 2.8 Schematic diagram of the crosssection of an A1GaAs/GaAs 2.9 11 16 22 23 els introduced by irradiation ..................... 25 HEMT ................................ 41 Energy band diagram of a HEMT in the vicinity of the heterojunction interface at equilibrium .................. 42 2.10 InP/InGaAs single heterostructure bipolar transistor structure. 45 3.1 Schematic diagrams of 5 and uniformdoped HEMT structures. 49 3.2 Schematic layer structures of a A1GaAs/InGaAs/GaAs HIGFET device ................................. 53 3.3 Crosssection of lightlydoped drain (LDD) HIGFET structure 54 3.4 Mask outline for fabrication of HEMT device ........... 55 3.5 Final mask design layout of unpassivated variable gate width FET structure. Upper five squares are source/drain pads while 3.6 lower four squares are gate pads .................. 56 Dimensions of unpassivated variable gate FET structure ..... 56 LIST OF FIGURES (Continued) Figure Page 3.7 Final mask design layout of unpassivated TLM structure ..... 3.8 Individual levels of three mask steps (top:level 1, middle:level 2, and bottom:level3 for fabrication of unpassivated variable gate 57 FET(left) and TLM(right) structures ................ 3.9 58 Mask layout of different type of devices for the fabrication of passivated HEMT devices ...................... 59 3.10 Composite mask layout design for fabrication of passivated HEMT devices ................................ 61 3.11 Mask level 1 for mesa isolation ................... 62 3.12 Mask level 2 for source and drain mesa ............... 63 3.13 Mask level 3 for gate and contact window opening in the passivation layer .............................. 64 3.14 Mask level 4 for final probe metal pad deposition ......... 65 3.15 Final mask design layout of variable gate width FET structure. 66 3.16 Four mask levels to fabricate passivated variable gate FET structure (left-top:level 1, right-top :level 2, left-bottom:level3, right- bottom:level 4) ............................ 66 3.17 Dimensions of passivated variable gate FET structure ...... 67 3.18 Final mask design layout of FatFET structure ........... 68 3.19 Four mask levels to fabricate FatFET structure (left-top:level 1, right-top : level 2, left-bottom: level3, right-bottom: level 4). . . . 3.20 Final mask design layout of Transmission Line Model structure. 3.21 Four mask levels to fabricate Transmission Line Model structure (left-top:level 1, right-top:level 2, left-bottom:level3, right- bottom:level 4) ............................ 68 69 70 3.22 Composite layout of RF device structure .............. 70 3.23 Final mask design layout design of Inverter structure ....... 71 LIST OF FIGURES (Continued) Figure Page 3.24 Alignment marks used for fabrication of passivated HEMT devices. 72 lapping ................................. 3.25 Schematic cross section of a HEMT structure after cleaving and 3.26 Schematic cross section of a HEMT structure after etch for mesa active layer formation ........................ .................... .................... ............................. 3.27 Schematic cross section of a HEMT structure after metalization for source and drain contacts 3.28 Schematic cross section of an unpassivated HEMT structure after metalization for gate contact 3.29 Schematic cross section of a HEMT structure after passivation and dryetch ........... 3.30 Schematic cross section of a passivated HEMT structure after final metalization for contact pads and liftoff 3.31 Unpassivated HEMT Device Fabrication Flowchart ........ 3.32 Passivated HEMT Device Fabrication Flowchart ......... 3.33 Currentvoltage measurement setup consisted with PC, HP 4145B parameter analyzer, and probestation ............... ........... 3.34 Hall measurement setup consisted with gpib interface PC, current and voltage sources, and 3200 Gauss magnet 3.35 Contact configuration of sample for Hall measurement ...... .......................... ............................ ............................... 3.36 Current, voltage and magnetic field configuration for Hall coefficient measurement 3.37 Schematic crosssection diagram of electron radiation source and sample setup 3.38 Betaemitting transition in Sr° beta source. Initially 75 76 78 80 81 82 83 84 85 85 87 88 Y9° is cooled down by emitting gamma particle whose halflife time is 3.19 hours 73 89 LIST OF FIGURES (Continued) Figure .......................... 4.1 Schematic layer structures of uniformly doped (a) and delta doped (b) HEMT devices 4.2 Energy band diagrams of uniformly (a) and delta (b) doped 4.3 .......... Energy band diagrams of uniformly (a) and delta (b) doped ............ HEMT structures without applied gate voltages Page 95 96 HEMT structures with applied gate voltages 104 4.4 Channel voltage along the FET gate 109 4.5 Twopiece linear approximation for velocityfield characteristic 110 4.6 Source and drain resistances effect on IV characteristics 115 5.1 'D 5.2 'D 5.3 Threshold voltage extraction from 'D VGS characteristics of A1GaAs/GaAs HEMT devices before and after neutron irradiation. 120 5.4 Neutron induced shifts of the threshold voltages on HEMT and HIGFET devices 120 5.5 Transconductance as function of gate voltage of HEMT devices before and after neutron irradiation 121 ................ ..... .................... ............. characteristics of A1GaAs/GaAs HEMT devices before and after neutron irradiation 119 characteristics of A1GaAs/InGaAs/GaAs HIGFET devices before and after neutron irradiation 119 VGS VGS ........................... ................ 5.6 Transconductance of HIGFET devices as a function of gate volt- 5.7 Currentvoltage characteristics of a HEMT device before and after neutron irradiation measured at gate voltages from 0.6 V to 123 0 V with a step of-0.1 V 5.8 Normalized mobility (Square) and source resistance (Triangle: experiment, Circle: TLM) of HEMT devices as a function of age before and after neutron irradiation .............. 122 ...................... neutron dose. Lines are best fits .................. 125 LIST OF FIGURES (Continued) Figure 5.9 Page Mobility (a) and carrier concentration (b) degradation in two GaAs samples with different initial carrier concentrations (Square: 10'6/cm3) as a function of neutron 3 x 10'5/cm3, Circle: 1.26 x dose .................................. ............... 126 5.10 Normalized drain saturation current as function of neutron dose 127 from experiment and model simulation 5.11 Normalized drain saturation current as function of neutron dose from experiment and model simulation ............... 128 5.12 Calculated transconductance (Square) and the experimental data ........................... ................... (Triangles) before and after neutron irradiation with the dose of 4.86 x 10'4n/cm2 characteristics of A1GaAs/InGaAs/GaAs HIGFET devices for different neutron doses 5.13 'D VDS 5.14 Breakdown IDVDS characteristics for 2m LDD spacing HIGFET devices after different doses of neutron irradiation ......... 5.15 Breakdown ID-VDS characteristics for 5m LDD spacing HIGFET devices after different doses of neutron irradiation ......... 128 129 131 132 5.16 Carrier concentration recovery after annealing of the neutron ir- ............................... radiated GaAs samples: Square: 0.54 x 10'4n/cm2 1014n/cm2 , Circle: 4.86 x 5.17 'D VDS characteristics recovery after isochronal annealing on 5.18 'D VDS characteristics recovery after isochronal annealing on 5.19 'D VGS characteristics recovery after isochronal annealing on 5.20 'D VGS characteristics recovery after isochronal annealing on neutron irradiated HEMT devices .................. neutron irradiated HIGFET devices ................ neutron irradiated HEMT devices .................. neutron irradiated HIGFET devices ................ 132 133 134 134 135 LIST OF FIGURES (Continued) Page Figure 5.21 The effect of acceptors in A1GaAs layer, in GaAs layer, and in both layers of uniformly doped HEMT structure on 2DEG concentration from analytical model .................. 138 5.22 The effect of acceptors in different layers of uniformly doped HEMT structure on 2DEG concentration from numerical model. 139 5.23 The effect of acceptors in different layers of 6 doped HEMT structure on 2DEG concentration from numerical model ..... 140 5.24 The 2DEG concentration as function of induced acceptor concentration for different spacer layer thickness ........... 5.25 The 2DEG concentration as function of induced acceptor concentration for different values of conduction band discontinuity. 142 5.26 The 2DEG concentration as function of induced acceptor concentration for different doping concentration in AlGaAs layer. 141 . 5.27 The simulated carrier mobility vs. radiation induced total defect concentration in GaAs layer ..................... 5.28 Measured sheet carrier concentration and mobility as functions of neutron fluence for the 6doped HEMT structure ....... 5.29 Measured sheet carrier concentration and mobility as functions of neutron fluence for the nGaAs control sample ......... 142 144 145 145 5.30 2DEG concentration of 6doped HEMT structure as a function theory ................................. of net acceptor concentration in GaAs from experimental and 146 5.31 2DEG concentration as a function of background acceptor con- lution ................................. centrations on HEMT devices from selfconsistent numerical so- 5.32 2DEG concentration as a function of background acceptor concentrations on HEMT devices from analytical solution ...... 147 148 5.33 2DEG concentration as a function of background acceptor con- solution ................................ centrations on HIGFET devices from selfconsistent numerical 149 LIST OF FIGURES (Continued) Page Figure 5.34 Threshold voltage shift as a function of background acceptor concentration for both HEMT and HIGFET devices ......... 150 5.35 Potential profiles of HEMT structure from selfconsistent numerical solution for different acceptor doping concentration ...... 151 5.36 Potential profiles of HIGFET structure from selfconsistent numerical solution for different acceptor doping concentration. 5.37 I . . 151 characteristics of a HIGFET (LDD = 3.5 pm) for in156 creasing electron doses at VDS = VGS 0.5 V ............... characteristics of a HIGFET (LDD = 3.5 pm) for increasing electron doses at VDS = 1.0 V ............... 5.38 'D VGS 5.39 '.c VGS characteristics of a HIGFET (LDD = 12.5 pm) for 5.40 'D VGS characteristics of a HIGFET (LDD = 12.5 pm) for increasing electron doses at VDS = increasing electron doses at VDS = 0.5 V .............. 1.0 V .............. ................. ................. 5.41 Threshold voltage shift as a function of electron doses for four different devices with VDS of 0.5 V 5.42 Threshold voltage shift as a function of electron doses for four different devices with VDS of 1.0 V 156 157 157 158 159 5.43 Threshold voltage shift as a function of LDD region in linear and saturation region of VDS ....................... 160 5.44 Ij VDS characteristics of a HIGFET with LDD of 12.5 pm before electron irradiation ...................... 160 VDS characteristics of a HIGFET with LDD of 12.5 pm after the accumulated electron radiation with a dose 161 characteristics of a HIGFET with LDD of 3.5 pm before and after the accumulated electron dose of 162 5.45 ID ofl.11x1016e/cm2. 5.46 'D VDS 1.11xlO'6e/cm2. 5.47 'D VDS . . characteristics of a HIGFET with LDD of 3.5 pm for increasing electron irradiation dose ................. 162 LIST OF FIGURES (Continued) Page Figure 5.48 Normalized drain saturation current as a function of accumulated electron dose at the drain voltage of 2.5 V ............. 163 5.49 Normalized drain saturation current as a function of accumulated electron dose at the drain voltage of 2.5 V ............. 163 5.50 The method for extracting resistance and mobility from current voltage characteristics ........................ 164 5.51 Mobility and resistance degradation showing on device with LDD 3.5 and 7.5 am as a function of electron doses ........... 165 5.52 Mobility degradation parameter extraction for devices with different LDD regions .......................... 166 5.53 Normalized transconductance as a function of gate voltage of a HIGFET device (LDD = 12.5 rim) before and after electron 166 irradiation at VDS = 0.5 V ..................... 5.54 Normalized transconductance as a function of gate voltage of a HIGFET (LDD=12.5 urn) before and after electron irradiation at VDS = 1.0 V ............................ 167 6.1 Energy band diagrams of (a) bipolar junction transistor and (b) 6.2 Energy band diagrams of (a) abrupt SHBT, (b) graded SHBT, (c) abrupt DHBT, and (d) graded DHBT ............. 171 6.3 Single heterostructure bipolar transistor structure ......... 172 6.4 Schematic diagram of emitter, base, and collector contacts. 173 6.5 Circuit configurations for (a) common emitter I 6.6 Collector currentvoltage characteristics of the unexposed devices. 175 6.7 Collector currentvoltage characteristics after electron accumu- abrubt junction single heterojunction bipolar transistor ...... . . 169 . V characteristics, (b) Gummel plots, and (c) inverse Gummel plots ...... lated dose of 11.6x iü' cm2 .................... 174 176 LIST OF FIGURES (Continued) Figure ..................... Page 6.8 Collector currentvoltage characteristics after electron accumu- 6.9 Collector currentvoltage characteristics for a base current of 50 ,uA before and after different doses of electron irradiation ..... 177 lated dose of 26x 1015 cm2 ) of device as a function of cumulative elec tron doses ............................... 6.10 Offset voltage (V01f 6.11 Commonemitter current gain as a function of base current after different levels of electron irradiation at VCE=l.5 V ........ 176 178 179 6.12 DC current gain (/3) and reciprocal gain (1/3) with electron dose. 179 6.13 Gummel plot for the collector current as a function of base emitter voltage before and after electron irradiation ........ 6.14 Gummel plot for the base current as a function of baseemitter voltage before and after electron irradiation ............ 6.15 Inverse gummel for the base current as a function of basecollector voltage before and after electron irradiation ............ 6.16 Inverse gummel for the emitter current as a function of base collector voltage before and after electron irradiation ....... 6.17 Gain (IE/IB) and reciprocal gain (IB/IE) from inverse Gummel plots as a function of cumulative doses ............... V characteristics for different cumulative electron fluences. The insert shows the linear 181 182 182 183 183 6.18 Semilog plot for a basecollector diode I I V plots .............................. V characteristics for different cumulative electron fluences. The insert shows the linear 184 6.19 Semilog plot for baseemitter diode I I V plots .............................. 6.20 Diode characteristics of basecollector diodes for applied reverse 185 bias after different levels of electron exposure ........... 186 6.21 Collector currentvoltage characteristics after annealing at temperature TA = 187 200°C for 10 minutes ................ LIST OF FIGURES (Continued) Page Figure 6.22 Collector currentvoltage characteristics after annealing at temperature TA = 300°C (=450°C inserted figure) for 10 minutes ............................ 188 6.23 Basecollector diode characteristics before and after isochronal annealing process ........................... 189 6.24 Baseemitter diode characteristics before and after isochronal annealing process ............................ 189 6.25 Currentvoltage characteristics of a SHBT device before neutron irradiation measured at base current from 60 A to 0 tA with a step of -10 1iA ............................ 190 6.26 Currentvoltage characteristics of a SHBT device after neutron irradiation with dose of 6.48 x 1014 n/cm2 measured at base current from 60 pA to 0 pA with a step of -lOpA ........... 191 6.27 Currentvoltage characteristics of a SHBT device after neutron irradiation with four different doses at base current 'B = 60 pA. 191 6.28 Collectoremitter offset voltage increase as a function of neutron dose. Inserted figure shows the absolute V01fset values found from Fig. 6.27 ............................... 192 6.29 Inverse DC gain (1/8) and gain (/3) of the device at VCE = 1.0 V as a function of neutron dose ................... 193 6.30 Gummel plots of 'B as a function of VBE at VBC = OV for different neutron irradiation dose ....................... 194 6.31 Gummel plots of I as a function of VBE at VBC = 0 V for different neutron irradiation dose .................. 194 6.32 Base current dependence of low frequency ac gain for different neutron irradiation dose ....................... 195 6.33 BaseCollector breakdown voltage characteristics after three different neutron irradiation ...................... 196 6.34 IV characteristics of baseemitter diode before and after neutron irradiation .............................. 197 LIST OF FIGURES (Continued) Figure Page 6.35 IV characteristics of basecollector diode before and after neutron irradiation ............................ 198 7.1 HEMT degradation simulated using radiation induced effects on model parameters .......................... 204 LIST OF TABLES Page Table 2.1 Damage function (Kerma for InGaAs and InP), NIEL, displacement damage dose rate, and 1 MeV neutron flux. Reactor was operated at 100 kW ......................... 29 3.1 Layer parameters of the MBEgrown 5 HEMT structure. 50 3.2 Layer parameters of the MBEgrown uniformlydoped HEMT . . . structure ............................... 51 3.3 MBE grown single layer A1GaAs structure parameters ...... 51 3.4 MBE grown single layer GaAs structure parameters ........ 51 3.5 MBE grown HIGFET heterostructure structure parameters. 52 3.6 RIE process parameters used for passivation layer etching of silicone nitride, silicone dioxide and polyimide ............ 3.7 PECVD process parameters used for passivation layers with silicone nitride and silicone dioxide .................. 5.1 6.1 76 79 Simulation parameters for SchrodingerPoisson selfconsistent numerical simulataion for HEMT structure .............. 139 Schematic cross section with layer properties of a SHBT ..... 172 RADIATION EFFECTS ON Ill-V HETEROSTRUCTURE DEVICES CHAPTER 1 INTRODUCTION Even though 111V compound semiconductors are versatile and have been applied to various kinds of electronic devices drawing nearly equivalent attention as Sibased technology and devices, major technical difficulties in the fabrication and engineering of ITTV heterostructure devices were not solved until molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) growth techniques were developed in the late 1970s and early 1980s. Owing to the possibility of controlling the bandgap of material during the material growth and with the excellent electron transport properties, a large number of GaAs and InPbased devices and other material systems such as A1GaAs/GaAs, A1GaAs/InGaAs, InGaAs/IriP, and other latticematched or pseudomorphic structures have been studied in various fields exhibiting superior properties for highspeed circuits/devices applications. Two major types of heterostructure ITTV compound semiconductor devices, high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs), are used for the study of the radiation degradation effects appearing in these devices. With a large bandgap material for emitter layer, HBTs, fabricated by either MBE or MOCVD, are often used in communication systems used in mu- 2 itary and commercial space satellites. These devices not only have superior electronic transport properties but also offer higher radiation hardness as compared to Si based devices/circuits [1]- [5]. Due to the matched lattice constants, A1GaAs/GaAs heterostructures have received considerable attention among other semiconductor compounds. Although a latticematched structure is highly desirable, a moderate lattice mismatch ( < 8 %) can be also used in practical devices without a discernible degradation of crystalline quality. We call this kind of latticemismatched combination of compounds pseudomorphic. InP/Ino53Gao47As system is another promising replacement for typical A1GaAs/GaAs structure, not only because of lattice constant match of the two materials but also because of the following advantages: (1) smaller effective mass (m*/mo = 0.032 for 1n053Ga047As vs. 0.067 for GaAs); (2) higher mobility p at low field cm2/Vsec (.tO = 7800 cm2/Vsec for 1n053Ga047As vs. for GaAs); and finally (3) higher electron saturation velocity 2.1 x 107cm/s for In0 53Ga0 47As 4600 (V3at = vs. 1.8 x lO7cm/s for GaAs). High electron mobility transistors, which are also called as a modulation doped field effect transistors (MODFETs) or selectivelydoped heterojunction transistors (SDHTs) and twodimensional electron gas FETs (TEGFETs), 1 are used in microwave low noise amplifiers (LNAs) and digital circuits in a number of applications in a radiation environment. With a highlydoped larger bandgap material such as A1GaAs on the top and an undoped smaller bandgap material such as GaAs below forming a heterostructure, a two-dimensional electron gas is formed at the heterointerface. This induces a high sheet carrier concentration 1 Origin: HEMT: Fujitsu, MODFET: Cornell, U.of Illinois, Rockwell, SDHT: Thomson CSF and TEGFET: AT & T Bell Lab. in a triangularshaped potential profile. Confined electrons in the potential well have high mobility without experiencing Coulombic scattering in the potential well. High sheet carrier concentration also produces a screening effect which can further reduce Coulomb scattering [6]- [10]. Complementary heterostructure insulated gate FETs (HIGFETs), whose typical materials are pseudomorphic Al Ga1 _As/InGai _As/GaAs, were more recently developed to provide a potential profile for a highspeed and low power operation with higher electron velocity and electron mobility in the InGaAs channel [14]- [17]. Unlike the triangular potential well in the case of A1GaAs/GaAs, the potential well formed by A1GaAs/IriGaAs/GaAs system is more like an asymmetric square well. This difference in shape of the well plays a significant role in the radiation hardness of the two types of devices. 1.1 Motivation Semiconductor devices have been used in the military and commercial satellites for several decades. Especially, satellitebased global communication has remarkably grown during the past few years. Consisting of highly sophisticated electronic systems requiring expensive preparing, launching, and maintaining the satellitebased resource, the issues related to the reliability and lifetime of the electronic systems on board the satellite deserve careful consideration. Since many of the satellite orbits pass through natural radiation belts which become major sources of radiation discussed in Ch. 2, it is very important to investigate radiation effects on electronic systems in terms of degradation mechanisms as well as related characteristics of devices/materials. Ever since the electronic devices in satellitebased resources exhibited failure 4 in electrical characteristics due to various kinds of radiation sources in space in early 60s, enormous efforts have been made to characterize degradation mechanism as well as recovery of the damage effects on devices/circuits. ITT-V compound semiconductors such as GaAs and InP are more frequently used than siliconbased devices because they are known to be better in terms of radiation hardness as mentioned earlier. Among the ITT-V compound devices including Metal Semiconductor FETs (MESFETs), HEMTs, HIGFETs, HBTs, Resonant Tunneling Diodes (RTDs), and optoelectronic devices such as lasers and photodetectors, only MESFETs have been investigated extensively for radiation effects. In this study radiation effects induced by various sources of radiation environments on semiconductor compound devices are investigated especially on HEMTs, HIGFETs, and HBTs. Neutron irradiation effects on A1GaAs/GaAs HBT devices/circuits have been studied by numerous groups [18] - [22]. Radiation effects on InP/InGaAs devices induced by 'y and /3 irradiation have also been reported by several groups [24]- [29]. Threshold voltage changes induced by neutron irradiation of HEMTs and related devices have also been studied [30] [36]. However, there has been some controversy about the physical mechanisms responsible for the threshold voltage shift in HEMTs. Understanding the exact physical mechanism responsible for the radiationinduced degradation is important for the development of improved radiationhard device designs. 1.2 Thesis Structure In chapter 2, several major radiation sources are briefly discussed followed by detailed investigation of damage mechanisms including displacement damage 5 effect, ionization degradation, total dose effects, and single event effects. Device structures, theoretical backgrounds, and modeling for electrical characteristics of HEMT device devices are discussed in chapter 3. Detailed device fabrication steps and methods are explained in chapter 4 for HEMT devices fabricated by using OSU cleaningroom facilities. In chapter 5 neutron irradiation effects on HEMT and HIGFET device performances are characterized. Theoretical simulation results of irradiation effects on device characteristics are also included in this chapter. Electron and neutron irradiated degradation effects on HBT devices are discussed in chapter 6. Finally, annealing effects on displacement and electron damaged HEMTs and HBTs are described followed by conclusion and further scope for research in this area. CHAPTER 2 RADIATION EFFECTS FUNDAMENTALS The primary purpose of this study is to develop a microscopic understanding of the major radiation effects in semiconductor devices, in particular ITTV semiconductor HEMTs and HBTs. Fundamental knowledge related to radiative particles and their origins as well as their interaction mechanisms with semiconductor materials is essential for an understanding of the radiation effects in devices. Therefore, various radiation sources and their induced damage on semiconductor materials as well as electronic devices fabricated with those materials are studied in this chapter. Starting with a discussion of the fundamental radiation quantities, radiation environments involving electrons, neutrons, protons, gammas, and other heavy ions are introduced. The focus is narrowed to the interest of verifying the relationship between a particular radiation source and its induced damages on materials/devices. We will discuss the experimental techniques used for the characterization of various radiationinduced defects. Radiation effects on ITTV compound semiconductor and basic mechanisms of radiation effects in bulk devices researched by numerous people are reviewed. Finally, a brief discussion of annealing behavior of radiation induced defects is presented. 7 2.1 Radiation Fundamentals Several radiationrelated fundamental quantities are introduced in this section. . Radioactive decay rate : The decay rate of a radioactive nuclide is defined by (2.1) dN/dt = \N where ) is the decay constant defined as the probability per unit time for the decay of the nucleus and N is the number of the radioactive nuclei. The solution of Eq. 2.1 is given by (2.2) N(t) = N(0)e_t A very useful parameter in radiation community is half life, ti,,2, which is the time for half of nuclear activity to decay. It is related to ) by = 1n2 . (2.3) Activity, A, is a related parameter, defined as the number of disintegrations per second (SI unit=Bq). The traditional unit of activity is known as Curie which is equal to the number of disintegrations per second occurring in 1 gram of Ra226, i.e. 1 Curie=3.7 x 1010 Bqs. . Exposure (X) is defined as x- (2.4) where zQ is the sum of the electrical charges of total air ions, regardless of their sign (electrons and positrons), created when photons liberate electrons in a unit air volume whose mass is Lm. Although, the traditional unit, roentgens, is no longer used as a standard radiation unit since the 1960s, it is still used for a measure of radiation exposure (R 2.58 x i0 Coulomb/kg). flux (unit: cm2s') is defined as the number of particles passing through unit area per unit time and can be written as JJ d/dt where is the fluence. Alternatively, fluence is the time integrated flux of particles (unit cm) and can be defined as (2.5) dN0 dA where N0 is a total number of particles passing through an area. By using differential fluence, dq5/dE, the fluence is also written as =f dE (2.6) Also ICRU (International Commission on Radiological Units and Measurements) developed the following equations: . absorbed dose (D): DD (2.7) where LED is the imparted energy to the matter in a unit volume by ionizing radiation and Lm is the mass element in the unit volume. The ICRUdefined unit of absorbed dose (D) is rad (1 rad = 100 ergs/gram). The SI unit of dose is grey (1 grey = 1J/kgm) kerma (K) is defined as K=" Lm (2.8) ionizing radiation is any radiation induced by direct ionizing particles, e.g., electrons, protons and indirect ionizing particles, e.g., uncharged particles like neutrons, photons. 1 EK is the sum of the initial kinetic energies of the charged particles induced by the indirect ionizing radiation in a unit volume element of the material, and is the mass element in the unit volume. According to ICRU, kerma is also defined as K = 4' /1,./f) where 4 is the energy fluence. tr/P (2.9) is called mass energy transfer coefficient, which is defined as Etr /NE ,LLtr/P (2.10) where LEtr/NE is the fraction of the part of the kinetic energy the incident particles transferred to the kinetic energy of liberated charged particles. p represents material density, and LIx is the thickness of the target material. Similarly, mass energy absorption coefficient can be defined as the absorbed energy in layer thickness x, expressed as absorbed/NE pLx Finally, for compound materials, (t/p)c (2.11) is obtained by weight summa- tion of each (p/p) for all components, that is, (/i/p)com = 2.2 Wj(/1/p)z (2.12) Radiation Environments Radiation environments that are capable of causing degradation effects on electronic materials/devices have various kinds of source particles distributed in all regions of the universe with energies ranging from keV to GeV and perhaps even 10 beyond. Generally, particles present in the earth's natural space environment are grouped into two categories: 1) particles from space environment trapped by earth's magnetic field that form the radiation belts (Van Allen belts) consisting primarily of electrons of energies up to a few MeV and protons of energies up to several hundred MeV; 2) cosmic rays that consists of heavy ions and protons of galactic or of solar origin. The charged particles gyrating spirally along the earth magnetic field lines reflect back and forth between the poles giving rise to bands or domains of electrons and protons and form the earth's radiation belts. These domains can be divided into five zones having an earth radius as a criterion [37]. Figure 2.1 illustrates a schematic diagram of these zones at the equator. These domain boundaries vary with latitude due to the variation of magnetic field lines with latitude. Protons originating from solar flares are mainly present in the regions four and five and extend from 5 earth radii to beyond 14 earth radii. The trapped protons with energies as high as 500 MeV are distributed mainly through regions one and two that reach above 1 earth radius to 3.8 earth radii. The electron domains are divided into inner and outer zones. The inner zone extends up to 2.8 earth radii, and the outer zone goes from 2.8 to 12 earth radii having 10 times higher electron fluxes than that of the inner zone. The maximum energy of the trapped electrons in the outer zone is 7 MeV whereas the maximum energy of electrons in the inner zone is less than 5 MeV [37]. For electrons with energies larger than 1 MeV, the peak in the electron flux is located between 3 and 4 earth radii [38]. Cosmic rays originate from two sources, the solar and the galactic. Galactic cosmic rays originated outside of the solar system, provide continuous, low-flux of protons (85 %), c particles (14 %), and heavy particles with energies up 11 -2$ Region --3.8 -12.0 -5.0 213 1 4 Solar Flare Protons Trapped Protons Outer Zone Electrons Inner Zone Electrons 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Earth Radii ______ Inner Zone Outer Zone Electrons ______ Electrons Solar Flare Trapped Protons ______ Protons FIGURE 2.1: Domain boundaries for protons and electrons in earth's radiation belts. (After reference [37]) to 1 GeV (1 %). UV and x-rays are also produced by the solar cosmic rays originated within the solar system. The amount of the solar cosmic rays is dependent on the solar activity. Solar flares are random in nature and account for a large part of solar cosmic rays. During the solar storms, solar flares consisted mainly of protons (90-95 %) including small amount of electrons and alpha particles are emitted by sun. Heavy ions constitute only a small fraction of the emitted particles. However, in a large solar flare, the protons and alpha particles can be greatly enhanced ('-' i0 times) over the galactic cosmic ray background whereas the number of heavy ions approaches up to 50% of the background cosmic concentration [39]. Associated with a solar flare is the solar wind or solar plasma. As the solar wind strikes the magnetosphere, it can cause 12 disturbances in the geomagnetic fields. The solar activity varies in cycles with peaks in intensity approximately every eleven years [39]. The solar wind during the periods of high solar activity reduces the galactic cosmic ray flux. Thus, the minimum in galactic cosmic ray flux occurs during solar maximum and the maximum in galactic cosmic ray flux occurs during solar minimum. Terrestrial cosmic rays are caused by the penetration of galactic cosmic rays transforming to a cascade of secondary radiation in particular neutrons. Radiation environment other than space includes nuclear reactors, nuclear weapons, highenergy particles accelerators and medical electrons (e.g. radiation therapy). In particular, the devices used in safety equipment that is required to operate during and after an accidental release of radiation need special consideration. For military purpose, nuclear weapons based on fission fusion reactions expose about 90 % of neutrons in earth's atmosphere. The prompt gamma particles, which are emitted when an exited nucleus falls back to a ground state and the delayed gamma particles generated by the decay of radioactive fission fragments comprise a significant part of the radiation source from the military weapon environment. Despite the wellproven fact that the radioactive particles from numerous sources cause electronic degradation effects on semiconductor devices, especially in space environment, radiation processing with selective particles has been used for beneficial purposes, for instance, sterilization and cancer treatment in medical field, eradication of harmful organisms in food industries, and even semiconductor as well as polymer industries have started using radiation process for modification of starting materials. 13 Radiation Effects on Semiconductor Devices 2.3 There are three major degradation effects appearing on materials: (i) total dose effects; (ii) transient dose or dose rate effect; and (iii) single event effects. The total dose effect is studied by subjecting the semiconductor devices to ionizing radiation (e.g. 7rays, Xrays, etc) and determining the total radiation dose at which the device performance degraded below a certain acceptable limit. This effect primarily created by electrons and protons (charged particles), is responsible for creating charge and charge center generation as well as threshold voltage shifts, increasing leakage current, and deteriorating overall device functions. Especially this effect is important for the devices sensitive to surface condition. Dose rate characterization is appropriate for devices meant for space applications, which receive relatively low dose ratestypically less than one rad per second. On the other hand, devices used in a nuclear weapons or reactor environment can encounter bursts of radiation impulses of width ranging from sec to msec. Such devices may fail long before the device received the radiation dose specified by the total dose tests. Hence, the devices meant for such applications should be characterized for high dose rate conditions. On the other hand, enhanced degradation of silicon devices under low dose rate conditions has also been observed and has received considerable attention in the last few years. This socalled ELDRS effect has been attributed to the delayed production of defects following irradiation [40]. Radiation testing under very low dose conditions similar to that encountered in space has invoked a considerable challenge. Single event effects (SEEs) are caused by charge collection at sensitive nodes due to particle radiation such as cosmic rays. As the name implies, SEE can be 14 caused by a single energetic particle that generates sufficient amount of charge from its interaction with the semiconductor. The protons and the heavy ions are the sources of SEE in space. The SEE has also been observed in terrestrial systems. The source of such failures in silicon devices has been attributed to alpha particles emitted by the trace elements in the packaging material. SEE can also induce different types of damages in devices. Single event upset (SEU) brings normally soft errors (nondestructive) whereas single event latchup (SEL) cause potentially destructive damage. Burnout of MOSFETs and Gate Rupture are part of SEE family and cause severe damage, called hard errors. Radiation damage in semiconductor devices can be grouped into two categories: i) ionization damage and ii) displacement damage. Ionization damages relate to the production of charges by the interaction of radiation with the different materials in the device structure. The dominant mechanisms by which most siliconbased devices degrade in the radiation environment are through the generation of electronhole pairs in insulating Si02 layers, and the creation of interface states at the oxidesemiconductor interface. Ionization damage is caused by electromagnetic radiation (gamma rays and Xrays) and by charged particle radiation (electrons, protons, alpha particles, etc.). To a good first approximation, the magnitude of these effects is proportional to the total energy (or the "total dose") deposited by the radiation, independent of the type of radiation. On the contrary, most IllV compound semiconductor devices do not incorporate insulating layers in their active volume. The charges produced by radiation in the quasineutral regions of the device recombine very fast. Hence, the ionization damage can only give rise to transient effects and no permanent change in the device characteristics. This accounts for their relatively high tol- 15 erance to total dose effects. The primary damage mechanism in these devices is through the creation of crystal lattice defects, or "displacement damage". This is a permanent damage that can be removed only by annealing. Displacement damage is generally caused by particle irradiation (electrons, protons, neutrons, and ions). Highenergy electromagnetic radiation (e.g. gamma) can also give rise to a relatively small displacement damage through the secondary particles (Compton electrons) resulting from the interaction of photons with the host material. Even though the major damage due to incident neutrons might be from atomic displacement, neutrons are also capable of inducing ionization in the lattice. Neutrons can not interact with charged particles because they are neutral, but ionization can be produced by secondary processes: (i) the recoil atoms or ions collided with incident neutrons produce ionization if they are highly energetic; (ii) the excited atomic nuclei returning to the low energy levels emitting gamma rays, which can ionize atoms in the lattice; and (iii) the target atomic nucleus absorbed incident neutrons emits ionized charged particles, in turn producing ionization. Therefore even in case of neutron bombardment, ionization is present accompanying gamma radiation or electronic excitation. 2.3.1 Total Dose Effects and Induced Damage in Semiconductors Ionizing radiation from various environment sources also presents total ionizing dose, which is the dose of radiation for a device can hold its electrical characteristics before it fails to provide reliable values. Different energy of incident gamma rays and xrays induce the three principal ways of interaction with materials: photoelectric effect A low energy (of the order of a few keV) photon pen- IEi1 etrates deeply into the atomic shells of the target material and transfers all its energy, E7 to an electron residing on a shell, giving it a sufficient energy to escape from the shell. The schematic illustration is shown in Fig. 2.2. The electron escaping from the shell has a kinetic energy given by Te=EyEB (2.13) where ER is a binding energy of the electron. This is the socalled photoelectric effect. This phenomenon explains the solar cells which convert thermal energy from sun directly into electrical energy. As a consequence of this photoelectron emission, the atom becomes excited and unstable. To reach a more stable state, either the atom can release an other electron, socalled Auger electron, then de-excite, or the vacancy can be filled by an other electron in the higher atomic shell by emitting an Xray photon. The X-ray fluorescence is the consequence of this procedure. source Detector FIGURE 2.2: Photoelectric effect inducing total dose damage on electronic devices by incident photons. 17 Unlike Compton effect discussed in the next section, the photoelectric effect strongly depends upon the energy of the incident photon, E, and the atomic number of the target atom, Z, in other words, atomic shell structure. The photoelectric crosssection including these parameters can be approximated as Upe OC (2.14) Z5/E/5 up to a few hundreds of keV of the incident photon energies. Compton effect of i0 106 when an incident photon has an energy in the range eV, it transfers a part of its energy to a weakly bound electron, and it scatters out from the atom at an angle 8 with respect to its incident direction leaving ionized atom behind. The photon will have longer wavelength and less energy. This is called Compton Shift. Scattered Photon hu' Incident Photon hu Target Atom -.. Pe .. FIGURE 2.3: Compton effect inducing total dose damage on electronic devices by x-ray beams. jI Rayleigh Scattering can be considered as a special case of compton effect since scattered photons experiencing Rayleigh scattering keep same value of wavelength. Atomic electrons in a target absorb energy from incident photons, then they scatter it in all directions. When the wavelength of the incident beam, A, is large enough to ignore compton shift value, zA, Compton effect merges into Rayleigh scattering. If we express the incident photon energy, E- = hi' and the scattered photon energy, E. hJ, from the energy and momentum conservation, the electron energy and momentum have the following relations in terms of the energy and momentum of the incident photon. E.y Te (pc)2 = + E2 Ey = Ee 2EE.),cos9 = E mc2 m2c4 (2.15) where Ee is the recoil electron energy and mc2 is the rest mass energy of the electron. In terms of the scattered angle, 9, Compton shift and the direction of the recoil electron are expressed by A=A'-A=A(1--cos6) = = 24.262 x 10" cm mc tançb- pair production (2.16) cot(O/2) hw A mc2 A (2.17) this process needs a highly energetic photon, whose entire energy is transferred in the field of an atom to create an electron- positron (e-p pair) pair. 2 2 The typical threshold energy of the incident A positron has identical properties as electron's except the opposite polarity of its charge. 19 photon is larger than 106 eV, and the schematic picture of the pair is shown in Fig. 2.4(a). By the law of energy conservation, the kinetic energy of the pair is given by K+K =E-2mc2 (2.18) where K_ and K+ are the kinetic energies of electron and positron, respectively, E.. is again the energy of the incident photon, and 2mc2 is the threshold energy in order for the atom to create an ep pair. Since the rest mass energy, mc2, is 0.511 MeV, The crosssection for pair production is not significant with photon energy less than several MeV. Pairproduction shows a moderate dependency of atomic number of the target material, but it is not as strong as that of the photoelectric effect. K --' Incident Beam hu nucleus + - -7r LJ' (b) (a) FIGURE 2.4: Pair production (a) and pair annihilation process (b). Another interesting process is pair annihilation shown in Fig. 2.4(b), describing an inverse process of pairproduction. When an electron and a positron reside near each other at stationary state, they can unite and annihilate producing a 20 pair of photons (most likely) or three photons (with less probability) to satisfy momentum conservation p.3.2 SingleEvent Phenomena When energetic particles strike on the device/circuit, there may be a measurable effect/change shown on their characteristics. The major sources of these energetic particles in space environments related to this single event phenomena are: 1) Earth Van Allen Belts protons and electrons; 2) magnetosphere for heavy ions; 3) cosmic ray protons and heavy ions; and 4) solar flares for protons and heavy ions. Unlike total ionizing dose effect, single event effect, is an individual event when a single, energetic ionizing particle induces an abnormal effect on a device. We discuss the single event effects in terms of definitions and terminologies wellknown in the radiation community [411. Single event effects on devices/circuits manifested by SEU (or soft error) usually do not bring fatal damages on devices/circuits, and normal device behavior can be restored by resetting or removing power. Especially, Single Event Functional Interrupt (SEFI) refers to the condition of the device showing its abnormal functions due to radiation and resuming its normal operations after resetting the power of the device. On the other hand, single hard error (SHE) is also considered as SEU yet causes a permanent damage to the characteristics of devices. Additionally, Sin- gle Event Burnout (SEB) and Single Event Gate Rupture (SEGR) are also destructive conditions occuring in power MOSFETs with highly localized burnout The initial momentum of the system was zero requiring at least two photons moving with equal momenta in opposite direction to cancel out their momenta after the union. 21 in the drainsource area, and burnout in gate insulator, respectively. Linear Energy Transfer (LET) is defined as "a measure of the energy transfer to the device per unit length as an ionizing particle travels through a material." The common unit for this property is MeVcm2/mg for Sibased MOS device. Two major parameters are used to determine a device/circuit weakness to SEU. To obtain an experimental test result with a specific LET, o is expressed as of errors/ ion fluence. The unit of crosssection is crn2/device(bit). Threshold linear energy transfer (LETth): the minimum LET required to induce upset phenomena at a crosssection crosssection for SEB and SEGR is Asymptotic or saturation cross section iO cm2. The typical i05 cm2. (a8t): cross section value when the upset rate does not increase. In other words, the value of the crosssection after LET became very large. Threshold LET occurs when a charge deposition is comparable with the critical charge, which is a function of the volume of node and degree of charge funneling (see Fig. 2.5). The cross section includes the surface area of the node as one of the parameters. When a semiconductor device is no longer capable of responding to input signals, we call it latched up to anomalous states. Latchup induced by single event with an ionized particle is longliving effect bringing potentially destructive harderrors in consequence. A typical example of this latchup phenomenon is shown in a bulk pwell CMOS structure forming pnpn thyristor structure. HolmesSiedle [42] introduced the effect, and was subsequently well formulated by Messenger and Ash [43]. 22 Incident Incident parkle / //IP / / / I,: n_Si/...//fr> Funnelling 'II. : ri \Ionization Path Ionization Ptth Substrate I '. (b) FIGURE 2.5: Induced ionization path due to a heavy ion impacted on (a) p-n junction device and (b) MOS transistor. 2.3.3 Displacement Damage Effects A relatively heavy, uncharged particle like a neutron traversing material collides with the lattice atoms of the semiconductor, displacing these atoms from their normal sites in the lattice into interstitial positions producing vacancyinterstitial pairs. The displaced atoms are called interstitials, while their former sites, now dislodged, are called vacancies. With fairly low energy ( keV), the displaced atom, frequently referred to as primary knock-on atom (PKA), can reside at the interstitial positions producing vacancy-interstitial pairs (Frenkel defects). If the PKAs received an energy of few tens of keV from a sufficiently energetic incident neutron, they can propagate through the solid, combining with impurity atoms or clustering with other vacancies creating a cascade-cluster. Highly energetic neutrons can impart enough energy to PKAs to result in subcascades, which form rather small, localized regions containing a high density 23 of defects in the lattice structure. Summers et al. [44] specified the PKA energy values for each kind of damage shown in lattice: isolated defects and small cascades are produced by PKAs with energy up to 2 keV; a single sub-cascade and isolated defects are created by PKAs with energy in the range of 2-12 keV; finally, several subcascades and isolated defects are produced by high energy PKAs larger than 12 keV. Fig. 2.6 shows the formation of defects and cascades in solid as a function of initial PKA energy created by heavy particles propagating through the solid materials. He also indicated that the nature of defects are same regardless of their origins whether from sub-cascades or isolated defects. Initial PKA Energy Frenkel Defects (vacancy-interstitial pairs) cascade-clusters (- few keV) subcascade-clusters (- few tens of keY) FIGURE 2.6: The cross-sectional view of the formation of Frenkel defects and cascades as a function of initial primary knock-on atom energy created by heavy particles. Vook [52] discussed about damaged region of semiconductors and concluded that for fast neutrons, there is superimposition of various sizes of damaged regions with the background of Frenkel defects depending upon the PKA energy. 24 However, very little correlation between vacancies and interstitials is expected in the damaged regions. The recoil energy of atoms for high energy (> 250 keV) electron irradiation is in the range of a few tens of eV which is only slightly larger than the displacement threshold energy of the lattice atoms in common semiconductors (e.g. In 6.7 eV, Ga 10 eV, As 10 eV). Hence electron irradiation introduces only simple point defects. In the case of protons of energy less than '-- 5 MeV, the main interaction mechanism is through Rutherford elastic scattering and the recoil energies are less than 100 eV. For higher energies, nuclear elastic and inelastic (spallation reaction) interactions are also important resulting in higher recoil energies. Similarly, neutron irradiation also produces highenergy recoils since the only mechanisms involving neutrons are through nuclear interactions. Evidently, the displacement damage produced by highenergy recoils consists of both point defects and defect clusters due to collision cascade. These defects like vacancies, interstitials, and vacancy clusters are electron- ically active and act in the same way as dopants, which are sources of carriers in semiconductors. They also act as traps for carriers, increase the number of collisions in the material, and introduce new energy levels within the energy band gap of the semiconductor [46]. As shown in Fig. 2.7, the defect levels behave as: (i) Generation center, which generates electron-hole pairs when the free carrier concentration is lower than its equilibrium value, for instance, in the spacecharge region of reversedbiased pn junction or MOS capacitors; (ii) Recombination Center, which captures a positive and a negative carrier from valence and conduction bands, respectively, recombining an electronhole pair at the defect center when there are excess carriers in semiconductor; and (iii) Trapping center where carriers are captured and emitted back to their origi- 25 nal band levels. All these defects cause of increasing resistivity and decreasing carrier mobility. Generation Recombination Bc K' ET Ev FIGURE 2.7: Electron energy band diagram associated with defect energy levels introduced by irradiation. Additionally, these defects can become compensation centers by behaving as dopants that accompany with opposite sign carriers. These carriers can be compensated with majority carriers in the material resulting the reduction of their concentration. Tunneling centers might be another possibility for large densities. In summary, due to the bombardment of heavy, uncharged particles on semiconductor material, atomic displacement or dislodging induces carrier removal. The mobility of the carriers is also reduced due to scattering mechanisms, and minority carrier lifetime degradation appears due to recombination centers. These overall degradations are related to the particle fluence and can be expressed by ratebalance equations: 1i- = 1/To = l/io+K, N = NOKN4 where (2.19) is the minority carrier lifetime, N is the carrier concentration and j is the mobility as a function of incident particle fluence 4, and K,-, KN, and are called degradation (damage) coefficients. 2.3.4 NonIonizing Energy Loss and Displacement Kerma By knowing the energy spectrum of PKAs and the result of the secondary collision of the PKAs with lattice atoms in terms of degree of displacement, the non-ionizing energy loss (NIEL), the energy transferring into atomic displacements per unit length of an incident particle can be calculated. Electron NIEL may be obtained by calculating this integral: NIEL = 1800 f°min L[T(e)IT(e)d)dcl (2.20) where NA/A is the ratio of Avogadro's number to atomic number, T() is the transferred energy to the target atom nucleus by an electron scattered with an angle, e. The upper limit angle 1800 implies that maximum energy is transferred to nucleus when e = emin, 1800 (headon collision). The lower limit angle, is the scattering angle for the threshold recoil energy produce displacement. d()/dfl is the differential cross section for elastic scattering of electrons into a solid angle increment d1, and L[T()] is the Lindhard partition function described by Lindhard [54] and Doran [55] for evaluation of the fraction of PKA energy transferred into nonionizing energy. 27 The quantitative analysis of the bulk damage due to displacement induced by charged particles have been developed by calculating the NIEL. Despite the fact that PKA of a particle depends on the types of particles and sample materials, there is a good linear dependence of damage coefficient on NIEL for silicon material while a quadratic dependence manifests on ITT-V compound semiconductors. Detailed investigation appears in section 2.4 showing the relationship between types of radiation particles and their damage coefficient dependence on NIEL. On the other hand, for the characterization related to neutron damage, displacement kerma (KD) in the material is evaluated. Socalled "NJOY methodology" is employed to calculate KD, which is given by [49]. KD o(E) f K(E, T)L(T)TdT where . i th : neutron reaction. . E : neutron energy. . oj(E) : neutron crosssection of reaction. . T PKA recoil energy. K(E, T) L(T) Ed normalized distribution of PKA recoil energy. : Lindhard energy partition function. threshold energy for displacement. (2.21) This is a fairly standardized damage model suitable for characterizing neutron damage in silicon. In the case of GaAs sample irradiated by neutrons, a slight adjustment is required on Eq. 2.21. Griffin et al. [49] suggested a modified PKAdependent model for GaAs to be KD = u(E) fEd K(E, T)L(T)(T)TdT (2.22) with (T), a damage efficiency function depending on PKA. It is very useful to discuss at this point the "equivalent monoenergetic neutron fluence" frequently used in the radiation community. Since energy spectra of neutron flux from a facility such as an accelerator or a nuclear reactor is different from one another, there is a need for standardizing the effective fluence of neutrons for the case of comparison. This method only holds with the knowledge of full energy spectrum of neutron source when energy behaves as a variable for evaluating degradation effect. The equivalence monoenergetic neutron flux by using 1 MeV for the standard energy value is evaluated as [53] cbeq,1MeV J q(E) FD(E) FD(1MeV)° (2.23) where FD is a damage function, which has a linear and a quadratic relationship with displacement kerma for Si and GaAs, respectively [49]. The conversion between onionizing energy loss, (NIEL), and displacement kerma, KD can be found by performing unit conversion and is given by [26]. NIEL(E) 10_27KDNA or NIEL(E) = 1.602 x (MeV . cm2/g) 1035KDNA p (rad. cm2) (2.24) (2.25) By knowing displacement kerma, equivalent neutron flux for typical semiconductor materials have been calculated [53] and listed in Table 2.1. 29 Material KD (1 MeV) (MeV mb) NIEL dDd/dt (MeV cin2/g) (rad/s) (cm2s') Si 95 2.04x103 1.4 4.3x10'° GaAs 70 O.64x103 0.46 4.5x101° InGaAs 82 0.58x103 0.418 5.6x10'° ImP 78 O.64x103 0.46 8.lxlO'° TABLE 2.1: Damage function (Kerma for InGaAs and InP), NIEL, displacement damage dose rate, and 1 MeV neutron flux. Reactor was operated at 100 kW. 2.4 Radiation Effects on 111-V Compound Semiconductors From late 80s, radiation effects in ITT-V compound semiconductors have been investigated [44] [51]. Gallium Arsenide is the primary material among III V compound semiconductors, which received a major attention for commercial applications. InP and InGaAs slowly started receiving attention from early 90s, but not much has been reported related to these compounds. Damage correlation effects, induced by protons, deuterons, helium ions, and 1 MeV equivalent neutrons, have been established by Summers et al. on silicon and ITTV compound semiconductor materials [44] [47]. As far as silicon is concerned, they found for all charged particles and energies a linear dependence of displacement damage factors from experiment on nonionizing energy transfer on the sample. Also, they discovered that deuterons and helium ions for a given energy created twice and eighteen times larger damages, respectively, than that of protons. Displacement damage factors from 1 MeV equivalent neutron to the charged particles also fell into the common curve showing the same linear de- 30 pendence as other charged particles mentioned above. Consequently, regardless of the types of particles, the damage factors showed the first order proportional- ity to the produced defects number, while no proof of PKA energy dependence appeared. For GaAs and InP, on the contrary, the degradation constant shows different behaviors from that of silicon and are strongly dependent upon the types of dopants and radiation particles. From the study that Griffin et at. [49] performed with GaAs, nonlinear behavior of damage constant responding to the neutron energy used was observed. The thermal spikes showing in the damage coefficient with respect to the neutron energy tells that the local melting and material recrystallization due to high cluster energy density were the major cause. The interesting phenomenon related to this was that the higher PKA energy only increases number of defect clusters but the size of them was not affected by it. It has been shown by Burke et at. [58] that the degradation of the bulk electrical properties of semiconductor materials is proportional to the energy going into the production of atomic displacements, that is, to the nonionizing energy loss (NIEL). The concept of NIEL has been widely used over the past decade for correlating displacement damage in semiconductors introduced by particle radiation of different types, (protons, neutrons, etc.) and of different energies. Such correlation studies are extremely useful in predicting the expected damage in space or highenergy accelerator environment from laboratorybased test measurements. The use of NIEL concept for correlating displacement damage effects is emerging to be as general as the concept of "total dose" in correlating ionization damage effects in semiconductor devices. A fairly recent study done by Xapsos et at. [50] confirmed that ptype GaAs 31 and indium phosphide showed a quadratic dependence of displacement damage by electron and gamma fluence on nonionizing energy loss. Especially, they found ntype InP irradiated by electron and Co6° gamma showing more reliable results by applying a quadratic dependence of displacement damage on NIEL than by applying a linear dependence. For n and ptype silicon, damage coefficient shows a linear and a quadratic dependence on NIEL, respectively. Their conclusions mostly coincide with that of Summers [47], who found damage coefficient from electron irradiaton with a linear dependence for nSi, GaAs, and quadratic dependence for pSi on NIEL. Additionally, for Si, GaAs, and InP, they found a linear relationship between damage coefficients and NIEL after proton irradiation on solar cells. The relative damage coefficients from proton and electron showed a direct dependence on NIEL for nSi, GaAs, and InP, yet the absolute degree of damage on devices from electrons was much less than that from protons. They considered the smaller recoil energy created by electron was responsible for this difference. Hence, the application of NIEL in the study of damage correlation has been successful in silicon devices whereas there have been considerable discrepancies in GaAs devices in the regime of particle energies that produce highenergy recoils. The results from Barry et al. [59] showed that the minority carrier life time damage in LEDs for proton energy greater than 10 MeV is considerably smaller than that expected from the NIEL calculations. Initially, there was an attempt to explain this discrepancies by introducing "restricted NIEL" concept that basically assumes that the high energy recoils traveled out of the sensitive region of the device and hence did not contribute to the measured damage. This hypothesis lasted only until the discrepancy was found in other device geometries in which this "restricted NIEL" concept is not applicable. Alternatively, it 32 is possible that damage clusters created by highenergy recoils are less effective in the degradation of bulk properties. Therefore, it is necessary to establish the correct procedure for the calculation of NIEL and to improve our understanding of the radiation damage mechanisms for different types of materials and devices. 2.5 Basic Mechanisms of Radiation Effects in Bulk Devices There are two important mechanisms by which the ionization damage manifests in SiMOS devices: i) charge trapping in insulating Si02 layers, and ii) the creation of interface states at the oxidesemiconductor interface. On the other hand, GaAsbased devices, which are not incorporated with insulator layers, degrade mainly due to displacement damage. Some of the known basic mechanisms by which displacement damage affects the performance of bulk (nonheterostructure) devices are the followings: i) carrier removal; ii) mobility degradation; iii) carrier lifetime degradation; and iv) defect assisted tunneling. The carrier removal effect refers to the decrease of carrier concentration in the samples after irradiation and it has been observed for both ntype and p type samples [60]. Charge compensation by the radiationinduced defects is responsible for the carrier removal. Quantitatively, the solution of charge neutrality equation taking into account of all the (intrinsic and radiationinduced) defects present in the materials can model this carrier removal effect [61]. This analysis of course requires the knowledge of the energy levels and the charge state (donor/acceptor nature) of the defects. In general, radiation creates both donortype and acceptortype deep level defects. Deep acceptors are responsible for the compensation in ntype materials and deep donors in ptype material. Deep donors in ntype and deep 33 acceptors in ptype materials do not play a role in the compensation mechanism since they are generally unionized (or neutral) at normal temperatures of interest. Finally, the energy levels of the radiationinduced defects should be dealt carefully. A vast majority of the radiationinduced defects give rise to deep levels. However, occasionally some radiationinduced defects do have energy levels that lie close (within 0.1 eV) to the band edges. The behavior of these defects must be evaluated carefully. Some second order nonlinear carrier removal effects observed at high fluences of particle irradiation have been attributed to such shallow levels [75]. Similarly, some defects may have more than one energy level associated with them. Quantitative modeling of carrier compensation by such multilevel defects also deserves careful consideration [62]. Carrier removal effect manifests in a number of ways in bulk devices. For example, the radiationinduced threshold voltage (VT) shift in MESFETs/JFETs is caused by the carrier removal effect [63]. The increase in resistance of diffused/ion implanted resistors or the increase in the parasitic resistance in devices ( e.g. source and drain resistance in FETs and emitter, base and collector resistance in BJTs) after irradiation is also primarily due to the carrier removal effect. The second basic mechanism by which radiationinduced displacement damage affects device performance is the carrier mobility degradation [8]. The additional scattering of carriers by the radiationinduced defects causes this effect. Scattering by the charged and neutral point defects may be quantitatively modeled by using the standard formulism of ionized impurity scattering and neutral impurity scattering [64]. Mobility degradation effect is responsible for the transconductance deterioration in devices subjected to irradiation. The third basic mechanism of radiationinduced degradation in devices is the decrease of carrier lifetime. It is caused by the additional channel of gen- 34 eration and recombination involving radiationinduced defects. Quantitatively, generation/recombination via deep levels associated with the radiationinduced defects can be modeled by the conventional ShockleyReadHall (SRH) mechanism [65]. Lifetime degradation causes a number of effects observed in devices including current gain degradation in bipolar transistors, light output degradation in light emitting diodes (LEDs) and lasers and increase of dark current in photodetectors and in solar cells. The final mechanism of interest that can cause degradation in devices is the defect assisted tunneling [66]. This occurs, for example, when there is a large density of defects generated in the depletion region of a pn junction. The electrons and/or holes can tunnel into the defect sites and eventually an electronhole pair may be removed. This gives rises to a current component. In bipolar transistors this is additional source of base current resulting in current gain degradation. 2.6 Defect Characteristics As discussed in previous section about the basic mechanisms of radiation effects in devices, it is found that the parameters related to the basic mechanisms are in turn controlled by the more fundamental defect characteristics. There are four crucial characteristics of defects that are important from the point of view of the quantitative modeling of the basic mechanisms: i) Energy level of the defect; ii) Charge state (donor/acceptor nature) of the defect; iii) Capture crosssection; and iv) Defect introduction rate (per unit fluence). The primary point defects that are generated by particle irradiation of GaAs are the Frenkel pairs related to Ga and As sites, namely, VGa, Ga2, VA8 and As2. 35 These point defects are quite mobile and hence a fraction of the initially generated defects may annihilate by recombination and also generate new defects including antisite defects (ASGa and GaA3) and possibly more complexdefects (e.g. VA8ASGa, VGaGaA8, etc.). Eventually a stable state is reached in which a set of defects with finite concentrations is left behind in the crystal that can be removed only by high temperature annealing. The energy levels and the chargestate of many of these defects have been topics of considerable debate in the past and only some of them have been reliably established. For example, VGa is believed to be an acceptor and VA8 a donor. A number of experimental techniques including Hall effect measurement, deep level transient spectroscopy, photoluminescance, photo-absorption, electron spin resonance, etc. have been used in the past to characterize the defects. Hall effect measurement provides quantitative information on the carrier concentration and carrier mobility. The introduction rate of donors and acceptors after irradiation may be determined by using ntype and ptype samples, respectively. Deep level transient spectroscopy is used to characterize the deep level traps and can provides information about the concentration, energy level and the capture crosssection of the traps. However, DLTS is not a convenient technique for shallow traps since very low temperatures are required to probe shallow levels by DLTS. Thus, Hall effect measurement and DLTS techniques work in somewhat complimentary regimes of energy levels of defects. Photoluminescence is a powerful technique to measure the energy levels very precisely but for only shallow levels and an absolute measure of concentration is difficult to obtain in PL measurements. Finally, it is important to note that some defect characteristics (e.g. energy Th level, charge state and capture crosssection) are independent of the type of radiation (electron, proton, etc) that created the defect. The defect introduction rate, on the other hand, is not only expected to be dependent upon the type of radiation but may also be a function of the particle energy, for a given type of radiation. Hence, the defect characterization must include a range particle energy of interest. 2.7 Thermal Annealing Effect of Bulk Damage Annealing refers to a partial or total recovery of electronic characteristics of materials/devices/circuits, which were exposed to damaging radiation sources. Annealing of radiation damage in semiconductors in contrast to metals, is likely to be affected by the charge states of the species that interact. This is especially important for processes that are detected by changes in carrier at various defects (because of the charge neutrality requirement). Although annealing study had started as early as early 50s when the degradation effects due to various radiation sources became major issues in radiation community, no established quantitative model has been discovered until now. Empirical data obtained from the experiment done for annealing study might be the only reliable source for further work on annealing process and anticipation. We will discuss about qualitative consequences of annealing process performed on irradiated materials in this section. It is plausible for the effect of annealing on an irradiated material to depend upon the concentration of defects and their spatial distribution through the material. Heavy irradiation would tend to create clustering and bimolecular recombination in the latter stages of annealing. As mentioned in previous sec- 37 tion, in case neutron radiation, most of the damage due to fast highly energetic neutrons appeared as displacement damage in the target materials. This implies that owing to the thermal motion of atoms expected from high annealing temperature, after a proper annealing process, part of these displaced defects may be annealed out through vacancy-interstitial recombination by breaking up and removing the defects created from radiation. On the other hand, annealing can also bring up undesirable disadvantages because of the possibility of the created defects forming associations with impurities previously existed in the lattices of the materials. Therefore, annealing can cause improvement in the post-radiation integrity of the lattice through recombination, but it is also possible for the defects to become trapping centers creating more defects on the materials. From the annealing studies on bulk silicon, silicon transistors, and silicon solar cells after pulsed neutron radiation, it has been found that a principal parameter affecting the rate of annealing, in the post-neutron-radiation phase, is "carrier density level within the active region, inside that portion of the device that determines its post-radiation characteristics." The annealing of Frenkel pairs has been discussed by Fletcher and Brown who divide the annealing processing occurring in the radiated material to three stages:(1) close pair recombination occurs as a first order; (2) an intermediate stage in which a mobile defect either recombined with its partner or permanently diffused away; and (3) recombination of randomly spaced vacancies and interstitials. Furthermore, observing fast neutron damages in terms of cascade theory (as contrasted to the thermal spike or displacement spike models), we can anticipate a high concentration of vacancies near the primary event with the interstitial atoms lodged somewhat farther away. On this basis, it is reasonable to assume that fast neutron irradiation (and to a lesser extent heavy charged particle bombardment) would be highly favorable for the formation of clusters of vacancies, or voids. Dislocation loops can result from the collapse of void regions. RadiationInduced displacement defects (bulk damage) in silicon do not anneal easily, because longlived vacancies and interstitials created by the radiation are usually complexed with an impurity atom. These defects are stable at room temperature through 200°C but the damage often anneals between 200°C and 450°C completely. In IllV compound semiconductors, on the contrary, even the simplest type of damage, resulting from low energy electron or gamma radiation, may be considerably more complicated than in single element semiconductor. In addition to the greater variety of intrinsic defects in compounds, because of misplace- ments, there is no assurance of the method for anticipating the numbers of created vacancies and interstitials even with the same type of radiation exposed to the materials. For instance, in IllV compounds low energy electron or gamma irradiation would tend to create not only pairs but also triplets as well. Fortunately, the triplets are probably easily converted into vacancyinterstitial pairs, for regardless of the energy of the primary knockon, we can expect a high degree of correlation between each misplacement and an interstitial. Khanna et al [51] studied displacement damage with neutron and gamma irradiated GaAs samples along with annealing effects on them by investigating photoluminescence spectrum intensity behavior. They observed the introduced defects and traps by neutron irradiation removed by isothermal annealing while different traps appeared after the procedure. Additionally, different defects formed when different radiation sources, electron, gamma, and neutron for their study, were used. According to the study done by Griffin et al. [49], recovery 39 of electronic characteristics of GaAs LEDs after isothermal annealing showed a dependence on types of radiation particles. They found almost full recovery of damage of Co6° gamma irradiated devices while at most half of damage were recovered on neutron irradiated samples. They concluded that the different mobility and annealing behavior of defects produced by each type of radiation were responsible for the discrepancy shown on recovery result since secondary electrons resulting from gamma photons create single point defects while damage clusters are common defects for neutron. In conclusion, annealing technique is valid for the recovery of introduced defects from various radiation sources when it is performed using properly determined method to avoid undesirable defects and traps through the process. 2.8 Basic Mechanisms of Radiation Effects in Heterostructure Devices The basic mechanisms of radiation effects in bulk devices discussed in previous section are also important in the case of heterostructure devices. However, the details of the manifestation of these effects in heterostructure devices may be different from that of the bulk devices. Some of the reasons for these differences may arise from the following: Heterostructure devices have layered structures often containing very thin regions. Since different layers of the structure are made from different materials, the nature and extent of damage in different layers may be different. The relationship between the overall device performance and the defects 40 in each layer of the device must be carefully investigated (nontrivial). . The band discontinuity at the interface between layers, carrier confinement and quantum effects may play a significant role in modifying the device degradation behavior. Other unusual effects such as the strain in the layers, polarization effects, etc. may also be important. Hence, the quantitative nature of the manifestation of the basic mechanisms of device degradation will be highly device dependent and can be addressed only in the context of specific devices. In this study, two different Ill-V compound semiconductor devices, high electron mobility transistor (HEMT) and single heterostructure bipolar transistor (SHBT), are chosen to address some of these issues in detail. 2.8.1 Degradation Effects in High Electron Mobility Transistors The schematic diagram of a crosssection of a typical A1GaAs/GaAs a HEMT layer structure is shown in Fig. 2.8. The larger band gap A1GaAs layer is doped and placed on the top of undoped, smaller band gap GaAs layer. The electrons diffuse from AlGaAs layer to the GaAs layer and are collected at the heterojunction forming a thin sheet of twodimensional electron gas (2DEG) in a quasitriangular potential well as shown in Fig. 2.9. Since the 2DEG is formed in the region of undoped GaAs where it is physically separated from the ionized donor impurities in A1GaAs region, the Coulomb scattering of electrons in 2DEG is drastically reduced. Furthermore, an undoped thin spacer located between doped A1GaAs and GaAs region further separate 2DEG from ionized donors at the heterojunction and in turn reduces Coulomb scattering. Hence 41 the 2DEG exhibits very high mobility giving the name of high electron mobility transistors for the devices made from these structures. n+GaAs ni-GaAs dopedAlGaAs undopedA1GaAs spacer layer 2DEG GaAs 30 cycles ( AIGaAs GaAs S.!. Substrate FIGURE 2.8: Schematic diagram of the crosssection of an A1GaAs/GaAs HEMT. As shown in Fig. 2.8, HEMT device is usually capped by a thin layer of doped GaAs for ohmic contact formation for source and drain contacts as well as protection of the more reactive AlGaAs layer from coming in contact with the ambient. For gate, on the other hand, Schottky barrier formation is performed on doped A1GaAs layer after recess etching the GaAs cap layer. There are several important device electrical characteristics of HEMT devices: 1) threshold voltage (VT), ii) transconductance (gm), iii) breakdown voltage, and iv) unity gain cutoff frequency (fT). The drain current (ID) of the device is determined by the bias conditions (drain and gate voltages) as well as 42 GaAs A1GaAs S.C. 1 ---r S.C. 2 A / EFO d4/ FIGURE 2.9: Energy band diagram of a HEMT in the vicinity of the heterojunction interface at equilibrium. by VT and g. As reported by a few researchers [30] [33], radiation damage causes an increase in VT and a decrease of g causing in turn a decrease on drain current under given bias conditions. The increase in the VT is related to the carrier removal effect whereas the decrease of g is related to mobility degradation. Detailed discussion for these effects will be presented in following chapters along with our experimental results. The carrier removal mechanism in HEMT structures is more complicated than that in the bulk. Since the origin of the 2DEG is the donors in the A1GaAs layer, the carrier removal rate in the 2DEG might be expected to be determined by the defects introduced in the doped A1GaAs. However, our experiment on neutron irradiation of HEMT structures show that the 2DEG 43 concentration begins to decrease at fluences that are too low to cause a significant reduction of carriers in the doped AIGaAs layer. On the other hand, the GaAs layer in which the 2DEG is formed is undoped and has a typical background doping concentration of ".' 1014 cm3 (ptype for MBE grown materials). Hence we believe that the carrier removal in 2DEG in our HEMT structures is more sensitive to the defects introduced in the GaAs region. Krantz et al. [30] also claimed the same observation as we have obtained. Detailed discussion supporting this claim will be presented in the chapter of experimental results. The electron concentration in the 2DEG is normally obtained by a self consistent solution of the Poisson and Schrodinger equations for quasitriangular potential well region. To determine the 2DEG concentration, a number of parameters including the Schottky barrier height, AIGaAs layer thicknesses for doped and undoped, conduction band discontinuity between AlGaAs and GaAs. Theoretical as well as experimental investigations on the dependence of 2DEG concentration on the above parameters have been reported in the literature [10] [12]. These conventional models normally ignore the effect of the background doping concentration ( 1014cm3) in the GaAs well region. This assumption, however, is not valid in the irradiated samples. We have developed a quantitative model relating the carrier removal in the 2DEC to the radiationinduced defect concentration in the GaAs region by performing selfconsistent Poisson Schrodinger solutions. We will investigate the carrier removal rate in 2DEG in terms of the radiationinduced defects introduced in each layer of the HEMT structure. Models for the charge control in the 2DEG through Schottky barrier gate voltage have been published [10], [11]. Threshold voltage of the device is calculated by using these models in terms of device structure parameters. These 44 models once again ignore the doping concentration in GaAs layer. However, we propose to modify these model by including the effect of the defect concentration in the GaAs layer and derive quantitative expressions for the dependence of VT on the radiation fluence for neutron irradiation on HEMT. Radiationinduced mobility degradation of 2DEG is qualitatively similar to the mobility degradation of carriers in the bulk. The additional scattering of carriers due to the radiationinduced defects in the 2-DEG region as well as in the spacer A1GaAs region is responsible for the mobility degradation. However, the quantitative formalism of scattering of 2DEG by the ionized impurities/defects is slightly different from that in the bulk since the two dimensional wave function of electrons is used in the solution of Boltzmann Transport Equation. But these details are straightforward and can be found in the relevant literature [13]. We will present the Hall measurement results performed on control samples of HEMT structures after neutron irradiation. The models for the 'D VDS characteristics of HEMTs have been discussed extensively in the literature [10] [12]. The transconductance regime of the 'D VDS (gm) in the linear characteristics of the device is directly proportional to the carrier mobility. Hence the mobility degradation leads to a gm degradation in the linear regime. We analyze this degradation of transconductance in the linear regime using the standard device models to determine the increase in the source/drain resistance and mobility degradation. In the saturation regime of the 'D VDS characteristics, the drain current is usually limited by carrier velocity saturation rather than by drain pinch off in most practical devices (except for very long channel devices). The saturation carrier velocity is not expected to be affected by radiation. Hence the intrinsic transconductance is not expected to be affected by radiation. However, the measured g g2 can be 45 lower than the intrinsic 9m if the parasitic source and drain resistances (R and RD) are significant. These resistances can be increased after irradiation due to mobility degradation and in turn cause a degradation of measured g. We analyze the data selfconsistently and develop quantitative models for device degradation parameters in terms of radiationinduced defect concentrations. 2.8.2 Radiation Effects in Heterojunction Bipolar Transistors A schematic diagram of an InP/InGaAs single heterojunction bipolar transistor is shown in Fig. 2.10. The baseemitter (BE) junction of this device is a InGaAs Contact Setback Layer I I InGaAs Grading I N.. I InP Emitter InGaAs Base InGaAs I InGaAs Collector InGaAs Subcollector InP substrate FIGURE 2.10: InP/InGaAs single heterostructure bipolar transistor structure. heterojunction (InP/InGaAs) and the basecollector (BC) junction is homojunction. The device that includes a heteroj unction also at the BC junction is called a double heterojunction bipolar transistor. The main advantage of the 46 BE heteroj unction is the emitter injection efficiency. The band discontinuity at the BE heterojunction presents a large barrier to the back injection of holes from the base to emitter doping and an increased base doping. Thus the emitter capacitance and the base resistance can be both decreased substantially which in turn increases the device speed considerably. The most important characteristics of HBTs are i) current gain, ii) offset voltage iii) breakdown voltage, and iv) unity gain cutoff frequency (fT). The physical mechanisms involved in the radiationinduced degradation of these SHBTs will be discussed in this section. There have been a few reports on the radiation effects in Si/SiGe HBTs [19], [20] , AIGaAs/GaAs HBTs [21] [23], and InP/InGaAs HBTs [26] [28]. The main physical mechanism associated with the current gain degradation in HBTs is the excess carrier lifetime in the base. The base current in a heterojunction bipolar transistor consists of three components: i) recombination in the neutral base region; ii) recombination in the BE junction space charge region (SCR); and iii) surface recombination at the BE junction periphery. Modern HBTs have very narrow base regions ( 500A). Hence, the contribution of the neutral base recombination is generally very small. Thus, most of the base current is contributed by the bulk and the surface recombination in the BE junction SCR. The offset voltage in HBTs is caused by the asymmetry between the B E heterojunction and the BC homojunction. The band discontinuity at the BE heteroj unction results in a smaller values of the BE junction saturation current as compared to that of the BC junction. Hence, in the common emitter configuration, the BC junction turns on before the BE junction is turned on giving rise to the offset voltage. The offset voltage has been observed to increase after irradiation [27]. We also have seen this from our study and shown that the 47 increase in the offset voltage is caused by the radiationinduced increase of the BC junction saturation current. In other words, the offset voltage degradations related to the life time degradation in the collector. We study this effect after electron and neutron irradiation in SHBTs. Having discussed radiation fundamentals and basic mechanisms of radiation effects on semiconductor bulk as well as heterostructure devices including re- views of related literature, we will introduce radiation experiments performed on IllV compound semiconductor devices in later chapters. In next chapter, especially, complete processing sequences of a HEMT device fabrication with optimized recipes for individual process step is included. It also includes a detailed description of electron and neutron irradiation processes performed on IllV devices. CHAPTER 3 DEVICE DESCRIPTION AND EXPERIMENT DETAILS In this chapter, we will introduce a description of layer structures of MBE grown HEMT and HIGFET structures. Growth parameters for MBE technique are briefly introduced. The growth layer and device structures of MBE grown HIGFET devices fabricated in Honeywell sensor Laboratory is also introduced. Especially, the HEMT device is thoroughly studied providing the optimized device fabrication techniques. Mask set designs created by "ic station" software 1 for an actual HEMT device without passivation and with passivation layer are discussed in detail. Dimensions of the several actual devices such as fat FETs, variable gate FETs, RF devices, and transfer length measurement (TLM) structures, as well as contact pads are specified for both unpassivated and passivated device mask sets. Measurement setups related to Hall (Van der Pauw) measurement and currentvoltage (I V) characteristics are described. Also this chapter introduces the detailed procedures of electron and neutron irradiation processes including appropriate methods of sample preparations. 1 This is Mentor Graphics software developed by Mentor Graphics Corporation. 49 3.1 HEMT Layer Structure A1GaAs/GaAs HEMT heterostructure layers were grown by Molecular Beam Epitaxy (MBE)2 technique in the Department of Electrical and Computer Engineering at Oregon State University (OSU). Devices with different gate lengths in the range of 6 to 20gm were fabricated at OSU using conventional photolithography technique. The schematic diagrams of A1GaAs/GaAs deltadoped and uniformlydoped heterostructures grown for this study are shown in Fig. 3.1. Each structure Drain Source I Source I Gate _2GaAs GaAs GaAs AIGaAs SI. GaAs Substrate ö GaAs AIGaAs - spacer 2DEG 2DEG GaAs Buffer Layer Gate _ A1GaAs - doped Sidçlta layer AIGaAs Drain GaAs Buffer Layer I I I I doped structure SI. GaAs Substrate uniform doped structure FIGURE 3.1: Schematic diagrams of 6 and uniformdoped HEMT structures. was grown on < 100 > LEC GaAs substrate which was indium soldered to molybdenum blocks during the growth process. Reflection highenergy electron 2 PerkinElmer PHI-425B solid source MBE system. 50 diffraction (RHEED) was setup for growth rate calibration and 1[tm/h was the nominal growth rate. During the MBE growth, the substrate temperature was maintained at 585°C. The layer parameters of the two types of structures are listed in the Tables 3.1 and 3.2. The top GaAs cap layer was heavily doped with silicon by 1 x 1018 /cm3 to reduce contact resistance for ohmic contact metalization. The grown layer was cleaved into 5 mmx 5 mm pieces for Hall effect measurement and 10 mm x 10 mm for device fabrication. Material Thickness Dopant Concentration Si (ntype) lx 1018/cm3 GaAs 100 A102Ga08As 300 A N/A intrinsic Si ö-layer Si 3x 10'2/cm2 Al02Ga08As 200 A N/A intrinsic GaAs 5000 A N/A intrinsic A102Ga0 gAs 100 A 30 cycles GaAs 100 A 30 cycles A <100> S.I.Substrate TABLE 3.1: Layer parameters of the MBEgrown 8 HEMT structure. Single layers of doped A1GaAs and GaAs grown by MBE on appropriate buffer layers were used as control samples for irradiation experiments. The layer parameters of these structures are listed in Table 3.3 and 3.4. The second type of HFET used in this study is A1GaAs/InGaAs/GaAs self aligned gate, high power HIGFET devices fabricated at Honeywell Sensor 51 Material Thickness Dopant Concentration GaAs 100 A Si (ntype) 1x1018/cm3 A102Ga08As 350 A Si (ntype) lxlO'8/cm3 A102Ga08As 150 A N/A Spacer intrinsic N/A intrinsic GaAs 5000 A A102Ga08As 100 A 30 cycles GaAs 100 A 30 cycles < 100> S.I.Substrate TABLE 3.2: Layer parameters of the MBEgrown uniformlydoped HEMT structure. Material Thickness (A) Dopant A102Ga08As 2500 lxlO'8/cm3 of Si A102Ga08As 2000 intrinsic GaAs buffer 5000 N/A A102Ga08As 100 30 cycles GaAs 100 30 cycles S. I. Substrate TABLE 3.3: MBE grown single layer A1GaAs structure parameters. Material Thickness Dopant GaAs 1im lxlO'7/cm3 of Si GaAs buffer 1m intrinsic S. I. Substrate TABLE 3.4: MBE grown single layer GaAs structure parameters. Laboratory. The gate length of these devices is 0.6 pm and a large number of 40 pmwide gates are connected in parallel for high current. These devices also incorporate a lightlydopeddrain (LDD) region of length 2 or 5pm between the gate and the drain to increase the breakdown voltage of the devices. The and LDD5 ' devices are 67360 pm and 46360 total gate widths for LDD2 pm, respectively. The crosssection of device structure of this HIGFET device including component of each layer is depicted in Fig. 3.2 followed by the illustration of the crosssection of LDD structure of HIGFET in Fig. 3.3. The structure parameters of MBE grown HIGFET devices are listed in Table 3.5. Material Thickness (A) Dopant 75As 250 intrinsic In0 25Ga0 75As 150 intrinsic delta layer doped Al0 25Ga0 Si GaAs buffer N/A S.I.Substrate TABLE 3.5: MBE grown HIGFET heterostructure structure parameters. LOT I.D.of dies: 5551-01-01-LDD2-2-54, 5551-01-01-LDD2-2-57, 5551-01-01-LDD2-2-58, and 5551-01-01-LDD2-2-54. ' LOT I.D. of dies: 5551-01-01-LDD5-4-46, 5551-01-01-LDD5-4-54, 5551-01-01-LDD5-5-54, and 5551-01-01-LDD5-5-58. 53 Source Gate Drain A1GaAs GaAs Buffer Layer S.I. GaAs Substrate I Implanted Region Delta Layer FIGURE 3.2: Schematic layer structures of a A1GaAs/InGaAs/GaAs HIGFET device. Mask Set Design 3.2 3.2.1 Unpassivated Devices To fabricate A1GaAs/GaAs HEMT devices, a threelevel mask set was designed. The design layout for a complete die is shown in Fig. 3.4. Each level of mask includes four unit cells which contains various kinds of FETs, TLMs, and Hall bars. . HHall Bars . VVariable Gate Width Transistors TTransfer Length Measurement structures 54 DG spacing Source I Gate 1k I LDD Drain N N Implant Implant GaAs Buffer InGaAs Channel Si 6 doped layer FIGURE 3.3: Crosssection of lightlydoped drain (LDD) HIGFET structure. . Sside Gate structures . TFat FET This mask set was designed to fabricate unpassivated HEMT devices with several dimensions and other supplementary structures. The following three levels of masking steps were used. . Level 1Mesa isolation for individual devices and structures: Each device and structure was fabricated on its own mesa island. The mesa island area where the each structure had built was shown as dark region. . Level 2Source and Drain formation: Au Ge Ni metalization was evaporated and patterned by conventional photolithography and liftoff techniques. The contacts were subsequently annealed in a forming gas ambient at 4000 for 3 minutes. 55 UEEThEE F1F3 ril I I II II ç, L_Lk .-I - 1IS1IIS2IIS3IE1E1IS6IIS7IIS8IftJI IS5IIS9IIS10I 14 ThEEThEE Si' EE1 1S14 I S12 S13 S16 EII1 515 1IS19 S17 S18 IS2O I FIGURE 3.4: Mask outline for fabrication of HEMT device. Level 3: Ti Au metalization was used to define the gate and the probe pads connecting to the source, drain, and the gate by liftoff technique. Among various kinds of device structures, variable gate structures with gate lengths of 6, 8, 12, and 20 1um were mainly used for the degradation effect characterization after exposure to various neutron flueces. Detailed description of each level of masking layout of various devices is described in next section. Fig. 3.5 shows variable gate FET structure after three making steps are performed. The schematic mask layout with dimensions of source, drain, and gate 56 FIGURE 3.5: Final mask design layout of unpassivated variable gate width FET structure. Upper five squares are source/drain pads while lower four squares are gate pads. terminals are shown in Fig. 3.6. Transfer length measurement (TLM) structures 120 Contact Pads Source & Drath FIGURE 3.6: Dimensions of unpassivated variable gate FET structure. 57 with gaps between adjacent pads of 6, 8, 12, and 20 m were also included in the mask set for contact/sheet resistance measurements. By knowing the sheet resistance from above method, sheet resistance increase on FET device after neutron irradiation can be easily determined. Therefore TLM structures were designed to be located close to variable gate FETs. Fig 3.7 shows a TLM structure after final masking step. Individual level layout pictures of the two devices (V and T) are shown in Fig. 3.8. FIGURE 3.7: Final mask design layout of unpassivated TLM structure. The top two mask layouts in Fig. 3.8 depict isolation mesa for device regions, two pictures in the middle are for ohmic contact metalization, and the bottom two pictures show layouts for metalization for both devices. 2 2 FIGURE 3.8: Individual levels of three mask steps (top:level 1, middle:level 2, and bottom:level3 for fabrication of unpassivated variable gate FET(left) and TLM(right) structures. 59 3.2.2 Passivated Devices Another mask set for the fabrication of passivated HEMT devices for future study was designed. Up to level 2, the masks for passivated devices are similar to the corresponding unpassivated device masks. However, level 3 of this set opens only gate windows and level 4 opens thick passivation layer to deposit a contact metal on top. The arrangement of each device and gaps between contacts were fixed for this layout design for the purpose of probe card usage. The overall layout is shown in Fig 3.9. Each legend shown on the figure represents p VW.WA V/,%dW/j// //////yJJ W4W//////////i5 mi 0 I 0 IIII I _III$ W//W////A W/V//// AV/////// V//,'%(&V//,'///,Z _._ 1_. rni V4Y7f% V//AW///////Z*5 0 0 II__I_ FIGURE 3.9: Mask layout of different type of devices for the fabrication of passivated HEMT devices. a specific type of device. Rectangular shapes of patterned bars indicate the alignment marks. . FFat FETs . VVariable Gate FETs . RRF devices TTransfer Length Measurement structures . INVInverters Fig. 3.10 shows a composite of all four levels of design layout for FET, TLM, inverter, and RF devices. As shown in the figure, metal contacts are located in a same row with same gap between contact pads. This is crucial for probe card usage. Alignment marks with several different dimensions are placed throughout the mask plate area for the sake of precise alignment. The individual levels of masks are shown in the next four Figs 3.11, 3.12, 3.13, and 3.14. The level 1 for the mesa isolation is shown in Fig. 3.11. This is a bright field mask. The regions where the devices are to be formed are dark where the photoresist is not affected by UV radiation. The bright areas where the photoresist is removed by the developer are etched down to the semiinsulating substrate region. The level 2 for source and drain contact metalization is shown in Fig. 3.12. Overall mask field is dark except the drain and source regions. The level 3 for contact and gate window opening is shown in Fig. 3.13. Again, mask field for level 3 is dark and the gate/contact metal is defined by liftoff. The level 4 for final metalization which connects the contact opening windows to probe metal pads is shown in Fig. 3.14. 61 FIGURE 3.10: Composite mask layout design for fabrication of passivated HEMT devices. 1. Variable Gate FET mask layout :The enlarged picture of variable gate FET layout is shown in Fig. 3.15 followed by each level of a mask layouts in Fig. 3.16. Isolation mesa is shown in the first level layout, source and drain regions for ohmic metalization is shown in the second level, contact window opening after passivation appears in the third level layout, 62 2 2 2 2 2 2 2 2 2 2 II I 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 DEJL1EIL1 II I I LI LI II _ I I _ II _ I 2 2 2 2 2 2 ______ I LI LI LI ___II___ I LII 2 2 I I LI LI LI LI LI LI LI LI LI LI LI LI I 2 2 2 2 2 2 2 2 2 2 21 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 ii I I I I 2 I _ II 2 I 2 2 2 2 2 2 2 2222222222222222222222 I FIGURE 3.11: Mask level 1 for mesa isolation. 63 000000000000 0000000 0000000 000000000000 000000000000 000000000000 0000000 0000000 00000m 0000000 000000000000 000000000000 Ii 0000000 00 0000000 000000000000 0000000 0000000 000000000000 0000000 00 0000000 0000000 DO 000000000000 0000000 000000000000 0000000 000000000000 0000000 00 0000000 0000000 0000000 0000000 0000000 0000000 000000000000 0000000 0000000 FIGURE 3.12: Mask level 2 for source and drain mesa. -issd U' uiuodo &opuiAt puo pu 0000000 0000000 oo DIDIDIDflD DOD cjo cUJ ojIoIaIctii LO DID OJ o ijJ DID ob ob ob obIoIoIc4oJ oDD ol]o 0000000 D DOD iOILDIEIEUJ ub olo 0000DDD 0000000 olD oIoIoIobfloo iio nUb ob Db ob DUb ioj 0J 0000000 DUO olo o1 olo aDo olo oDo otu OJ 0000000 DID olD 0Db 000 DUb DID aDo IAI JAJ :t 0000000 aDo DID ob ulu DD00000 DID DID aDo d]o o 11c1 UOIA oUu aDo ob ob ala DJ DDDODDD ob olu ob Db 010 olololobftnDo ololobIoOolo 000 rnlflDkT 0J 0000000 u iJu oIoIDlDDDo dDIDflEUJ OJL4DIDIJOD oj 0000000 DID obklolijDo oloIrIoIol1 ctb cflj ollo DUb oDo 000 J79 deposition. pad metal probe final for 4 level Mask 3.14: FIGURE 2222222 22222222222222 s LU 2 JJJLsJLJLJJL2JL s rr!rr! !ir!2 !!F.f 111,111 2 2 2 5 111,111 111,111 IIIIIIILSI T 1111111 2 I IIIiIiIIU [UJ s . IT uIIIIIUUuI I II Uj UJ 1UJ .j i Il lilullIlli I.E III 1111111 IIIuIUuU 2 t!M!2! t.M?.? 2 2 UJ t.MF.r 2 s Uj .l ;,; IiIiIiIIii 2 [UJ .I .l TEl Ui TEl Eli TEl TEl TEl 111 TEl TEl ;;i ;;i ;';'i ;;;1 ;;i ;;i ;;i ;;i I?PPPPIP IPIPIPIPPIP 2 £ PIPIPIPIPIF 2 2 2 LJJJL2Ji.h2hl rrrr. rrr2r!rr! rrr2irr.T I1IIIlI 2 2 hId 11111,1 1111111 1111111 2 5 2 1111111 I,IIuuuIul c.i iii r.i iii ii liii iii iii 2 2 JJL J7JL t!M?!! t!!! I 1,111,1 II Il II TI S uuIlu,luuIII 2 £uIuuuuuuIu uuiuim J7JL J7JL 2 J7JL IJL J'JL J7JL iii tM?.? tL!.? 2 UJ 2 2 2 5 2 2 2 5 2 111111111 1111111 iii iii iii iii 2 I iLUuj III III U U III III U I U I U 2 liii uuil 11:1 J7JL U .1 iii iii J'JIL 1111111111 IiIiIiIiI U I i.i 1c.i 1LU1I iii iii 1111111111 t!'.'!2! t!.?.!! U U 2 £ x S 2 2 x 2 2 2 S 2 2 z S 2 2 2 J7JL IIJL 11111111111 t!'M!!! 'L!M?2! 11211111121111112 I I I U U . 2 Sn 1111111111112,l,IlV,,I,112111111111,II2 ,,2 2 5252222 III Iii III III Iii III III III III III III III lii III 65 and finally the contact pad metalization for connection to device terminal region is shown in the fourth level layout. The dimensions of variable FIGURE 3.15: Final mask design layout of variable gate width FET structure. uuiiin FIGURE 3.16: Four mask levels to fabricate passivated variable gate FET structure (left-top:level 1, right-top :level 2, left-bottom:level3, right-bottom:level 4). 67 gate FET structure are also shown in Fig. 3.17. Several gate lengths in the range of 6 m-3O tm were designed to characterize the gatelength effect on DC characterization. Not only for variable gate FET but also for other device structures on this mask layout have same dimension for contact pad which is lOOxlOO source. 1um2 and 70x10 urn2 for both drain and HHH 1000 50 50 100 100 /5 50 50 50 50 FIGURE 3.17: Dimensions of passivated variable gate FET structure. 2. Fat FET mask layout: Another DC unit cell, that is, the three fatFET structures are designed with different gate lengths of 30 urn, 40 urn, and 50 urn which is depicted in Fig. 3.18. Metalization pads, source and drain areas are same as those of variable gate length FET's. FIGURE 3.18: Final mask design layout of FatFET structure. _H_H__ FIGURE 3.19: Four mask levels to fabricate FatFET structure (left-top:level 1, right-top : level 2, left-bottom: level3, right-bottom:level 4). 3. '&ansfer Length Measurement mask layout : Transfer length measurement (TLM) structures are designed with a gap between the adjacent contacts of 6, 10, 15, 20, and 25 pms. Fig. 3.21 shows the final form of layout followed by individual layout sheets associated with alignment marks. FIGURE 3.20: Final mask design layout of Transmission Line Model structure. 4. RF and Inverter mask layout RF structure with gate lengths of 6 and 8 pms were designed for the characterization of RF performances. The schematic picture of overall RF structure is depicted in Fig. 3.23. Also, RF structure along with the TLM structure is used to form an inverter structure which is shown in Fig. 3.23. 5. Alignment Marks Two types of alignment marks were designed to 70 FIGURE 3.21: Four mask levels to fabricate Transmission Line Model structure (left-top :level 1, right-top:level 2, left-bottom:level3, right-bottom: level 4). __HrO FIGURE 3.22: Composite layout of RF device structure. 71 FIGURE 3.23: Final mask design layout design of Inverter structure. assist improving alignment accuracy. As masking level increases, the size of the alignment marks also increases with steps of 5 jim and 2 jim, for large and small gap alignment marks, respectively. These two different gaps of alignment marks are useful and efficient since rough aligning can be performed first with large gap (5jim) alignment marks followed by more precise alignment with small gap (2jim) marks. Another efficient method of managing alignment marks for this layout design was that a new alignment mark is added for each new masking level. As shown in Fig. 3.24, the two alignment marks on left side with four layers start from the first level (inner most one) and go all the way up to level 4 (outer most). The two marks in the middle of the figure start from the second level. Similarly, the two marks on right start from the third level of masking step. This relay method of alignment marks location can avoid a massive confusion of aligning due to several overlapping alignment marks at the same location. Adding new alignment mark with new level of masking step can simplify the aligning procedure and yet results more 72 precise alignment. II =1111= _ ___L__ ii I FIGURE 3.24: Alignment marks used for fabrication of passivated HEMT devices. 3.3 Processing Description This section includes a description of the processing steps for HEMT devcies including lapping, cleaning, photolithography, etch, passivation, metalization and liftoff. 3.3.1 Lapping To remove indium from the backside of the cleaved wafer, lapping is performed by polishing the bottom surface in a silica carbide slurry. A 8doped HEMT layer structure after cleaving and indium lapping is shown in Fig. 3.25 73 n GaAs cap layer A1GaAs - layer A1GaAs GaAs Buffer S.I. Substrate FIGURE 3.25: Schematic cross section of a HEMT structure after cleaving and lapping. 3.3.2 Cleaning There are several cleaning procedures involved in the device fabrication. After lapping, the slurry is removed by simple DI water rinsing. The white wax which covers on the top surface of the sample is removed in the boiling acetone and cleaned in an ultrasonic bath in the mixed solution of tricholoro ethylene (TCE) and acetone. Acetone bath, methanol bath, followed by deionized water rinse and finished with nitrogen blow gun dry is known as a typical standard AMD clean procedure which is used in each processing step to remove organic and inorganic chemicals for whole fabrication courses. Acetone itself can remove photoresist layer after each level of photolithography step and also can lift off undesired metal in contact with the photoresist. 74 3.3.3 Photolithography The AMD cleaned sample is placed on the spinner as the first step of photolithography. Coated with positive photoresist5 onto the surface, sample is spun typically at 4000 rpm for 40 seconds and then followed by soft bake at 80- 90°C for about 10 minutes. The sample is put on the holder in Canon Model FPA-120 Mask Aligner for pattern printing. Mercury lamp (.A = 436 nm) is used as ultra-violet source to expose the patterns onto sample for about 10 seconds. Properly exposed sample is soaked in the developer solution6 to transfer the pattern onto the photoresist. The developer solution is cleaned by rinsing the sample with DI water. If an etching step follows the lithography step, the developed sample is hard baked at 110°C for at least 10 minutes. If a passivation layer needs to be etched, several hard baking procedures are performed for a strong adhesion of the photoresist to the passivation surface. 3.3.4 Etch Wet etch method is used to produce mesa active structure as shown in Fig. 3.26. A freshly mixed solution of H202:H3PO4:H20 in etching and its etch rate is the ratio 1:3:50 is used for the 1100 A/mm. The freshness of etchants for etch solution is crucial for correct etch rate of sample and also continuous stirring of the solution helps for uniform etch rate and surface. For oxide etch prior to metalization, different etchant is used. Placing sample in the mixture of 10 cc Shipley 6 Shipley Microposit S-1811 PPR. Microposit MF-321. 75 n GaAs cap layer A1GaAs - layer A1GaAs GaAs Buffer S.!. Substrate FIGURE 3.26: Schematic cross section of a HEMT structure after etch for mesa active layer formation. ammonium hydroxide and 5 cc DI water for 1 minute removes native oxide on GaAs layer. This recipe may not work to remove oxide on InGaAs layer. To etch thick passivation layers, (e.g. polyimide, Si3N4, and Si02), Reactive Ion Etching (RIE) is commonly used. The operating parameters developed by Shatalov [53] for each dielectric film are cited in Table 3.6. 3.3.5 Metallization and LiftOff Prior to each metalization, oxide layer which is possibly accumulated on sample surface should be removed by oxide etch procedure described previously. This helps better adhesion of the metal to the surface of sample. Two different metalization processing steps are performed during HEMT device fabrication. For source and drain ohmic contacts, Ni, AuGe, and Au are deposited in sequence with the thicknesses of 200 A, 1000 A, and 200 A, respectively. For Schottky gate contact, 200 A of Ti and 1000 A of Au are deposited in sequence. Especially, titanium is deposited to increase adhesion of Au to the A1GaAs surface 77 at a pressure 1 x 10-6 Torr. Reasonable amount of each metal is placed on the holder which is connected to current source. A tungsten basket is used for Ni while molybdenum boats are used for AuGe, Au, and Ti. The deposition rate and accumulating film thickness are monitored by quartz crystal monitor7. Metal deposited sample is now ready for lift off procedure. The sample is soaked in acetone and if necessary acetone is also squished on the sample using a syringe. This removes the excess metal in contact with the photoresist leaving the desired metal pattern on the sample. Annealing of the source and drain metal (AuGeNi) is necessary to achieve good ohmic contacts. The furnace is preheated to the set temperature of 450°C in a flowing forming gas ambient. Sample is annealed for about 4 minutes. By examining the contact pads through a microscope, fade color and crack pattern are seen if it is properly annealed. The time and temperature are optimized. The GaAs cap layer in channel region must be etched out through gate recess procedure before the final metalization. Final metalization (TiAu) is performed to create Shottky barrier between metal/A1GaAs layers. The HEMT device structure after the final gate metalization procedure is shown in Fig. 3.28. S. 3.6 Passivation The channel area of HEMT device is sensitive to surface effects. To avoid possible physical and chemical contamination, dielectric films are typically deposited in ITTV semiconductor processing. Although the HEMT devices used for this study were unpassivated ones, it might be very useful to develop the passivation ' Martex Model TMlOU 79 110°C) hard baking time after photoresist deposition. Patterning for contact windows can be completed with the third mask level. Silicon nitride and silicon dioxide, on the other hand, can be deposited on the sample through PlasmaEnhanced Chemical Vapor Deposition (PECVD) technique. Each control parameter for each material is cited from Shatalov [53] and summarized in Table 3.7. Typical cleaning and photolithography proce- Process Parameter Si3 N4 Si02 Power (%) 14 14 Pressure (mTorr) 700 700 Temperature (°C) 350 300 15 17 75 85 Sill4 flow (%) SiH4 flow N20 (sccm) 10 flow (%) N20 flow (sccm) N2 flow (%) N2 flow (sccm) Time (mm) Thickness Variation (A) (A) Index of Refraction 10 50 - 250 30 30 2450 2250 50 140 2.0 1.5 TABLE 3.7: PECVD process parameters used for passivation layers with silicone nitride and silicone dioxide. dures done on polyimide passivation layer can also be applied to these dielectric Si3N4 and Si02 dielectric layers deposited on the sample by PECVD. Fig. 3.29 shows the HEMT structure after RIE etching step for contact windows. A final HEMT device crosssection is shown Fig. 3.30 after gate recessing and contact metalization on the sample and final liftoff procedure were completed. The processing charts for the unpassivated devices and passivated devices are shown in Figs. 3.31 and 3.32, respectively. All the devices used in this study are unpassivated devices. Future study will be done on passivated devices. Ni/AuGe/Au Passivation Layer FIGURE 3.29: Schematic cross section of a HEMT structure after passivation and dryetch. Passivation Layer Ni/AuGe/Au Ti/Au FIGURE 3.30: Schematic cross section of a passivated HEMT structure after final metalization for contact pads and liftoff. 3.4 3.4.1 Measurement SetUp for Electrical Characterizations Current Voltage Measurement Setup Configuration A schematic diagram of the measurement setup used for the electrical characterization of SHBTs and HFETs is illustrated in Fig. 3.33. For SHBTs, this setup is used for currentvoltage characterization for a transistor as well as baseemitter and basecollector diodes. Gummel and inverse gummel characteristics are investigated. For HFET devices, on the other hand, this is mainly used for the measurement of 'D VDS characteristics as well as for the direct parameter extractions such as saturation voltage and threshold voltage. Three goldplated tungsten probes are used to make contact with the probing pads of the device on a probe station equipped with three micromanupula- apping Indium Bac AMD Cleaning AMD Cleaning AMD Cleaning Photoresist Photoresist Photoresist Softbake Softbake Softbake v1ASK#2 for MESA structure for Source and ) Drain Contacts / (MASK#3' for Gate Window \ & Metalizationj Developer Developer Developer Hardbake Native Oxide Etch Etch for Gate Recess Wet Etch Ohmic Metalization Oxide Etch Lift Off nnealin2 for Ohmic1 Metal Deposition Lift Off FIGURE 3.31: Unpassivated HEMT Device Fabrication Flowchart. tors and a viewing microscope. Measurement for desired characteristics are performed using HewlettPackard HP 4145B semiconductor parameter analyzer. This equipment has four channels of high precision source measure units (SMU). The measured data are retrieved to the PC via HPIB interface. Pro- AMD Cleaning apping Indium Bach AMD Cleaning AMD Cleaning Photoresist Photoresist Softbake Sofibake IASK#1 for MESA structure Developer for Source and Drain Contacts Developer Passivation layer Photoresist Softbake for Gate and Contact Windc Developer IRIEDryEtch I AMD Cleaning Photoresist Softbake v1ASK#4 for Metalization Developer Oxide Etch Hardbake Oxide Etch Wet Etch Metal Deposition Metal Deposition Lift Off LiftOff [4nnealing for Ohini FIGURE 3.32: Passivated HEMT Device Fabrication Flowchart. grams written with visual basic software, incorporated with HPIB interface command library, are used for measurement control as well as data storage processes. ] I PC with hpib Interface Probe Station 4145B Parameter Analyzer [1° DD [cci // DGD FIGURE 3.33: Currentvoltage measurement setup consisted with PC, HP 4145B parameter analyzer, and probestation. 3.4L2 Hall Measurement Setup Configuration To study carrier transport mechanism, Hall measurement set up has been used and it is depicted in Fig 3.34. Hall samples were prepared with MBE grown structure, which were also used for HEMT device fabrication along with several control sample structures. The grown structures were cleaved into 5 x 5 mm2 pieces for Van der PauwHall measurement. Indium on the back side of the structure was removed through lapping procedure. To make ohmic contacts on the samples, 1n0gSn0.1 alloy was soldered on the center of four edges of each sample. Second type of sample was prepared with Au Ge contact, which also produce ohmic contacts with A1GaAs surface. One set of Hall samples with a cross bar shape as shown in Fig. 3.35 was also prepared using photolithography and etching procedure. This type of patterning reduced errors due to geometric asymmetry. All Hall samples were sintered for 10 minutes at 450°C in a forming gas ambient to penetrate through the larger bandgap AlGaAs layer. The Hall measurement consists of IBM Interface computer, Keithley Model Sample holder Keithley Model 181 Nanovoitmeter PC with interface -it Keithley Model 220 Current Source I Switch Box Magnets FIGURE 3.34: Hall measurement setup consisted with gpib interface PC, current and voltage sources, and 3200 Gauss magnet. 181 programmable nanovoitmeter, Keithley Model 220 programmable current source, an electromagnet and a switch box. Sample holder is specially designed to hold the four contacts of samples. Typical contact configuration of the Hall measurement sample for this study is shown in Fig. 3.35. T T S S D D BCS A S I 5mm FIGURE 3.35: Contact configuration of sample for Hall measurement. ETh Low temperature measurement was performed by using liquid nitrogen, which is filled into a specially designed styrofoam container. Sample whose four corners are connected to the switch box is immersed into the container while the low temperature measurement is performed. Mobility and carrier concentration are obtained at both room and low temperature. As shown in Fig. 3.35 resistivity of sample is determined by passing positive and negative currents through (A,B) while voltage drop between (C,D) is measured at each current. Same procedure is repeated by passing currents through (A,C), (C,D), and (B,D) while voltage drop through (B,D), (A,B), and (A,C) are measured, respectively. The resistivity p of the sample is obtained by using Van der Pauw method [77]. ut (RAB,CD + RAC,BD + RCD,AB + RBD,AC) F P1(2) 4 (3.1) where RAB,CD = VCD/IAB, t is layer thickness, and F is Van der Pauw correction factor. The Van der Pauw Hall mobility and carrier concentration are also obtained from this setup by finding Hall coefficient RH for given geometry in Fig. 3.36. To find RH (rn3/C), fixed value of current is forced through B voltage across AD is measured. A magnetic field of 3200 Gauss is then applied perpendicular to the sample surface and the voltage across AD is measured again. Then the measurement is repeated after reversing the polarity of the magnetic field. Finally the Hail coefficient RH is determined from RH= where IVAD = VAD (for +Bfield) tIWAD 2B1 (3.2) VAD (for -Bfield) with current I flowing through (B,C), t is the layer thickness, and w is layer length. The Hall voltage is similarly measured across BC for current flowing through AD and the average FIGURE 3.36: Current, voltage and magnetic field configuration for Hall coefficient measurement. Hall constant RH,av is determined. The carrier concentration for ptype sample is given by, 1 p= (3.3) qRH,aV and for n type (3.4) qR for ntype samples. Hall mobility 1H is finally defined as ILH RH (3.5) =IRHk7. For an application to find sheet carrier concentration 2DEG on HEMT device structure, sheet Hall coefficient RH3 is used rather than RH, which is simply obtained from RH divided by layer thickness t. This Hall effect measurement by using Van der Pauw method for mobility and sheet carrier concentration is performed before and after the samples are exposed to radiation. 3.5 Electron Irradiation This section includes a description of the detailed electron irradiation procedures performed on heterostructure devices. Although same procedures of irradiation processes are planned for both SHBTs and HFETs, a careful monitoring of the degree of degradation on electrical characteristics of each device is required due to the different nature of device characteristics. For electron irradiation experiment on SHBT and HIGFET devices, 100 mCi active Sr9° SIF.1177 beta source 8 was used and the experimental setup, capable of this study, was provided also by nuclear engineering department in Oregon State University. Schematic diagram of the crosssection of the source setup is depicted in Fig. 3.37. Sample was placed directly underneath the source at a distance by 2 mm. Source Chamber I Active layer ( 5 mm diameter) sample ceremic substrate Ag window sample position controller FIGURE 3.37: Schematic crosssection diagram of electron radiation source and sample setup. source is supplied by Amersham Corp., currently AEA Technologies. Source compound containing Sr° is thinly deposited on a ceramic substrate encapsulated in a stainless steel capsule (X.117), which efficiently absorbs emitted beta particles. As shown as arrows in Fig. 3.37, radiation escapes directly downward on the sample through a silver window, whose thickness is 50 pin. c, 90 r38 90 139 Y -790 halflife 29.12 years halflife Stable State 64.0 hr FIGURE 3.38: Betaemitting transition in beta source. Initially Y9° is cooled down by emitting gamma particle whose halflife time is 3.19 hours. SrL° A radioactive Sr° element with a halflife time, to }73990 t1,i2, of 29.12 years decays by losing one beta particle at the cutoff energy of 0.546 eV. Y399°, highly active and unstable, is initially cooled down by emitting gamma photons whose energies are relatively low. This cooled down Y399° now has a halflife of 64.0 hours and emits a beta spectrum with a maximum energy up to 2.25 MeV to become a stable Zr°. Schematic picture of this beta emitting transition from Sr° to the stable state is shown in Fig. 3.38. The amount of electron flux can be calculated with the assumption that only half of the betas produced by the source can be reached to the sample due to the isotropic nature of radiation direction. In other words, only half of betas leaving the source will travel to the direction of the sample. From the sample configuration in Fig. 3.37, one can establish the electron flux, by 1 F,, and it is given dN (3.6) where dN represents the total number of 3 particles from the source element, dA is area of the thin Sr° source, and factor (1/2) accounts for the assumption that sample only covers half of the solid angle, that is, 4ir/2 = 2ir. The total electron fluence reaching the surface of the sample can be obtained as = where / 3.7 x 109s F(t) dt 2ird2/4 JO is the exposure time, Fig. 3.37), and 3.7 x i09 d is t (3.7) 9.42 x i09 ts'cm2 the diameter of the source (= 5 is the unitcorrected radioactivity (from 1 mm from Curie = 3.7 x 1010 disintegrations per second). 3.6 Neutron Irradiation Neutron irradiation of heterostructure SHBT and HEMT devices was performed using the Rotary Speciman Rack (RSR) facility of the 1 MW TRIGA Mark II nuclear reactor at OSU Radiation Center operated at 100 kW. The foil activation dosimetry technique was used to measure the total epithermal and fast neutron fluxes and had the results of 1010 cm2 for epithermal flux with the energy range of 0.5 eV < E < 10 keV, and 5x101° cm2 for fast neutron flux with the energy, E, larger than 10 keV. The differential neutron energy spectrum at the RSR, which were results of Monte Carlo Neutron Photon (MCNP) transport simulation, were integrated to 91 obtain fluxes for the fast neutron as well as epithermal parts by normalizing MCNP calculated spectrum to one and multiplying by the fast neutron and the epithermal flux values from the direct measurements. Therefore, the absolute values of these fluxes were determined from the dosimetry measurement while using the MCNP results for the shape of the spectrum. The 1 MeV equivalent neutron fluence is determined by using the following relation. &q,1MeV Jo ç(E) FD(E) FD(1MeV) dE Note that each semiconductor material has its own value of the damage function, which requires &q,1MeV to be calculated for each material of interest. Energy spectrum used for this radiation procedure ranges from 0.5 "-i 10 MeV. 3.6.1 Irradiation Process on HFET Devices Control Hall samples of single layers of GaAs (Table 3.4), A1GaAs (Table 3.3), and HEMT structure (Table 3.1) were also irradiated along with A1GaAs/GaAs HEMT devices for carrier transport mechanism study. Neuron Irradiation was performed on the samples using the following procedure. 1. After the device fabrication, the wafer was cut into smaller segments, each segment including at least 3 sets of unit cell (variable gate HEMT, TLM). Different pieces were used for different doses. 2. After the appropriate measurement and characterization were completed of all samples, the a HEMT sample, a GaAs layer, and a AlGaAs HEMT- like structure layer were encapsulated in cadmium boxes to block thermal neutrons (Eth < 0.5 eV) and to reduce ionization damage. 3. Neutron Irradiation was performed on samples at ambient temperature ( 200°C) without applying bias on the samples. 92 4. After neutron irradiation is performed on all samples followed by "cool down" step, electrical measurement and characterization of HEMT devices as well as Hall samples are repeated for radiation degradation study. Similar sequence of sample preparation for neutron irradiation was done with HIGFET devices and GaAs layers. One HIGFET device and two segments of GaAs layer which had Sidoping concentrations of 1.26 x 1016/cm3 and 3 x 10'5/cm3, respectively, kept in one cadmium box after completion of measurement prior to neutron irradiation. Four cadmium boxes which contained three segments of each type of structure were prepared for four different neutron irradiation doses. The fluence range used in this work is from to r' 1 x 1013 5 x 1014 n/cm2. 3.6.2 Irradiation Process on SHBT Devices After the completion of fabrication process, the wafer including SHBT devices were cut into smaller pieces, 4 small pieces for different neutron doses in this study. Each piece included several identical SHBT devices to obtain statistically reliable results. To proceed neutron irradiation on these SHBT pieces, following preparation steps were required: 1. All required electrical measurement and characterization as described in Sec. 6.3 are completed of all pieces as well as devices. 2. Cadmium boxes are used to contain sample pieces to prevent thermal neutrons and ionization damage. 3. Properly packed samples are separately irradiated for radiation durations of 7, 20, 60 and 180 minutes at ambient temperature ( terminals floating during irradiation. 200°C) with all 4. After irradiation, "cool down" procedure follows until it reaches acceptable safety limits. It usually takes two to three weeks. 5. Same electrical measurement and characterization are performed monitoring degradation behavior appearing on results. CHAPTER 4 HEMT DEVICE MODELS In this chapter, we will introduce a model describing currentvoltage (I V) characteristics and compare them with the experimental results. Starting with deriving an equation for the description of twodimensional electron gas (2DEG) at the interface of the heterojunction, we will discuss how the 2 DEG is controlled by the Schottky barrier gate potential by introducing chargecontrol model. Formulation used for derivations of these characteristics is semi empirical using an approximate triangular well model appearing the heterointerface. Wellknown published results are taken into account for corrections on modeling. A model for IV characteristics of a HEMT will be derived by applying the chargecontrol model, which controls 2DEG concentration with gate bias voltage. To describe electron velocity in the twodimensional layer, a two piece linear approximation model is used along with derivations of two different approaches for evaluating saturation current by assuming velocity saturation or pinchoff. The series resistance effects are included in both assumptions. The models of 2DEG concentration and IV characteristics are modified to incorporate the effects of neutron irradiation induced degradation in HEMT control samples as well as HEMT and HIGFET devices. 95 4.1 2Dimensional Electron Gas in A1GaAs/GaAs Interface Schematic diagrams of uniformly doped and deltadoped A1GaAs/GaAs heterointerface HEMT structures are shown again in Fig. 4.1. Drain SourCe1 n+ GaAs I I SI - Delta Laç n+GaAs intrinsicAIGaAs intrinsicAIGaAs t 2DEG GaAs 30 cycles( S.I. Substrate 5.1. Substrate (a) (b) AIGaAs GaAs FIGURE 4.1: Schematic layer structures of uniformly doped (a) and delta doped (b) HEMT devices. Due to the electron affinity difference between A1GaAs and GaAs, the free electrons from A1GaAs diffuse into GaAs and form a twodimensional gas at the heterointerface as shown schematically in Fig. 4.1. The diffused electrons are accumulated at the interface in the GaAs side and are confined to a potential well of width much smaller than the de Brogue wavelength' of the electrons 'de Brogue wavelength of a thermal electron is about 260A. in the potential well [57]. Fig. 4.2 shows the energy band diagrams of the A1GaAs/GaAs heterostructures at equilibrium without any external contacts. A1GaAs GaAs A1GaAs S.C. 2 GaAs S.C. 1 / I 02. (a) 3 EFO (b) FIGURE 4.2: Energy band diagrams of uniformly (a) and delta (b) doped HEMT structures without applied gate voltages. LIST OF FREQUENTLY USED S YMBOLS Total thickness of A1GaAs layer beneath the gate. d, Thickness of undoped A1GaAs layer (spacer). Ld Correction for gate metal to a 2DEG spacing. d D Twodimensional density of states (=3.24x 10' m2V') 97 LE Conduction band discontinuity at AIGaAs/GaAs heterointerface. E0 E1 The first sub-band energy in the potential well. The second sub-band energy in the potential well. EFO Fermi level. permittivities of GaAs and A1GaAs layers, respectively. F5 Critical electric field corresponding to velocity saturation. F1 Electric field. F21 Electric field at the heterointerface. gd Output conductance. Transconductance. g g1 Intrinsic transconductance. 61, E 6.625x i0 h Plank's constant h h/2ir (=1.015x io- Js) Js or 4.135x iü' eVs ) 'Yo, 'Yi adjustable parameters for sub-band energies, E0 and E1, respectively. I Drain current at saturation. Boltzmann constant (= 1.381x 10-23 J/K or 8.617x io eV/K) L Gate length m* Effective mass of electron in GaAs. ,u Low field mobility of the 2-DEG. energy sub-band. n2 Surface carrier density in n5 Surface carrier density. (NA), Nd (ND) Acceptor and donor concentrations in GaAs, respectively. Wavefunct ion of electron whose energy is E2. ç cm Schottky barrier height. q Electron charge. Q3 Total surface charge. R, RD Source and Drain resistances, respectively. T Absolute temperature. V Channel voltage. VD Drain voltage. VS Drain voltage at the saturation current. V, V Internal drain and source voltages, respectively. V9 Applied gate voltage. V01/ Threshold voltage. kB Na Gate voltage minus channel voltage Gate voltage minus threshold voltage (V V v3 Electron saturation velocity. v20 Conduction band bending showing at the interface at equilibrium. v2 Conduction band bending with applied gate voltage. w2 Space charge width in region 2 (A1GaAs region) Veff W Gate width. z Channel beneath the gate. The motion of electrons in the potential well is described by the envelope function F(r, z) and quantized subenergy level, E [76]. F(r, z) = q (z)exp where r is a 2dimensional (2D) vector, ke (ike . r) (4.1) is a wave vector describing the motion parallel to the interface, z is the distance from the interface to GaAs layer. The wavefunction '/ which characterizes electron motion in the well should satisfy the Schrodinger equation at the given potential energy V(z) 2 with the effective electron mass m*, h2 d 1 2dzm*(z) d(z) dz +V(z)q(z)=Eçb(z). (4.2) Also the potential energy V(z) is obtained by solving Poisson's equation with the permittivity of the GaAs layer 2 . d2V(z) qp(z) dz2 6 In this section V is defined as the potential energy (not electrostatic potential). (4.3) The space charge density p(z) is expressed in terms of electron concentrations ( n2) at various subband energies E2 in the quantum well and the concentrations of the ionized donors and acceptors (Nd and Na), in GaAs layer and it is expressed as Na) qnj p(z) = q(Nd energy subband, n2, which is given as with the surface energy density in ni = (m*kBT/lrh2) (4.4) I2 (z) In {1 + exp [(EF E2) / kBT]} (4.5) where m* is the electron effective mass in GaAs, kB is the Boltzmann constant, T is the absolute temperature, h is the reduced Planck constant, q is the electron charge, EF is the Fermi level, and E2 is the subband energy level. The expression of the subband energy, E in the triangular well approximation is, E where qF2iz F21 (h2/2m*)I3 [3qFii I/i + 2 )] (4.6) is surface electric field which provides the potential energy V(z) = in the vicinity of A1GaAs/GaAs interface. F21 is determined by Gauss's law with the surface carrier density n and the ionized impurities in the undoped GaAs layer Q22. = qn8 + Q. (4.7) where (4.8) with the value of n2 given by Eq. 4.5 and fWdep Qjj=qJ (NdNa)dZ. (4.9) 0 Typically, in the unirradiated samples, ionized impurities Q22 is small compared to the surface carrier density n3. Therefore, Eq. 4.7 can be expressed as EF21 = qn3. (4.10) 100 By substituting Eq. 4.10 into Eq. 4.6, the first two sub-band energy levels are obtained as P20 (eV) = yo (F3)213 (F3 in V/rn) (4.11) P21 (eV) = -y (F3)2 (F3 in V/rn) (4.12) where 'Yo = 1.83x 10-6 and 'yi = 3.23x 106 are adjustable parameters for the sub-band energies. The surface carrier density n3, taking into consideration the lowest two energy levels is 1E1 dE n3=D I JE0 1+exp JE1 where the density of states D = n3 dE +2D I rn*/irh2. 1+exp [E,_E,F1] By using a simple integral (4.13) formula3, is obtained as n3 Eq. 4.14 (DkBT) in {1 + exp [(EF E)/kBT]}. (4.14) is reduced at the low temperature limit as: n3 = D(EF E0) (4.15) for unoccupied second sub-band and n3 = D(E1 E0) + 2D(EF E1) for occupied second sub-band where the density of states, cyclotron mass, is 3 r dx J ri 3.24 x 1017 = _log(1+e_x) rn2V'. (4.16) D, with a measured 101 If the doping concentration in the small gap semiconductor (GaAs in our case) can not be neglected, the electric field, F1 can be derived by solving Poisson equation incorporating the acceptor or the donor concentrations. For ptype dopants, the Poisson equation is given as dF1 q dz where n(z) is (4.17) --[n(z) + NA1] free electron concentration, NA1 is ionized acceptor density in the GaAs (smaller energy gap) region, and i is the dielectric permittivity of GaAs layer Integration of Eq. 4.17 from the edge of depletion region GaAs to the interface (F1 = 0) in (F1 = F1) gives (4.18) = qn3 + qN1w1 with the 2DEG electron density, n8 and the space charge width, w1. In the case of ntype GaAs, the electric field, F1, can be expressed as: dF1 dz q = --[rt(z) ND1] (4.19) with ND1 being the ionized donor density in GaAs layer. Eq. 4.19 is integrated as = qn8 where 4.2 d1 qND1dl (4.20) is the depletion layer width in GaAs. Charge Control Model for HEMT Device Characterization This section introduces theoretical models for the charge control in the 2DEG through Schottky barrier gate voltage. It has been already studied and pub- "Generally, e for GaAs layer, and a large error on results. 2 for A1GaAs layer considered as same without giving 102 lished for a typical HEMT device [10], [11]. These models are used for the calculation of the threshold voltage of the device in terms of device structure parameters. Normally, these models ignore the doping concentration in the GaAs layer. We modified the model to include the effect of the defect concentration in the GaAs layer and derived the quantitative expressions for the dependence of VT on the radiation fluence. Series resistance increase after neutron irradiation is incorporated to find appropriate models for currentvoltage characteristics and transconductance. 4.2.1 Equilibrium The position of two subband energies E0 and E1 in triangular potential well in the interface region shown in Fig. 4.2 is only illustrative. Each subband energy level was obtained by solving the SchrödingerPoisson equation given in the previous section [6]. Also the potential V2 in space charge region in AIGaAs layer (semiconductor 2), with the total depletion approximation, should satisfy the Poisson equation as shown in Eq. 4.3, that is, d2V2 N2(z) (4.21) where N2 is the doping concentration of the AlGaAs layer. Take the heterojunction interface to be origin and the space charge width in region 2 to be w2. From the boundary condition at the origin and the w2, / dV2 \ ( dV2 \ V2(0)=0 =0 = F22. (4.22) 103 Now we can successively obtain expressions of the electric field, Ff2, and the potential, V2 (w2) by integrating Eqs. 4.21 and 4.22. F2 = (4.23) N2(z)dz q V2(n2) = v20 = F2w2 _f W2 PZ (4.24) dzj N2(z')dz' 0 where N2 (z) = 0, d < z < 0 f or N2 (z) = N2, for z < (4.25) d2. By using Eq. 4.25 to integrate Eqs. 4.23 and 4.24, we obtain e2F2 = qN2 (w2 V20 = Solving Eqs. 4.26 and 4.27 for = qN2 2e2 (w d) (4.26) d). (4.27) w2 gives /2qE2N2v2o + q2Nd (4.28) qN2d2 where the bandbending, v20, estimated from the Fig. 4.2 is given by qv20 = LEc 2 Neglecting the interface states, we can set (4.29) EFO. = qn8 = E1F1. Finally, using Eqs. 4.10, 4.14, and 4.28, we can evaluate the 2--DEG concentration leading the next relationship: V 2q2N2v2o = DkTln qN2d = + q2Nd [(1 + e(E_E0)T))(l +e F_El)/1T)] We take into account the spacer layer thickness, GaAs region. Eq. 4.30 promises to find n3 . d, (4.30) next to the doped Al- using empirical values of E0 and E1 by choosing arbitrary value of EF to start with and infinitesimally increasing EFO until n3 satisfies Eq. 4.30, which gives the 2DEG concentration in equilibrium. 104 .2.2 Charge Control Model When a Schottky barrier gate is placed on top of the doped large band gap material (uniformly doped case), the 2DEG sheet charge concentration is effectively controlled by appropriately applied bias voltage. By selecting appropriate thicknesses of the larger band gap semiconductor and spacer, A1GaAs in our case, and applied gate voltage, the maximum 2DEG sheet carrier concentration, n5, maintains the borderline between complete depletion state and onset of free carrier conduction in A1GaAs layer underneath of the gate terminal. Energy band diagram of A1GaAs/GaAs, heterostructure with applied gate voltage, V is depicted in Fig. 4.3. A1GaAs s.c. 2 GaAs S.C. i A1GL41 S.c. 2 g\ EF (a) (b) FIGURE 4.3: Energy band diagrams of uniformly (a) and delta (b) doped HEMT structures with applied gate voltages. 105 With the assumption that the interface between gate Schottky contact and the AIGaAs/GaAs heterointerface is completely depleted, the electric potential V2 again satisfies Poisson equation Eq. 4.21 in equilibrium condition discussed in previous section. Since the heterojunction interface is chosen as origin and electrostatic potential V2(0) is assumed as 0, N2 = 0 for d2 <z <0 d <z N2 = N2 for <d2. (4.31) Also the double integration applied on Eq. 4.21 with given integration limits defined in Eqs. 4.31 gives V2 (d) = v2 = F2 d Then the band bending v2 dz f N2(z') dz'. f (4.32) is expressed as V2 = qN2 2E2 (d d)2 F2 d (4.33) then = -(V2 V2) (4.34) where the total depletion voltage at the threshold bias point, V2 is expressed as V2 = (4.35) d)2, also the band bending, v2, can be examined from Fig. 4.3 and is given as qv2 = qçb qV9 + EF iEc (4.36) where cm is the metal-to-A1GaAs Schottky barrier potential, V, is the applied gate voltage, and EF is Fermi level with the gate bias. Substituting Eq. 4.36 into Eq. 4.34 produces (4.37) 106 As shown in Eq. 4.10, since E2F2 is equal to according to Gauss law in the qn8 absence of interface states, the total surface charge Q8 is Q8 EF depends upon = n3 62 qn8 = 71 EF + Ec + V). (V2 and hence Eq. 4.38 (4.38) is nonlinear. Using a numerical solution and obtaining linear dependence of on EF Eq. n8 4.38 is rewritten on [11]. 62 d+M (V V0ff) (4.39) with the offvoltage, which annihilates the 2DEG in the well as: V01 = V2. The correction term Ld in the denominator of Eq. dence of .2.3 n3 4.39 (4.40) accounts for the depen- on EF. Radiation Effects on Charge Control Model So far, we developed 2DEG sheet carrier concentration without considering background acceptor concentration in GaAs region since it is usually negligible compare to 2DEG concentration. However, it is not valid for neutron irradiated devices because neutron irradiation introduces acceptorlike defects in all the layers of the device and in particular, the GaAs region. We will discuss this with the Hall measurement results performed on control samples for the neutron irradiation effects on carrier concentration and mobility in Ch. 5. When acceptorlike defects are introduced, the electric field at the interface, F1 is determined from Eq. 4.18 rather than Eq. F1 = qn3 4.10 + qNwl (4.41) 107 where NA is the acceptor concentration in GaAs and e is the dielectric constant of the semiconductor (GaAs). The depletion layer width w in GaAs can be determined by noting that =kTin INA1 n] (4.42) I q gives the Fermi level position in GaAs relative to the intrinsic Fermi level, hence w is given by w + c7bu1k + EF) 'I2(Eg/2 (4.43) qNA Finally, the sheet carrier concentration is related to the gate voltage V9 with the following relation. 62 qn8 = where 62 EF + iE + g) (vp2 qNAW (4.44) is the permitivity of AlGaAs, d is the thickness of the A1GaAs region (including spacer), zEc is the conduction band discontinuity at the heterointerface and m is the Schottky barrier height. T42 in Eq. 4.44 is the band bending in A1GaAs. For 8doped HEMT, it is given by the following equation = qnD(dd) qNAd2 62 262 (4.45) where D is the 8doping concentration (per unit area) and d is the distance of the doping plane from the heterostructure interface. For a uniformly doped HEMT (A1GaAs region), it is given by = qNd(d 62 d2)2 qNAd2 (4.46) 262 where Nd is doping concentration (ntype dopant) and (d d2) A1GaAs layer thickness. The second term in both Eq. 4.45 and Eq. for the background (neutroninduced) acceptors in the A1GaAs. is the doped 4.46 accounts Analytical solution of n3 as a function of V for a given value of the acceptor concentration NA is generated as follows. First, by assuming an arbitrary value of n8, the Fermi level EF can be calculated from Eq. 4.5 to Eq. 4.43. Using this value of EF in Eq. 4.44, the corresponding gate voltage V is calculated. This is repeated for several values of n8 to generate the n8 curve. The whole procedure is repeated for other values of acceptor concentrations. Detailed simulation results are presented in Ch. 5. 4.3 Application to FET Devices Having been acquainted with charge control model, it is appropriate to use this knowledge for the modeling of a currentvoltage characteristics of a HEMT device. Wellknown, published results of such models are introduced in this section [9], [10]. Fig. 4.4 shows the potential distribution along the channel in a HEMT device with external gate voltage in addition to drain voltage. Taking into account the channel voltage V(z), the effective charge control voltage is changed to Veil V V(z) (4.47) and the total surface charge given in Eq. 4.39 in previous section is modified to Q3(z) 4.3.1 = --(V V(z) V011). (4.48) CurrentVoltage Characteristics without series resistance effect The channel current at z can be expressed as: I = Q3(z)Wv(z) (4.49) 109 undoped GaAs S.I. Substrate FIGURE 4.4: Channel voltage along the FET gate. where W is gate width and v(z) is electron velocity. For the device with fairly short gate dimension which is on the order of 1 pm or less, velocity saturation at the drain end leads to drain current saturation. In the twopiece approximation, model for the velocityfield characteristics, the electron velocity is assumed to be proportional to electric field until the velocity saturation occur and the constant velocity will be maintained in the saturation region. Twopiece linear approximation for velocityfield characteristic is depicted in Fig. 4.5 showing the electric field at velocity saturation as F3 and By taking F3 v3 as the saturation velocity. as a critical value of the electric field, where the velocity saturation occurs, v = pFforF<F3 v = v3 for F > F3. (4.50) In linear regime, where the field is smaller than F3, using Eqs. 4.39, 4.48 and 110 Experiment vs Saturation Region > Linear Region F Electric Field FIGURE 4.5: Twopiece linear approximation for velocityfield characteristic. 4.50, the expression of the current Eq. 4.49 becomes, = [IW d V(z) + Ld V011] as 3 and (17 Using Eq. 4.38, defining " _2__i) L dV I/off) (4.51) as V, 'D can be simplified as: 1D = /3L(Vg V(z)) dV(z) dz . (4.52) Integrating Eq.4.52, fz IDdZ = eLf vc (V IDZ = /3L(VV V2 2VV + 2 V)dV (4.53) -) (4.54) =0. (4.55) Solving the quadratic equation in Eq. 4.55 gives the channel voltage as a function of channel length. Without considering series resistance effect, it is given by VV+ V'V12_2 9L (4.56) 111 At z = L, V = VD so that Eq. 4.54 is simplified as = fi(V VD (4.57) ). Substituting Eq. 4.57 into Eq. 4.56 will give the expression of the channel voltage, V(z), V(z) (4.58) Eq. 4.58 gives the potential distribution in the channel for a given VD and without series resistance effect taken into account. We can also derive saturation voltage, VDS, when the drain current saturates due to velocity saturation short gate approximation. From Eq. 4.52, dV dz (4.59) Drain current becomes saturated from velocity saturation when 1TD = V1 = F8 x L where F3 is saturation electric field. The twopiece model used for velocityelectric field characteristics satisfies v3 = tF3. Combining Eq. 4.57 and Eq. 4.59 gives I_I dz = F8 = vs Vsl Let's solve Eq. 4.60 for saturation voltage, iDS. L(V'V1 Simply, =0 V1 4.3.2 = (V + Vs1) - (4.60) + V11. (4.61) (4.62) Current Voltage Characteristics including series resistance effect We will introduce the series resistances of source and drain to the current voltage characteristics. Although recent fabrication technology have reduced 112 this effect on device characteristics creating almost no effects due to contact series resistances, it is still important for our study because it becomes one of major parameters of degradation on device characteristics after irradiation by energetic particles. A slight modification of IV model equation developed in previous section is necessary to include this effect and it starts with finding the net channel voltage, V(z). Again, from Fig. 4.4, channel voltage can be defined as R5I V(z=O) = V(z = L) = VD RDI. (4.63) Integrating Eq. 4.52 by using limits of voltage values shown in Eq. 4.63 gives pV(z) / IDdz = 3L / Jo Jv(o) IDZ = 3L (VVc(z) (V V(z) V(z)) dV(z). VV(0) + 1V2(0)) (4.64) (4.65) By substituting RsI to V(0) from Eq. 4.63, Eq. 4.65 can be expressed in terms of source series resistance, R5, ZID 2VV(z) + 2VRSID RI + 2L = 0. V2(z) (4.66) By solving the quadratic equation, Eq. 4.66, the channel voltage, V(z) is obtamed and is given by V(z) = + (V RSID) 2 2IDZ (4.67) If the drain current saturation occurs due to velocity saturation (short gate length case), then F(z = L) F F3 I [ = dV(z)/dzIZL. - RsI)2 --j (-1/2) (4.68) 113 and Vs1 F., x L is given by [S R5I)2 V1 = -i] (-1/2) (4.69) Eq. 4.69 can be solved for saturation current ig due to velocity saturation. Then R5I)2 S2 0. (4.70) Solving the quadratic equation of Eq. 4.70 leads to, 1 R/32V2 k/' + 2/3RsV + (V/V51)2 (1+ /3RsV)]. (4.71) If the source series resistance is assumed to be zero, Eq. 4.71 is simplified as: ig=av1 [/1+(v/vs1)2_1]. On the other hand, the saturation current I is given by Eq. 4.59 at dV =F., dz ig /3L(V91V) (4.72) z = L as: (4.73) or I = /3Vs1(V Vi). (4.74) Combining two same saturation current equations,Eq. 4.72 and Eq. 4.74, provides intrinsic saturation voltage expression, that is, VS =1/; +V1 \fr2+v2 (4.75) which is similar to Eq. 4.62 derived earlier. Together with source and drain series resistance, Eq. 4.75 becomes V=V+Vsl_v/V2+Vl+I(RD+Rs) (4.76) In case, the gate length is long enough to induce saturation drain current through channel pinchoff before electrons reach to the saturation velocity value, 114 the IV characteristic model including series resistance effects can be modified as follows. By using same limitations described in Eq. 4.63, = 13L(V [L IDdZ = 3L f Jo / [v;vj dV Va) dz (4.77) VD-IDRD V)dV (V JIDRS ID(RS + RD)) = [V IDRD)2 (4.78) IR]] - V2]] V) (4.79) (4.80) where V = IDRD is internal drain voltage and V = IDRS is internal source voltage. In the linear regime, the second term of Eq 4.87 is negligible compared to the first term, then 'D can be rewritten as ID(RS + RD)] ID[1+/3V(RS+RD)] (4.81) VVD. Rearranging Eq. 4.88 gives I3VVD D- 1+V(RD±Rs) ( 4 .82) Series resistance effect appearing on the IV characteristic is depicted in Fig. 4.6. And the output conductance g is easily obtained from Eq. 4.89 by differentiating it with respect to drain voltage, VD and is given by (.8) 43 In saturation regime (assuming saturation is due to pinch off), saturation occurs when V = 1/. From Eq 4.87, I = fi [V(1/ V) '(1/2 - V2)]. (4.84) 115 I3Vg'VD 1 + I3Vg' (RD+Rs) Voltage FIGURE I 4.6: Source and drain resistances effect on IV characteristics. 0 or R will remain constant only when V 0. When we consider the series resistance effect on the saturation current due to pinch off, ig = = 1 VIR + I2R)] [iv 7jS 2VIR8 + I2R = (V I2R 2I( VR5 + By solving the the quadratic Eq. I; (T'Rs+ Finally, reorganizing Eq. 4.86, I + IR5)2 = 0. (4.85) (4.86) becomes V ) (4.87) \J 4.87 gives (1+ f3VRs)±JT2fiVRs (4.88) For small Rs, we may neglect R term in Eq 4.86 and it changes the equation to be I[1 + I3VRs] (4.89) 116 or D2(1+R) And the saturation voltage VE = 490 + IRD where V = V'. The total series resistance R + RD can be determined from the linear region of the characteristics by rearranging Eq. 4.89, 4.91 VDS that is, =RD+Rs + Eq. 'D 1 (4.91) shows the controllable resistor behavior of FET with small value of drain voltage (in linear region). Source and drain resistances are easily estimated from yintercept when this VD/ID is plotted as a function of 1/V in the linear region of IV characteristics. Additionally, with intrinsic transconductance gm2 as 3V, the maximum device transconductance, gm including series resistance effect, is given by 9m; 1 + /3RV (4.92) Theoretical models for electrical characterization of HEMT devices were derived in this chapter. Starting with Schrodinger and Poisson equations, the models of sheet carrier concontration of 2DEG were derived at equilibrium and also with applied bias condition (charge control model). For HEMT device characteristics, we developed the models of currentvoltage and transconductance characteristics for short and long channel HEMT devices. These models were modified to incorporate the radiationinduced effects in the device characteristics. 117 CHAPTER 5 IRRADIATION EFFECTS IN HFETs In this chapter we present the results of the radiation effects in heterostructure field effect transistors (HFETs). Electron and neutron irradiation effects on the DC characteristics of these devices are carefully examined. We introduced detailed information of HEMT device fabrication steps with the optimized processing recipes, radiation procedures for electron and neutron, and electrical characterization methods of both devices in Ch. 3 and theoretical model for currentvoltage mechanism in Ch. 4. Experimental results of both electron and neutron irradiation on HFETs are presented in this chapter. Currentvoltage, transconductance, channel mobility, and series resistance of the devices are mainly investigated. Carrier concentration and channel mobility of control samples are examined by Hall effects measurement experiment to extract fitting parameters for 2DEG concentration and currentvoltage models under radiation influence. Analytical as well as numerical models developed for 2DEG sheet carrier concentration are simulated including radiationinduced acceptorlike defects. We also select the device structure parameters (e.g. doping concentration and thicknesses of various layers) as control variables to establish the dependence of 2DEG sheet concentration on these parameters. 118 Neutron Irradiation Effects on HFETs 5.1 We will present the experimental results of irradiation performed on Hall control samples as well as both HEMT and HIGFET devices. From current voltage characteristics, the radiationinduced effects on threshold voltage shifts, transconductance and drain current degradation will be described. We also examine mobility and carrier concentration degradation using Hall control samples to extract fitting parameters for 2DEG models. 5.1.1 Threshold Voltage Shift The I V characteristics of the different wafer segments used for the different doses were slightly different from one another before irradiation. Hence we have used the normalized values so that these parameters (e.g. drain current, ID) with respect to the corresponding unirradiated values can be plotted on the same scale. The 'D VGS characteristics of the AIGaA.s/GaAs HEMT devices and those of the HIGFET devices are shown in Fig. 5.1 and Fig. 5.2, respectively. Threshold voltage was determined by extrapolating the 'D istics to 'D VGS character- = 0 as shown in Fig. 5.3 for a HEMT device after the largest dose of neutron fluence. It is seen that the threshold voltage always shifts to a more positive value after neutron irradiation. Neutron induced shifts of the threshold voltage for the two types of devices are summarized in Fig. 5.4. It is clear from Fig. 5.4 that the threshold voltage shift of the HIGFET is much smaller. The principal reason for this behavior will be discussed in discussion section in detail, which tells the difference in the threshold voltage shift of the two types of devices arises from the difference in the sensitivity of the quantum well shapes in the two cases to the radiation induced charges in the channel region. The fit of the threshold voltage shift as a function of neutron dose to a linear 119 1.1 1.0 0.9 -U- 0.54x1 &4n/cm2 -0- before_dose2 0.8 -- 1 .62x1 014n/cm2 0.7 8 -a- before_dosel 0.6 before_dose3 -A- 4.86x1 014n/cm2 - 05 0.4 0.3 VDS=2.25V I 0.2 0.1 (a) HEMT 0.0 -ni -1.5 -1.0 -0.5 0.0 0.5 1.0 V35(V) 'D VGS characteristics of A1GaAs/GaAs HEMT devices before and after neutron irradiation. FIGURE 5.1: 1.1 0.9 -0- dosel _before -U- 0.81 xl 0'4nIcm2 -0- dose2_bef ore 0.8 -'- 3.24x1 014n/cm2 1.0 0.7 dose3_bef ore -A- 4.86x1 014n/cm2 0.6 0 0.5 0.4 0.3 v0= O.25V 0.2 0.1 (b) HIGFET 0.0 _r I 0.1 0.2 0.4 0.3 0.5 0.6 VGS (V) FIGURE 5.2: 'D VGS characteristics of AIGaAs/InGaAs/GaAs HIGFET devices before and after neutron irradiation. 120 1.0 - D Before irradiation U 4.86 x 0.8 8 1014 1.0 0.8 n / cm2 0.6 0.6 HEMT 0.4 0.4 0.2 0.2 I>TOV . 0.0 -0.2 I;=.0 -0.2 1.0 0.5 0.0 -0.5 0.0 VGS (V) FIGURE 5.3: Threshold voltage extraction from 'D VGS characteristics of AIGaAs/GaAs HEMT devices before and after neutron irradiation. 0.30 HEMT HIGFET linear fit 0.25 - - - -. parabolic it linear fit 0.20 VT=1.1x10112 > -0.15 - -_ . . AVT=4.62XlO 0.10 0.05 :' '. (r 0 1 2 3 Neutron Fluence 4 5 6 (1 014n/cm2) FIGURE 5.4: Neutron induced shifts of the threshold voltages on HEMT and HIGFET devices. 121 equation of the form, ZWT = VT VTO = açb yields the value of the degradation coefficient, a, to be and 0.64 (5.1) 4.62 x 1016 Vcm2/n, x 10_16 Vcm2/n, for the HEMT and the HIGFET devices, respectively. A parabolic dependence of ZWT on the neutron fluence given by the equation, (5.2) is actually found to give a better fit of the experimental data of 1VT shift in HEMT devices as shown in Fig. The transconductance imum transconductance (gm) 5.4. of the HEMT device normalized to the max- (gm,maxo) neutron doses is shown in Fig. 5.5. as a function of gate voltage for different The transconductance (gm) of HIGFET de- 1.2 Opens : before 1.0 - U 0.54x1 014 n/cm2 -.- 1 .62x1 014 n/cm2 0.8 9 A 4.86x1 014 n/cm2 c E 0.6 E E 0.4 0) 0.2 VDS=2.25V 0.0 -0.2 L.. -1.5 -1.0 0.0 -0.5 0.5 1.0 VGS(V) FIGURE 5.5: Transconductance as function of gate voltage of HEMT devices before and after neutron irradiation. 122 vice normalized to the maximum transconductance before radiation (gm,maxo) with respect to the gate voltage for different neutron doses is shown in Fig. 5.6. As shown in 'D VGS plots for both devices, threshold voltage shifts on HEMT 1.2 -D- before_dosel o -U-- 1.0 =O.81 xl 014n/cm2 before_dose2 -.- 2=3.24xl 014n/cm2 before_dose3 0.8 -A- 43=4.86x1 O'4n/cm2 D) 0.6 E 0.4 V=O.25V 0.2 0.0 0.1 0.4 0.3 0.2 0.5 VGS (V) FIGURE 5.6: Transconductance of HIGFET devices as a function of gate voltage before and after neutron irradiation. device is larger than those of HIGFET device for each neutron dose. Also, the decrease in maximum transconductance is smaller for HIGFET than HEMT. 5.1.2 CurrentVoLtage Characteristics Devices of ALGaAs/GaAs HEMT VDS characteristics of a HEMT device were measured before and after the neutron irradiation with doses of 4.86 x 1014 n/cm2 and shown in Fig. 5.7. Gate voltages are changed starting 0.6 V with a step of -0.1 V. The drain saturation current for a long gate length transistor, that is, saturation due to drain pinch 123 4.0 Pre-Irradiated 3.5 3.0 ..U ..uU 2.5 E 1.5 U ..U UU . 2.0 0 ..U .....UuU ... V00.6V... . 4.86x1014n/cm2 U. .UU ......U uuUU U. .. UU . U U flU. U.. .... ....UU ..UUU .. .U 1.0 0.5 0.0 0.0 0.5 1.0 2.0 1.5 2.5 VDS (V) FIGURE 5.7: Currentvoltage characteristics of a HEMT device before and after neutron irradiation measured at gate voltages from 0.6 V to 0 V with a step of-0.1 V. off, is given by [10] I(1/VOff_VC(0))2 (5.3) where V(0) = IR3 is the potential drop across the source resistance R5 between the source and the gate as used in Ch. 4. ,8 is also defined in the same section Eq. 4.51 as E2 W (5.4) d+zd L where is the channel carrier mobility, E2 and d are the dielectric constant and the thickness of the AlGaAs layer beneath of gate, respectively, is a correction term that accounts for the variation of the Fermi level at the A1GaAs/GaAs interface with the sheet concentration of the 2DEG [9] and W/L is the gate width/length ratio of the device. From Eq. 5.3 and Eq. 5.4, it is clear that the radiationinduced decrease 124 in drain current is determined by the combined effects of the increase in the threshold voltage, decrease in the channel carrier mobility and increase in the source resistance. Mobility degradation with dose is generally expressed in the form = 1+b. (5.5) Ii If the series resistance is small, then the following simple equations for the normalized drain current and transconductance in the saturation region can be derived using Eq. 5.3, Eq. 5.4, and Eq. 5.5. I(çb) 1 1+b JSO 11-aq { VTO 2 1 (5.6) VVTo] and gm(cb) VTO açb VTO (5.7) ] VgVTOj is the threshold voltage before radiation. However, the series resisgm,o where 1 1+b L tance of the HEMT devices was quite significant and hence we used the following simple analysis to separate the effects of mobility degradation from the series resistance. The drain resistance in the triode region (small VDS of the characteristics) was shown in Eq. 4.91 'D [101, =Rs+RD+ (VV) VDS VDS 1 (5.8) where RD is the drain resistance of the channel between the gate a Thus a plot of VDS/ID vs. 1/(V VT) is linear with an yintercept of (R8 +RD) and the slope of 1/3. Hence, the channel mobility can be determined from the slope, /, using Eq. 5.4 and the source resistance from the intersection with the yaxis by assuming R5 = RD. The normalized mobility and series resistances obtained from this analysis are shown in Fig. 5.8. The value of the mobility degradation parameter b deter- 125 1.05 1.8 ): HEMT filled symbols & ( : HIGFET open symbol & ( 1.00 1.6 0.95 R/R0 1.4 0.90 0 : 0.85 1.2 .__-__ 0.80 -A 1.0 0.75 /0=1/(1+b) ('R 0 I I I I 1 2 3 4 Neutron Fluence 0.70 5 (1014n/cm2) FIGURE 5.8: Normalized mobility (Square) and source resistance (Triangle: experiment, Circle: TLM) of HEMT devices as a function of neutron dose. Lines are best fits. mined from the best fit of the data in Fig. 5.8 is found to be 7.36 x 10'6cm2/n. As shown in Fig. 5.8, the series resistance of HEMT devices determined from the above analysis is also found to be consistent with the measurements made on transmission line model (TLM) structures adjacent to the actual HEMT devices. The increase in resistance Rg and RD is caused by a decrease in the shee carrier concentration as well as mobility degradation in the extrinsic channel regions (source to gate and drain to gate) of the device. In order to assess the defect concentrations introduced by neutron radiation in the different layers of the device, we also performed Hall measurements on control samples of AlGaAs and GaAs single layers irradiated along with the test devices. The results of carrier concentration and mobility degradation in GaAs as a function of neutron dose are shown in Fig. 5.9. From these measurements we determined a carrier removal (or acceptor introduction) rate of 20 cm1, 126 I.' I.' 1.0 1.0 0.8 0.8 0 C C 0.6 0.6 0.4 0.4 0.2 0.2 (H, 0.0 0 1 2 3 4 5 0 Neutron fluence 2 1 3 4 5 (1014n/cm2) FIGURE 5.9: Mobility (a) and carrier concentration (b) degradation in two GaAs samples with different initial carrier concentrations (Square: 3 x 1O'5/cm3, Circle: 1.26 x 1O'6/crn3) as a function of neutron dose. nearly same in both AlGaAs and GaAs. Once the threshold voltage, the channel carrier mobility and the series resistance values are known, the drain saturation current, ig and the transconductance, g can be calculated using the following expression derived from Eq. 5.3. [s/i + 2/3RsV /3V 74 where V = V9 1+fiR9V (1 + /3R8V)] (5.9) (5.10) VT. The experimental results of normalized 'D VDS characteristics of the HEMT devices with gate voltage of 0.4 V for different neutron doses are plotted along with the simulated results from Eq. 5.9 in Fig. 5.10. The normalized drain saturation current at VDS = 2.25V for different neutron doses are shown in 127 1.2 D Pre-irradiated O.54x1 014n/cm2 1.0 1.62x1014n1cm2 A 0.8 8 4.86x1O14cm2 _.;;;,.....oorn V0 = O.4V 0.6 4;. 0.4 0.2 fin 0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 VDS(V) FIGURE 5.10: Normalized drain saturation current as function of neutron dose from experiment and model simulation. Fig. 5.11. The calculated drain saturation current using Eq. 5.9 as a function of neutron dose is also shown in Fig. 5.11. Good agreement between the theory and the experiment in both Figs. 5.11 and 5.10 confirms the validity of the mobility and the series resistance values extracted earlier from the simple analysis. The transconductance, values using the above model in Eq. 5.10 has been calculated. Once again, good match between the calculated and the experimental 9m values is observed for 9m < 9m,maxO as shown in Fig. 5.12. It is believed that the decrease of 9m in the experiment for large Vg is caused by the onset of parallel conduction in the A1GaAs which is not modeled in the simulation [12]. The transconductance degradation is predominantly due to the channel mobility degradation. 128 1.2 o Experiment Simulation 1.0 VDs=2.25V _O U, I 0.8 0 0.6 - 0.4 - 0 0.2 L 0 2 1 4 3 5 Neutron Fluence 4) (1014 n/cm2) FIGURE 5.11: Normalized drain saturation current as function of neutron dose from experiment and model simulation. 1.2 Triangles: Experiment Squares: Model Before 1.0 - -A- 4.86x1 014n/cm2 -D- Before - -.- 4.86x1 014n/cm2 00 -0.8 £ A A_- 1 A A /A A A A -0.2 -0.4 -0.6 0.0 0.2 0.4 0.6 0.8 VGS (V) FIGURE 5.12: Calculated transconductance (Square) and the experimental data (Triangles) before and after neutron irradiation with the dose of 4.86 x 1O'4n/cm2. 129 5.2 Neutron Irradiation Effects on HIGFET Device We will now discuss neutron irradiation degradation on VDS characteristics including breakdown voltage degradation after neutron irradiation. Annealing experiment performed on HIGFET as well as Hall control samples will be also briefly discussed. 5.2.1 Current- Voltage Characteristics The normalized 'D VDS characteristics of the HIGFET devices at a gate voltage of 0.24 V for different neutron doses are shown in Fig. 5.13. The gate voltage for this measurement was chosen to be very low (in the subthreshold region) in order to keep the drain saturation current small. The drain current decrease is due to 1.2 V08=O.24V 1.0 0D0 00 0.8 .....uuuuuuuI 0 8 _c 0.6 0.4 .....S. 0 Before 0.2 0.81 xl 014ri/cm2 -.- 3.24x1 O'4n/cm2 0.0 A 4.86x1 014n/cm2 -0.2 0.0 0.5 1.0 1.5 2.0 2.5 3.0 VDS (V) FIGURE 5.13: 'D VDS characteristics of A1GaAs/InGaAs/GaAs HIGFET devices for different neutron doses. 130 the increase in the threshold voltage. Even though the drain current degradation in Fig. 5.13 appears to be severe, since the device is operated in the subthreshold region even a small change in VT causes a large change in 'D On the other hand, the drain current degradation (Fig. 5.2), as well as in the higher VGS region (VGS >> VT), gm degradation (Fig. 5.6) are much less than that for the HEMT devices. Unfortunately, the poor heat sinking of these unpackaged high power devices prevented us from applying large drain voltage and large gate voltage at the same time. Hence we could not measure the drain saturation current I for VG >> VT. The validity of the analysis underlying Eq. 5.8 is questionable under subthreshold conditions and hence we could not use this analysis to separate the mobility and the series resistance effects. However, since the HIGFET devices are fabricated by selfaligned source/drain ion implantation, there are no extrinsic regions and hence the series resistance effects are expected to be small. Hence we believe that the transconductance degradation caused by neutron radiation is due to the mobility degradation. 5.2.2 Breakdown Voltage Degradation As mentioned earlier, the HIGFETs were designed for large breakdown voltages by incorporating a lightly doped drain region between the gate and the drain. The schematic crosssection diagram of a HIGFET device incorporated with a LDD is shown in Fig. 3.3. The breakdown 'D VDS characteristics of devices with two different LDD spacing are shown in Fig. 5.14 and 5.15. The LDD5 device (5 jim LDD spacing) does not show any evidence of breakdown up to the maximum drain voltage used in this study (35V) even after the highest neutron dose. On the other hand, the breakdown voltage of the LDD2 device (2 jim LDD spacing) is unaffected only for doses less than 3.2 x 1014 n/cm2 and then 131 reduces to '- 25 V for a dose of 5.0 x 1014 n/cm2. It is believed that the decrease in breakdown voltage for high doses is caused by increased generation current due to the radiationinduced traps. 0.20 A A A A A A A A A before irradiation A 0.15 : 3.24x1014n/cm2 before_irradiation 4.3 : 4.86x1 014n1cm2 .0.10 A GS=OO2VI!A 0.05 n 0 5 10 15 20 25 30 VDS (V) FIGURE 5.14: Breakdown 'D VDS characteristics for 2pm LDD spacing HIGFET devices after different doses of neutron irradiation. 5.3 Annealing Study Isochronal annealing experiments on neutronirradiated GaAs samples were performed in the temperature range from 220°C to 520°C at intervals of 20°C. The recovery of carrier concentration of the neutron irradiated GaAs sample as a function of annealing temperature is shown in Fig. 5.16. Two distinct annealing stages at 320°C and 450°C are clearly seen from the carrier concentration plot. The maximum annealing temperature of HEMT and HIGFET devices was 132 0.07 o before_irradiation - 0.06 V0=0.02V . n ).fAIU 11jUl11 before_irradiation v-3:4.86x1O14n/cm2 0.05 00000 oo00000o 0.04 0003 0 0 I. 0.02 - 0 . ',vyvvvyyv, 0.01 v.yvvv,v., (b)51.iLDD SPACE yy 0.00 0 yvYVY vyvyvyvyvy 5 30 25 20 15 10 VDS (V) 35 FIGURE 5.15: Breakdown 'D VDS characteristics for 5jtm LDD spacing HIGFET devices after different doses of neutron irradiation. 1.2 . Annealed after 4 = O.54x1 014n/cm2 1.0 -0- Annealed after 4 = 4.86x1 014n/cm2 C C 2-0.8 C 0) 0 C 0.6 0) 0 Ce 0.4 ooo--0oo 0.2 150 200 250 ooooo--o 300 350 400 450 500 Annealing Temperature ( °C) FIGURE 5.16: Carrier concentration recovery after annealing of the neutron irradiated GaAs samples: Square: 0.54 x 10'4n/cm2, Circle: 4.86 x 10'4n/cm2. 133 restricted to 340°C to avoid permanent damage to the metalization layers of the devices. The partial recovery of the 'D VDS characteristics of the HEMT and HIGFET devices after annealing at 340°C is shown in Figs. 5.17 and 5.18. 1.2 -.- before irradiation 1.0 (a) HEMT 3=4.86x1 014n/cm2 after annealed 0.8 VGS= O.4V 8 0.6 0.4 0.2 340°C 0.0 0.0 0.5 1.5 1.0 2.0 2.5 VDS (V) FIGURE 5.17: 'D VDS characteristics recovery after isochronal annealing on neutron irradiated HEMT devices. Similarly the partial recovery of the threshold voltage and the transconductance of the devices after annealing at 340°C is seen from the 'D VDS plots shown in Figs. 5.19 and 5.20. 5.4 Modeling for Carrier and Mobility Degradation in HEMT Carrier removal is an important physical mechanism at the material level, which is responsible for a number of radiationinduced degradation effects observed in semiconductor devices. Examples of device level degradation effects caused 134 1.2 (b) HIGFET before irradiation 1.0 -A- 486x1Ol4n/cm2 after annealed 8 I_IIlI lull. I.. 08 lull 0.6 04 34O0C 0.2 0.0 0.0 O.24V 0.5 V03 FIGURE 5.18: 'D VDS characteristj neutron irradiated HIGFET devices. 30 2.5 2.0 1.5 1.0 (V) recovery after isochronaj annealing on 3.5 30 -._ before irradiation 486x1oIcm2 2.5 ----- after annealed <2.0 O°c __P 1.5 VDsr225V - -. -S -S 1.0 (a)HEMT 0.5 0.0 -0.6 -0.4 -0.2 0.0 0.2 V (V) 0.4 0.6 0.8 1.0 characteristic recovery after isochronal annealing on FIGURE 5.19: 'D neutron irradiated REMT devices 135 0.12 0.10 -- before irradiation ---- q,=4.86x1 014n/cm2 after annealed 0.08 0.06 340°C VDs=O.25V 0.04 0.02 (b) HIGFET 0.00 0.1 0.2 0.3 0.4 0.5 VGS (V) FIGURE 5.20: 'D VGS characteristics recovery after isochronal annealing on neutron irradiated HIGFET devices. by carrier removal include increase in parasitic series resistance in bipolar transistors and field effect transistors (FETs), and threshold voltage shift in FETs. Carrier removal effect in bulk devices is easily understood in terms of charge compensation due to the radiationinduced defects in the active volume of the device. However, the carrier removal effect in heterostructure devices is more complex due to the presence of a number of layers of different materials in the active volume of the device and due to the quantum effects caused by very small thickness of one or more layers of the device. Mobility degradation is another important physical mechanism, which con- tributes to the degradation of drain current and transconductance in FETs. The mobility degradation is caused by the enhanced scattering of carriers by the radiationinduced defects. Even though there is no fundamental difference in this behavior between the bulk devices and heterostructure devices, the 136 quantitative modeling of the effect in the two types of devices is different. In the previous section, we observed that radiationinduced carrier removal and mobility degradation were primarily responsible for HEMT device characteristics, such as threshold voltage shift, drain current and transconductance degradation. With the help of the qualitative analysis performed on the results from neutron irradiation experiment in HEMT device, a more fundamental level of carrier removal and mobility degradation is studied in this section and also some results of the investigation is presented because they are ultimately required for a quantitative modeling of the device level effects. 5.4.1 Theoretical Model for Carrier Removal As mentioned earlier, the carrier removal in bulk materials is easily understood. If the starting material is ntype, the carrier concentration after irradiation is given by n=nO+NDNA--noa'cb (5.11) where NA is the concentration of the radiationinduced acceptorlike (deep and shallow level) defects. The deep donors do not contribute to the carrier removal since they are neutral in ntype material. If (NDNA) varies linearly with the particle fluence, , as assumed in Eq. 5.11, then the carrier removal rate, a' is constant. The radiationinduced net acceptor concentration NA ND can be easily determined from the experimentally measured carrier removal rates in bulk ntype materials using Eq. 5.11. In the case of heterostructures, the carrier removal cannot be described by a simple expression as in Eq. 5.11. There are several layers in a heterostructure device and the acceptorlike traps introduced in each of these layers may contribute to carrier removal. The twodimensional electron gas (2DEG) carrier 137 concentration in a HEMT structure is a complicated function of a number of parameters including the doping concentration in the A1GaAs layer, the thickness of the doped layer, the thickness of the spacer layer and the doping concentration in the GaAs layer as shown in Eqs. 4.30 and 4.44. Again, the carrier concentration can be determined by a selfconsistent solution of the Poisson's equation and the Schrodinger equation introduced in Sec. 4.1. If a triangular potential well approximation is made for the 2DEG region, and analytical solution for the 2DEG concentration maybe obtained. Delagebeaudeuf and Linh [10] first published an analytical model using this approach. However, their model ignores the effect of the doping concentration in the GaAs region. This approximation is quite valid for the unirradiated samples since the background doping concentration in the GaAs (buffer) layer in high quality HEMT structures is quite small. However, it turns out that the radiationinduced acceptors in the GaAs region have the most significant influence on the 2DEG concentration. Hence, it was important to develop a modified model that takes into account the effects of acceptors in the GaAs region as well as those in the A1GaAs regions. We already discussed about the importance of radiationinduced acceptor concentration in GaAs layer and found out the effects on threshold voltage shift in HEMTs in the previous section. The device structure parameters of the 5doped and uniformly doped HEMT devices along with the layer properties are described in Tables 3.1 and 3.2. Some of these parameters such as A1GaAs layer thickness, doping concentrations of the 8doped and uniform AlGaAs layer as well as other constants are used for used in our analytical models. Fig. 5.21 shows the influence of acceptors in all the layers, as calculated from 138 the analytical model It is seen that the acceptors in the A1GaAs have only a 1.1 . ---------- 1.0 --- 0.9 0.8 C -. 0.7 C C,, Uniform : Analytical 0.6 0.5 0.4 Na in GaAs only Na in AIGaAs only Nafl Both 0.3 1 0.1 10 Na (1015 cm3) FIGURE 5.21: The effect of acceptors in AlGaAs layer, in GaAs layer, and in both layers of uniformly doped HEMT structure on 2DEG concentration from analytical model. negligible effect on the 2DEG concentration. On the other hand, the acceptors in the GaAs region have a much stronger effect. The 2DEG concentration can also be obtained from a numerical self consistent solution of PoissonSchrodinger equations and the detailed derivation procedures of 2DEG concentration in the heteroj unction can be found in Sec. 4.1. Also, the details of this numerical simulation procedure are described elsewhere [80], [81]. Other parameters related to selfconsistent numerical simulation are summarized in Table 5.1. This numerical approach is necessary for those hererostructures for which closed form analytical solution cannot be obtained. The results obtained from 139 Quantity Parameter ptype doping concentration 1 x 1 x 1016 1013 Unit cm3 Schottky barrier height 0.9 eV acceptor binding energy 50 meV LEc (A1GaAs/GaAs) 300 meV TABLE 5.1: Simulation parameters for SchrodingerPoisson selfconsistent numerica simuataion for HEMT structure. the selfconsistent solution for the HEMT structure with uniform doping in the AlGaAs layer are shown in Fig. 5.22. The selfconsistent solution gives a slightly 1.1 A''A........_ . 1.0 0.9 o 0.8 C 0., (I, 0.7 0.6 0.5 Uniform: Numerical 0 Na in GaAs only Na in AIGaAs only __ Na in Both 0.4 0.1 1 10 Na (1015 cm3) FIGURE 5.22: The effect of acceptors in different layers of uniformly doped HEMT structure on 2DEG concentration from numerical model. 140 smaller carrier removal than that obtained with the analytical solution. It is believed that the selfconsistent solution results are more accurate. The self consistent results for the case of HEMT structure with a deltadoped A1GaAs layer are shown in Fig. 5.23. The delta doping concentration and the position 1.1 1.0 - 0.9 C 0.8 U, C 0.7 -dopedHEMT I 0 0.6 Na in GaAs only Na in AIGaAs only Na in Both 0.5 0.1 1 10 Na (1015 cm3) FIGURE 5.23: The effect of acceptors in different layers of 8 doped HEMT structure on 2DEG concentration from numerical model. of the 8doped plane in the AlGaAs were chosen so that 2DEG concentration is the same in both the (uniformly doped and 8 doped) structures before irradiation. It is seen that the carrier removal in the 8doped HEMT structure is slightly smaller that that of the uniformly doped HEMT structure. The device structure parameters (doping concentration and thickness of various layers) have been optimized for the best performance in the unirradiated samples. In order to investigate if these parameters are also optimized in terms 141 of radiation performance, the influence of these parameters in the carrier removal is studied. The results of the 2DEG concentration as function of the radiation induced acceptor concentration (assumed to be present in all the layer) for different spacer layer thickness are shown in Fig. 5.24. The effect of AlGaAs 0.9 0.8 0.7 --- ...............-____ 0.6 E 0 0 A................ A.... A.................... 0.5 A CI) c 0.4 0.3 0.2 A .Ad=5nm A.... A d=15nm 0.1 0.01 0.1 1 10 N (1015 cm3) FIGURE 5.24: The 2DEG concentration as function of induced acceptor concentration for different spacer layer thickness. composition was modeled by varying the conduction band discontinuity between AlGaAs and GaAs. The acceptor concentration dependence of the 2DEG concentration for different values of the conduction band discontinuity is shown in Fig. 5.25. Finally, the effect of the doping concentration in the AlGaAs on the carrier removal due to acceptor incorporation is illustrated in Fig. 5.26. It is seen from Fig. 5.24 to Fig. 5.26 that the density of carriers removed (per unit area) is nearly independent of the spacer layer thickness, the AlGaAs composi- 142 0.7 A ............ A .... A A. 0.6 A ---. -------------- 0.5 A cJ E 0 0.4 0 1 0) C 0.3 -U0.2 = 0.15 eV --E=02OeV A......... = 0.25 eV 0.1 0.01 10 0.1 Na (1015 cm3) FIGURE 5.25: The 2DEG concentration as function of induced acceptor concentration for different values of conduction band discontinuity. 0.60 0.55 A ............ A .... A . -A. A .A 0.50 0.45 cJ E 0.40 0 'b 0.35 C,) C 0.30 0.25 0.20 .Nd=5x1017cm3 ---.---- Nd= 1x1018cm3 A Nd = 2x1 018 cm3 0.15 0.01 10 0.1 Na (1015 cm3) FIGURE 5.26: The 2DEG concentration as function of induced acceptor concentration for different doping concentration in A1GaAs layer. 143 tion and the A1GaAs doping concentration. The carrier removal depends only on the radiationinduced net acceptor concentration. The decrease in carrier concentration is found to be nearly equal to depletion charge (per unit area) due to the defects introduced in GaAs. 5.4.2 Modeling for Mobility Degradation Effect on HEMT We analyzed the influence of the radiationinduced defects on the carrier mobility using standard theoretical models [78]. The following scattering mechanisms were included in the analysis of mobility: polaroptic phonon scattering, acous- tic phonon deformation potential scattering, piezoelectric scattering, Coulomb scattering due to the remote impurities in the A1GaAs region and the ionized impurity scattering due to radiationinduced impurities/defects in the GaAs region and in the spacer AlGaAs region. It may be noted that the ionized impurity scattering is proportional to the combined donor and acceptor (ND + NA) defects whereas the carrier concentration is determined by the net acceptorlike defect concentration (ND NA) in the GaAs channel region. The simulated results of carrier mobility versus the radiationinduced total defect concentration (ND + NA) in GaAs are shown in Fig. 5.27. The 300 K mobility is mainly limited by polar optic phonon scattering and hence it is only weakly dependent on the radiationinduced defect concentration in GaAs since ionized impurity scattering plays a more dominant role at low temperatures. Hence the 77 K mobility has been used for comparison between theory and experiment. On the other hand, the 300 K values of carrier concentration are more relevant since the carrier freezeout may occur at low temperature. 144 106 1 .' 0 E C, 1 1 1o3 '- iü' 1013 Nd 02 101 1016 1017 Na (cm3) FIGURE 5.27: The simulated carrier mobility vs. radiation induced total defect concentration in GaAs layer. 5.4.3 Experimental Results and Analysis The measured values of normalized sheet carrier concentration (n/no) at 300 K and normalized reciprocal mobility (0/t) at 77 K as a function of neutron fluence for the 5doped HEMT structure are shown in Fig. 5.28. The dependence of normalized carrier concentration and mobility for the bulk nGaAs control sample is shown in Fig. 5.29. The net acceptorintroduction rate due to neutron irradiation can be easily calculated from the measured carrier removal in nGaAs control sample using Eq. 5.11. Assuming the net acceptor introduction rate in the GaAs region of the HEMT structure to be the same as that measured in the control GaAs sample, the measured 2DEG concentration in 6doped HEMT structure as a function of the expected radiationinduced acceptor concentration in GaAs is shown in Fig. 5.30 and compared with the total radiationinduced defect concentration in the GaAs region. The total defect 145 1.1 o tat77K nat300K 1.0 5 I, 4 0.9 0 C C 3 0.7 2 HEMTI 0.6 fl-c 0 1x1014 2x1014 3x10'4 4x1014 5x104 Fluence 4, (n / cm2) FIGURE 5.28: Measured sheet carrier concentration and mobility as functions of neutron fluence for the 8doped HEMT structure. 4.5 1.1 --o--at77K 1.0 --nat300K 4.0 0.9 3.5 0.8 3.0 o 0.7 C 2.5° 0.6 2.0 0.5 1.5 0.4 - GaAs 0.3 1.0 ('9 0.5 0 lxlO4 2x1014 3x10" 4x1014 5x10" Fluence 4) (n / cm2) FIGURE 5.29: Measured sheet carrier concentration and mobility as functions of neutron fluence for the nGaAs control sample. 146 concentration (NA + ND iO' cm3) required to explain the mobility degradation is nearly an order magnitude larger than the net acceptor concentration (NA ND 1016 cm-3) determined from the carrier removal data. 1.00 0.95 IHEMTI 0.90 "SO 0.85 00.00 C c 0.75 0.70 Experiment ---o--- Simulation 0.65 0.60 0.55 0.50 1 2 3 4 5 6 7 8 9 10 Na (1015 cm'3) FIGURE 5.30: 2-DEG concentration of 6-doped HEMT structure as a function of net acceptor concentration in GaAs from experimental and theory. 5.4.4 Modeling for Radiation-induced Threshold Voltage Shift In this section we present the results of numerical as well as analytical simulation performed incorporating the radiation induced effects on the threshold voltage shift of HEMT and HIGFET devices. Numerical models are developed by solving Schrodinger equation and Poisson equation self-consistently similarly to the carrier removal model discussed in Sec. 5.4.1. The well-known analytical model of sheet carrier concentration of HEMT device is modified to incorporate the effect of increase of acceptor concentration in the device as well. 147 Simulation parameters required for numerical and analytical models of a HEMT device were already introduced in Table 5.1 in Sec. 5.4. For a HIGFET device, most of the parameters are set to be same as those of HEMT except 330 mel/ for the conduction band offset between A1GaAs/InGaAs and 152 meV for the conduction band offset between InGaAs/GaAs. Results from Simulation 5.4.5 The sheet carrier concentration of the 2DEG in the single interface deltadoped A1GaAs/GaAs HEMT as a function of gate voltage for different background acceptor concentrations as obtained from the selfconsistent numerical simulation is shown in Fig. 5.31. The threshold voltage was determined by extrapolating uN= 1x10'3cm O---3x1013 A lxlO'4 0.1 v-3x10 14 cJ E 0 +-3x1015 0 '.4 HEMT:Numerical X-7x1015 // *-1x1016 i'/ - 0.01 1E-3 L. -1.2 -1.1 -1.0 -0.9 -0.7 -0.8 -0.6 -0.5 -0.4 -0.3 VGS (V) FIGURE 5.31: 2DEG concentration as a function of background acceptor concentrations on HEMT devices from selfconsistent numerical solution. n3 versus V9 linear plots (for large V) to n8 = 0. This is consistent with the pro- 148 cedure used for the determination of the threshold voltage in the experiments discussed in next section. The results for the gate voltage dependence of the sheet carrier concentrations as calculated by the analytical model are shown in Fig. 5.32. The results of the analytical model are found to be in fairly good I. N=1x10 cm 13 -3 0---- 3x1 013 A 1x1014 v 3x10 lxi 0.1 cJ E 0 + 3x10'5 0 - HEMT:Analytical X 7x1 1 1X10l6// / 0.01 1E-3' -1.3 -1.2 1.1 -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 VGS (V) FIGURE 5.32: 2DEG concentration as a function of background acceptor concentrations on HEMT devices from analytical solution. agreement with the selfconsistent simulation. This validates the numerical procedure used for the selfconsistent simulation. The gate voltage dependence of the sheet carrier concentration in the case of HIGFET devices for different background acceptor concentrations as obtained from the selfconsistent numerical simulation is shown in Fig. 5.33. The threshold voltage shift relative to a minimum acceptor doping concentration of 1013 cm3 as a function of background acceptor doping concentration are shown in 149 11 UN9 = lxi 013 cm 0 lxi 014 A lxi 013 v-3xiO 0.1 - 15 ---7xi015 0 09 .2 +ixlO'6 E 03 /+ 4Q7 /// C'4 0 00 Q5 1!) 54 0.01 53 02 51 / 1 -0.1 0.0 0.1 O I I I 0.2 0.3 0.4 JI 00 I 0.5 32 01 I 0.6 04 03 05 06 I I 0.7 0.8 01 06 0.9 09 1.0 VGS (V) FIGURE 5.33: 2DEG concentration as a function of background acceptor concentrations on HIGFET devices from selfconsistent numerical solution. Fig. 5.34 for the two types of HFET structures. It is clearly seen that the HIGFET shows a much smaller threshold voltage shift as compared to the conventional single interface heteroj unction device for the same increase in the acceptor doping concentration. 5.4.6 Results from Experiment The 'D VGS characteristics of HEMT and HIGFET devices before and after neutron irradiation were already shown in Fig. 5.1 and Fig. 5.2. The threshold voltage was determined by extrapolating the 'D VGS characteristics to 'D = 0. As mentioned earlier, the Hall effect measurement on controlGaAs samples irradiated along with the HFET devices determined a carrier removal rate of 20 cm1. Using this value of the acceptor introduction rate, the experimental values of the threshold voltage shift have plotted in Fig. 5.34 as a function of acceptor concentration for both types of HFET devices shown in the previous 150 0.35 0.30 0.25 I I I HEMT: Analytic HEMT:Numencal HEMT: Experimental HIGFET: Numerical HIGFET: Experimental t * X 0.20 U 0.15 * 0.10 * U 0.05 C, 0.0 . x x x i 2.OxlO'5 4.OxlO" I I 6.OxlO" 8.OxlO'5 1.OxlO'° Na (cm3) FIGURE 5.34: Threshold voltage shift as a function of background acceptor concentration for both HEMT and HIGFET devices. section. Good agreement is found between the theory and the experiment. 5.4.7 Discussion It is clear from both the experimental and simulation results that the shift in the threshold voltage of HIGFETs is smaller than that of the conventional single interface HEMTs for the same increase in the background acceptor concentration. This difference in sensitivity of the two types of devices may be understood with the help of the potential profiles obtained from the selfconsistent simulation, shown in Figs. 5.35 and 5.36, for the HEMT and HIGFET device, respectively. The potential profiles are shown for two different acceptor concentrations in each case and both the profiles are for the same value of the gate voltage near the threshold voltage. The zero of the vertical scale coincides with the energy of the Fermi level. In the case of single interface HEMT, it is seen in Fig. 5.35 that the shape 151 1250 \ 1000 > 1xi013/cm3 HEMTI lxi 016/cm3 AIGaAs ci E -;ç750 00) 0c a) 500 GaAs 0 0 0 0 / 0 Si-delta layer I I 400 600 I 0 200 I I 800 1000 1200 I 1400 1600 1800 2000 Layer depth (Angstrom) FIGURE 5.35: Potential profiles of HEMT structure from selfconsistent numerical solution for different acceptor doping concentration. 800 - lxi 700 HIGFET ________________ 0'31cm3 600 E 0) 0) 500 400 C 0 300 g200 0 100 0 Si-delta layer lnGaAs nfl I 0 250 500 I I 750 1000 1250 I I 1500 1750 2000 Layer depth (Angstrom) FIGURE 5.36: Potential profiles of HIGFET structure from selfconsistent numerical solution for different acceptor doping concentration. 152 of the triangular quantum well and the energy position of the bottom of the well are both affected by the background acceptor concentration in the GaAs region. The increase in the acceptor concentration increases the electric field at the interface (as evident from Eq. 4.41) that sharpens the well. This in turn increases the values of E0 and relative to the bottom of the well as seen from E1 Eqs. 4.11 and 4.12. Secondly, the bottom of the well is also pushed up relative to the Fermi level with the increase in acceptor concentration. This arises due to the negative acceptor charges in the GaAs depletion region (qNAW term in Eq. 4.44). As a result of these two combined effects, as the acceptor concentration increase both (E0 EF) and (E1 EF) increase and the value of n decreases (as evident from Eq. 4.30) at a given value of V. Equivalently, the threshold voltage corresponding to n8 = s,th increases with acceptor concentration. On the other hand, in the case of HIGFET device, it is clearly seen from Fig. 5.36 that shape of the well is hardly affected by the acceptor incorporation in the GaAs region. This is not surprising since the well shape is primarily determined by the heterojunction band discontinuity on both sides of the well unlike by the electric field at the interface in HEMT devices. Hence, the quantized energy levels in the well are also not affected by the increase in the acceptor concentration. Secondly, this simulation shows that the bottom of the well relative to the Fermi level position is not significantly affected by the increase in acceptor concentration. As a result, the net change in the threshold voltage with increase in acceptor concentration is much smaller in the case of HIGFET devices. The analysis of the 'D VDS characteristics and the transconductance data consistently shows the degradation of channel mobility with neutron dose. The mobility degradation is due to the additional scattering by the neutroninduced 153 defects in the channel/buffer GaAs region. This may be related to smaller displacement damage in InGaAs as compared to GaAs since the displacement kerma in InGaAs is slightly larger than in GaAs [53]. Neutron radiation induced defects in GaAs have been studied extensively in the literature by a number of experimental techniques. The carrier removal rates reported for ntype GaAs in the literature vary over a wide range from 3 cm1 to 30 cm1 [36], [73] [75]. The measured carrier removal rates in our control GaAs samples lie within this range. Radiation in general introduces both donor type and acceptor type deep level defects. The deep acceptors are responsible for compensation in ntype material and deep donors in ptype. Thus, the material always becomes more resistive after radiation (except at very low doses), irrespective of whether it is ntype or ptype to start with. The deep donors in ntype and deep acceptors in ptype do not play a significant role since they contribute very little charge due to their large ionization energies. The MBEgrown GaAs buffer layer is normally ptype and hence it would appear that only deep donors are of importance in this material. While this is correct in the bulk ptype material, the situation is very different near the modulationdoped heterojunction region. In ntype modulationdoped structures, the Fermi level is close to the conduction band in GaAs near the heterojunction. In other words, the GaAs surface is inverted and hence only the acceptors play a role in the removal of carriers. This is the reason why we used ntype control GaAs samples to determine the acceptor introduction rate responsible for the threshold voltage shift of HEMT structures. Experimental results of carrier removal rates determined by Hall effect measurements and a quantitative analysis of carrier removal rate in our HEMT structures are presented in Sec. 5.1.2. Similarly we predict that only deep donors will be involved 154 in carrier removal in ptype modulation doped structures. Finally the physical nature of the acceptorlike traps introduced by neutron radiation is commented. One of the most plausible candidates is Ga vacancy (VGa) which is known to be an acceptor with an energy level at E + 0.04 eV as revealed by photoluminescence experiments on neutron irradiated GaAs [8]. DLTS experiments on neutron irradiated GaAs reveal three deep levels, EL6, Uband and EL2 with energies at 0.35eV, 0.5eV and 0.8eV below the conduction band [74]. Of these EL2 is known to be deep donor but the chargestate of the other two levels has not been clearly identified. Williams et al. have proposed EL6 and some other shallower traps to be responsible for the carrier removal rate in neutron irradiated GaAs [75]. Even though the exact physical nature of these defects is not presently known, they are also clearly plausible candidates for the acceptorlike traps responsible for the carrier removal in GaAs and for other observed degradation effects in our devices. As a conclusion of the investigation of neutron irradiation effects in delta doped A1GaAs/GaAs HEMTs and A1GaAs/InGaAs HIGFETs devices, three main degradation effects observed are discussed, that is, increase in threshold voltage, decrease in drain current and transconductance, and increase in parasitic source and drain resistances. It is conclusively shown that the acceptorlike traps introduced in the channel GaAs region are responsible for the increase in threshold voltage. The HIGFETs show less threshold voltage shift than conventional HEMTs for the same dose and this difference is shown to be due to the carrier mobility degradation in the channel. Quantitative models were developed for the degradation of all the device parameters and compared with the experimental results. 155 Electron Irradiation Effects in HIGFETs 5.5 In this section, electron irradiation effects on A1GaAs/InGaAs/GaAs HIGFET devices are investigated. Four devices are incorporated with different lightly doped drain (LDD) regions as 1.5, 3.5, 7.5, and 12.5 pm between gate and drain. All of these devices have the same gate length of 0.6 pm and the width of 10 pm. Schematic crosssection diagrams of these devices were already introduced in Figs. 3.2 and 3.3. Electron irradiation was performed in the manner that was described in Sec. 3.5. Due to the limited crosssection area of electron flux from source element, only four devices, which are located close, have brought major attention to degradation study. We studied currentvoltage as well as transconductance characteristics as a function of cummulative electron dose. 5.5.1 'D VGS Characteristics Drain current was measured by VDS in the range of 0 to 1.0 V. All four devices show threshold voltages in the range of 0.35-0.45 V when the threshold voltage was determined by extrapolating linear region of 'D Fig. 5.37 and 5.38 show 'D VGS VDS curve to 'D = 0. characteristics with LDD region of 3.5 pm before and after four different electron fluences while sourcedrain voltage was kept to be 0.5 V and 1.0 V, respectively. VGS characteristics of the device with larger LDD region, 12.5 pm, for same electron flunces are shown in Figs. 5.39 and 5.40 for VD = 0.5 V and VD = 1.0 V, respectively. Threshold voltages shift to the positive direction with increasing electron fluence. Threshold voltage shifts of four devices with different LDD regions as a function of electron fluence are plotted in Figs. 5.41 and 5.42 for linear and , 0.12 U Unirradiated 0.10 0.08 156 o D1:8.24x1014e/cm2 A D2:3.97x1015e/cm2 v D3:7.07x1015e/cm2 0 0 A A A ° A 0 A v V 0AV lAy iV. v 2 D4:1.11xl&6e/cm V. V. 0.06 A A V=O.5Vl 0.04 LDD=35l.ml 0.02 A AA . 0.2 0.3 I I 0.4 I 0.7 0.6 0.5 0.9 0.8 1.0 VGS (V) FIGURE 5.37: 'D VGS characteristics of a HIGFET (LDD = 3.5 pm) for increasing electron doses at VDS = 0.5 V. 0.225 U 0.200 Unirradiated 0.175 0.150 A o D1:8.24x1014e/cm2 A D2:3.97x1 015 e/cm2 v D3:7.07x1 015 elcm2 0.125 OAA £v V V o Ay A y .,. D4:1.11x1016e/cm2 E . "V. 0.100 0.075 VDS10V 0.050 LDD=35rnl 0.025 A AAA 0.2 I 0.3 0.4 0.5 0.6 0.7 i.:. 0.8 0.9 1.0 1.1 VGS (V) FIGURE 5.38: 'D VGS characteristics of a HIGFET (LDD increasing electron doses at VDS = 1.0 V. 3.5 pm) for 157 0.06 0.05 . L- Unirradiated Io_Dl:8.24xlO14 D2:3.97x1 015 0.04 v-D3:7.07x1015 D4:1 .11 xl 016 E 0.03 0 V0 = 0.5 V 0.02 AAaA1.AAM AVV .4. LDD=l2.5.LmI 0.01 O 00 0.3 0.2 I I I 0.4 0.5 0.6 0.7 0.8 VGS (V) FIGURE 5.39: 'D VGS characteristics of a HIGFET (LDD = 12.5 tim) for increasing electron doses at VDS = 0.5 V. 0.12 0.10 0.08 Before irradiation o 8.24x1 014 e/cm2 A 4: 3.97x1 015 e/cm2 V : 7.O7xl 015 elcm2 A t1A Av ,AV $4:1.11x1016e/cm2 0.06 0 WV A AAA VV V VDSr1l.0V 0.04 LDD=12.51.irn 0.02 n fin 0.2 0.3 0.4 I I I I 0.5 0.6 0.7 0.8 0.9 1.0 V0 (V) FIGURE 5.40: 'D VS characteristics of a HIGFET (LDD = 12.5 tim) for increasing electron doses at VDS = 1.0 V. 158 saturation region of drain currents, respectively. 10.0 LDD:1.5i.un 7.5 TV0s05V o LDD:3.5jim A LDD:7.5tm LDD:12.5tm v A U 5.0 E A >'_ 0 2.5 A V A 0 0 V 0.0 -2.5 U, 0 I I 2 4 Electron Fluence 10 8 6 4) (10 15 e/cm 12 2 FIGURE 5.41: Threshold voltage shift as a function of electron doses for four different devices with VDS of 0.5 V. Although a strong linearity between threshold voltage shift ( IVT ) and the electron fluence for both cases is not showing on plots, considering experimental errors such as probing effect and the subjective method of extracting threshold voltages, the general trend of these curves show linearity of LVT with electron fluence. The magnitude of the threshold voltage shift is in general quite small (< 10 mV) as also obtained in the case of neutron irradiation. The LDD region dependence of IVT is investigated in Fig. 5.43 for both linear and saturation values of VDS for two electron fluences. With a larger sourcedrain voltage, threshold voltages tend to be shifted more than those with a smaller VDS voltage. Also, IVT shows less change as LDD region increases. 159 15 o 12 A v LDD:1.5ini LDD:3.5jm Vos=1.OVI LDD:7.5jim LDD:12.5jim . 9 A > 0 6 V 3 6 V V 0 0 I 2 0 I I 4 6 E'ectron Fluence 8 10 12 (1015 e/cm2) FIGURE 5.42: Threshold voltage shift as a function of electron doses for four different devices with VDS of 1.0 V. The dependence of LWT on LDD spacing is not presently understood. 5.5.2 'D VDS characteristics The drain current characteristics as a function of drainsource voltage (VDS) for the device with LDD region of 12.5 im before and after the total electron dose of 1.11 >< 1O' e/cm2 are shown in Figs. 5.44 and 5.45, respectively. As found from the neutron irradiation effects on HFET devices, the threshold voltage shift, decrease in the channel carrier mobility, and source resistance increase are believed to be responsible for the electron irradiationinduced degradation. For the device with LDD region of 3.5 jtm, 'D VDS characteristics are shown in Fig. 5.46 before and after a cumulative electron fluence of 1.11x 1016 e/cin2. Saturation drain current decrease and resistance increase in the small VDS region are clearly seen in this figure as well. We plotted 'D VDS characteristics of 160 14 A filled: VDS = 0.5 V 12 open: V05 = 1.0 V 10 0 A E 6 A A A A 0 0 : 7.07x1 015 0 : 7.07x1 A 4,4:1.11x1016 A 4,: 1.11x1016 4 . 4 2 A . 0 4 2 0 LDD 14 12 10 8 6 (l.Lm) FIGURE 5.43: Threshold voltage shift as a function of LDD region in linear and saturation region of VDS. 0.09 Before Irradiation 0.08 V,35 = 0.8 V - LDD=12.5pm 0.07 0.7V 0.06 < 0.05 E 0.6V 0.04 0.03 0.5 V 0.02 0.4V 0.01 0.00 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 VDS (V) FIGURE 5.44: 'D VDS characteristics of a HIGFET with LDD of 12.5 /m before electron irradiation. 161 0.09 0.08 0.07 0.06 0.05 E 0 0.04 0.03 0.02 0.01 0.00 0.00 0.25 0.50 0.75 1.00 1.25 1.75 1.50 2.00 2.25 2.50 VDS (V) FIGURE 5.45: 'D VDS characteristics of a HIGFET with LDD of 12.5 tm after the accumulated electron radiation with a dose ofl.11 x 1016e/cm2. the same device with the gate voltage of fluence in Fig. 5.47. VGS = 0.8 V with different electron Gradual degradation of drain saturation current is clearly seen as the electron fluence increases. Normalized drain saturation current for four devices as a function of electron fluence in subthreshold (V < VT) and linear (V > VT) regime of gate voltages are shown in Figs. 5.48 and 5.49. All devices clearly show a linearly decreasing trend as a function of electron fluence and there is no direct relationship between LDD region and saturation current degradation appearing on the plots. Again, drain resistance and mobility values are separated by using Eq. for a small drain voltage region (VDS=0.5 V), that is, =Rs+RD+ ,3(V9VT) VDS 5.8 1 (5.12) 162 0.13 before inadiation 0.12 0.11 44= 1.11xlOt6e/cm2 LDD = 3.5 V0 =0.8 V 0.10 0.09 / 0.08 ...... 0.07 .T.. .. .. .. .... o 0.06 0.05 0.04 .. 0.03 0.02 0.01 n fin 0.00 0.25 0.75 0.50 1.25 1.00 V 1.50 1.75 2.50 2.25 2.00 (V) FIGURE 5.46: 'D VDS characteristics of a HIGFET with LDD of 3.5 1um before and after the accumulated electron dose of 1.11xlO'6e/cm2. U. I 0.12 o 0.11 0.10 U ..000000000 000000 a £ a a a a a a a a a a a a a . v . U . . . V V V v V V V V V V V V V V 0.09 OV. a 0.08 . 0 0.07 a. V 0.06 before irradiation 0.05 0.04 0.03 . a V 0 = 8.24x1 0 e/cm a = 3.97x1 015 e/cm2 v 0.02 LDD=3.5tm 0.01 14 2 4, -707x10 e/cm 2 4, 1111x1016e/cm2 15 fl On 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 V0 (V) FIGURE 5.47: 'D VDS characteristics of a HIGFET with LDD of 3.5 pm for increasing electron irradiation dose. 163 1.02 1.00 VG=O.4V V0=1.5V 0.98 0 0.96 * 0.94 0.92 A, 0 I 0.88 LDD:1.5)m 0.86 0.84 0.82 0.80 0 LDD:3.5j.tm A LDD:7.51m V v LDD:12.5i.tm 0 . - A 7 I I I I I 0 Electron Fluence (1015 elcm2) FIGURE 5.48: Normalized drain saturation current as a function of accumulated electron dose at the drain voltage of 2.5 V. 1.05 1.00 V 0.95 A VG=0.7V VDs=l.5V 0 0.90 A -. 0.85 V 0 A 0 U)0 (no . V A 0 0.80 LDD:1.51im 0.75 0 0.70 A v A LDD:3.5im I LDD: 7.5 irn LDD:12.5tm I I I 0 Electron Fluence 4 eIcm) FIGURE 5.49: Normalized drain saturation current as a function of accumulated electron dose at the drain voltage of 2.5 V. 164 where /3 is as W E2 (5.13) as shown in Eq. 5.4. From y-intercept at x = 0, source and drain resistance can be evaluated and the slope of the linear part of the curve extracts channel mobility. Fig. 5.50 shows five such curves for a device for LDD region of 3.5 for different electron fluences. From both the y-intercept and slope increases with increasing electron fluences, the source resistance increase as well as channel mobility decrease can be evaluated as a function of electron fluence. 30 dose 0: Unirradiated linear fit for dose 0 25 o 0.824x10'5e/cm2 A 3.97 x 1015 e/cm2 v 7.07x1015e/cm2 v V 11.1 x1015e/cm2 . A VA .g... U V .. VDS/ID=l.92x[l/(VG-VT)] + 0.3481 ... 10 L0D3.5m1 c 3 4 5 6 7 l/(VG-VT) 8 (V) 9 10 11 12 FIGURE 5.50: The method for extracting resistance and mobility from current voltage characteristics. Channel mobility and source resistances for all four devices are extracted and plotted data of two devices with LDD 3.5 and 7.5 pm are shown in Fig. 5.51. Mobility degradation parameters for three devices are shown in Fig. 5.52 and 165 1.00 1.9 LDD 3.5 m 0.98 1.8 LDD 7.5 im 0 0.96 1.7 0.94 1.6 0.92 1.5 0.90 1.4 L/IAo Fl 0.88 1.3 .0 0.86 1.2 0.84 1.1 / 0.82 R0 1.0 no fl Rn 0 4 2 6 Electron Fluence 4) (10 8 10 12 e/cm2) FIGURE 5.51: Mobility and resistance degradation showing on device with LDD 3.5 and 7.5 pm as a function of electron doses. these parameter values are in the range of 1.82 x 1017 cm2/e. Although it is clearly shown from Figs. 5.51 and 5.52 that degradation effects appearing on HIGFET test device characteristics are mainly due to channel mobility decrease and source resistance increase as a function of electron doses, there is no strong dependence of LDD region on the degree of degradation effects. 5.5.3 Transconductance Normalized transconductances radiated values (gmo) (gm) of the LDD region of 12.5 pm to the unir- for different electron doses for VDS of 0.5 and 1.0 V are shown in Fig. 5.53 and Fig. 5.54, respectively. Threshold shift to the positive direction and decreasing the maximum g values are clearly seen as the electron fluence increases. Regardless of LDD regions, same effects are appearing on all four devices. 166 1.25 - 1 + 1.82x1O7--.- .: 1.20 1.15 / /i=1+1.84x1O174) 1.10 .0 S 1.05 - '-/I.=1+O85x1O174) U 1.00 0 4 2 6 10 8 Electron Fluence 4) (10 15 e/cm 12 2 FIGURE 5.52: Mobility degradation parameter extraction for devices with different LDD regions. 1.1 1.0 Before irradiation 0.9 o 0.8 A 0.7 V 0 4 !12Vvv : 8.24x1014e/cm2 3.97x1015 e/cm2 4): 7.07x1015 e/cm2 4). 1.11x1Oe/cm E C) 0.6 E C) 0.5 0.4 LDD=12.5m 0.3 0.2 V=O.5V 0.1 0.0 I 0.1 0.2 0.3 0.4 0.5 0.6 VGS (V) FIGURE 5.53: Normalized transconductance as a function of gate voltage of a HIGFET device (LDD = 12.5 m) before and after electron irradiation at VDS =0.5 V. 167 1.1 1.0 LDD=12.5im 0.9 0.8 vos=1.0v I 0.7 0 E 0.6 C) E 0.5 C) Before irradiation 0.4 o 0.3 : 8.24x1 014 e/cm2 : 3.97x10'5e/cm2 V 0.2 : 7.07x1015 elcm2 44:1.11x1016e/cm2 0.1 fin 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 VGS (V) FIGURE 5.54: Normalized transconductance as a function of gate voltage of a HIGFET (LDD=12.5 tim) before and after electron irradiation at VDS = 1.0 V. CHAPTER 6 IRRADIATION EFFECTS IN SHBTs The underlying concepts of heterostructure bipolar transistor(HBT) were first introduced by Shockley and later developed by Kroemer in 1950s, almost soon after homojunction bipolar transistors were invented. However, only after the invention of modern epitaxial growth techniques, such as molecular beam epitaxy (MBE) and metal organic chemical vapor phase deposition (MOCVD), the possibility for integrated circuit (IC) manufacture with HBTs became realistic. Due to the lattice constant match to be developed for A1GaAs/GaAs, HBTs were the first highspeed and high current gain applications. In this chapter single heterojunction bipolar transistors (SHBT) made with IriP/InGaAs material system are studied for electron and neutron irradiation effects. Devices with InP/InGaAs materials exhibits higher performance than AIGaAs/GaAs HBTs due to smaller electron effective mass, higher mobility 1a0 at low field, and higher electron saturation velocity. HBT fundamentals are briefly discussed by introducing their energy band diagrams. Device structure of the InP/InGaAs SHBTs fabricated at Crawford Hill Laboratory which are used in this study is described. Finally, we investigate electron and neutron irrdiation effects followed by annealing effects on the radiationdamaged devices. 169 6.1 HBT Fundamentals Single heteroj unction bipolar transistor consists of an emitter whose energy bandgap is larger than that of the base while both base and collector are made of the same material (homojunction). Energy band diagrams for a typical npn Si BJT and npn InP/InGaAs/InGaAs SHBT are shown in Fig. 6.1. In SHBTs hole injecting from base to emitter is surpressed by the larger potential barrier in valence band leading to higher emitter injection efficiency. Owing to the additional barrier, base can have a higher doping concentration and emitter can have a lower doping concentration resulting in base resistance, lower emitterbase junction capacitance, shorter emitter charge storage time, and higher Early voltages. Additionally, base thickness can be reduced to decrease base transit time. emitter - nSi base collector emitter ___ pSi (a SiBJT n-Si -S.- S:. ..___ nInP collector base InGaAs n InGaAs (b' SHBT FIGURE 6.1: Energy band diagrams of (a) bipolar junction transistor and (b) abrubt junction single heteroj unction bipolar transistor. I] The conduction band discontinuity results in an abrupt spike at the heterointerface. Properly selected grading layer deposited between emitterbase can smooth out the spike to improve electron injection. A typical characteristic of SHBT is a nonzero turn on voltage due to the asymmetry between emitter base and basecollector junctions. Double heterojunction bipolar transistors (DHBTs) with heterojunction for both baseemitter and basecollector could significantly reduce this effect. Fig. 6.2 shows SHBT and DHBT with abrupt and grading junction features. Among ITTV compound semiconductors, A1GaAs/GaAs system was mostly used for SHBT and DHBT structures in 1980s. The stablized high performace A1GaAs/GaAs HBT devices showed cutoff oscillation (frna) (fT), and maximum frequency of in the range of 100-200 GHz [82]. InP/InGaAs became next promising material system to achieve even higher performance with GHz and cutoff fT fmax = 236 = 200 GHz [83]. The high speed application of Si/SiGe HBTs have recently received enormous attention due to their compatibility with Si processing. So far, IBM has standardized SiGe BiCMOS technology with a fairly high 6.2 fT (up to 120 GHz). SHBT Device Structure The InP/InGaAs SHBTs used in this study were grown by MOCVD technique at Crawford Hill Laboratory. A schematic cross section and layer of this device structure parameters of a SHBT are shown in Fig. 6.3 and Table 6.1, respectively. The large area devices (emitter size 50x50 pm2) used for this study were also fabricated at Crawford Hill Laboratory using conventional photolithography 171 emitter base collector Ec _In:_ Ev InP InGaAs InP (c) Abrubt DHBT Ec E (b) Graded SHBT FIGURE 6.2: Energy band diagrams of (a) abrupt SHBT, (b) graded SHBT, (c) abrupt DHBT, and (d) graded DHBT. process. Schematic diagram of the emitter, base, and collector layout is shown in Fig. 6.4. 172 InGaAs Contact Setback Layer InGaAs Grading I I InP Emitter InGaAs fnGaAs Base InGaAs Collector InGaAs Subcollector semiinsulating InP substrate FIGURE 6.3: Single heterostructure bipolar transistor structure. Material Thickness Dopant Concentration InGaAs(EmitterContact) 500 A 2x10'9/cm3 InP(EmitterGrade) 750 A 5x10'8/cm3 InP(Emitter) 750 A InGaAs(BaseSet back) 50 A n 5x1017/cm3 n intrinsic InGaAs(Base) 10000 A p 2x10'9/cm3 InGaAs(collector) 5000 A n intrinsic InP(Subcollector) 100 A 5 x 10'8/cm3 TABLE 6.1: Schematic cross section with layer properties of a SHBT. 173 micron 40 micron 50 micron 10 micron I FIGURE 6.4: Schematic diagram of emitter, base, and collector contacts. 6.3 SHBT Electrical Characterization Setup Configuration To obtain the common emitter I V characteristics of single heterojunction bipolar transistors, first, collector current, I is measured at different base currents, for instance, from 0 to 60 pA with an increment of 10 pA while collectoremitter voltage, is swept from 0 to 2 V. Basecollector (B C) and baseemitter (BE) diodes were also characterized in the forward bias VCE, mode to measure the resistances of collector and emitter, respectively. In the reverse bias mode, the breakdown voltage of BC junction is also obtained. For extractions of modeling parameters as well as for a better understanding of device operation, Gummel and inverse Gummel plots of each terminal current are measured. The baseemitter junction voltage is swept in the range of 0 1.0-1.3 V while basecollector terminals are shorted for Gummel plot. On the other hand, for inverse Gummel plots, basecollector bias is swept in the same range while baseemitter terminals are shorted. Circuit configurations for I V 174 characteristics, Gummel and inverse Gummel are shown in Fig. 6.5. (a) (b) 'C 'c (c) ,J + 'B 'B 7 -±--+ EE VBC CE VBE-- IFIGURE 6.5: Circuit configurations for (a) common emitter I tics, (b) Gummel plots, and (c) inverse Gummel plots. 6.4 V characteris- Electron Irradiation Effects in SHBT Devices Electron irradiation experiment was performed by using source. Sr90/Y9° 100 mCi All device terminals were unbiased and left floating during irradiation and were at the ambient temperature. Electron irradiated samples were characterized typically after 1 to 2 hours after irradiation. Although some self annealing is possible during this period, we believe that this delay brings stable condition of the devices for reproducible characterization. The results of electron irradiation effects in the InP/InGaAs/InGaAs unpassivated SHBT devices are presented in this section. BC and BE junction diodes characteristics are briefly discussed followed by annealing effects on the irradiated devices. 175 6.4.1 Current Voltage Characteristics Figs. 6.6, 6.7, and 6.8 show the DC commonemitter currentvoltage (IcVcE) characteristics of unexposed, after cumulative electron dose of 11.6x 1015 cm2, and 26.Ox i0' cm2 respectively. 9 Unexposed 8 7 4OjA 6 3OA 5 <4 201.iA 2 lOpA 1 0 0.00 I I I 0.25 0.50 0.75 I I 1.00 1.25 1.50 1.75 2.00 VCE (V) FIGURE 6.6: Collector currentvoltage characteristics of the unexposed devices. The prominent degradation effects of the devices shown in Figs. 6.6, 6.7, and 6.8 are: (1) decrease of the collector current (Ic) for a constant base current (IB); (2) increase of the collector output conductance (/.Ic/IVcE) in the active region of 'c VCE curves; (3) increase of the collectoremitter voltage at which the emitter current begins to saturate (Vsat); (4) increase of the offset voltage. Finally, the typical transistor characteristics of the devices disappeared after a 176 6 dose: 11.6x1015e/cm2I 5 4 40 pA 30 -o 2 20 1 10 0 0.0 0.2 0.4 0.6 1.0 0.8 1.2 1.4 1.6 1.8 2.0 VCE (V) FIGURE 6.7: Collector current-voltage characteristics after electron accumulated dose of 11.6x lOis cm2. 0.35 0.30 dose: 26.0 x 1 015e/cm2 I 0.25 'B = lOj.tA 0.20 'B = 20j.tA 'B = 30jtA 0.15 1B40 0.10 'B = 50p.A 0.05 0.00 VcE -0.05 offset _n in 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 VCE (V) FIGURE 6.8: Collector current-voltage characteristics after electron accumulated dose of 26x lOis cm2. 177 final dose of 26.Ox i0' cm2 (Fig. 6.8). Collect current decrease, saturation voltage increase, and offset voltage shift are also shown in electron irradiated passivated InP/InGaAs SHBT devices while output conductance of passivated devices are unaffected through the entire course of irradiation [28]. All of the above effects are also seen from the I VCE characteristics for different cumulative electron doses shown in Fig. 6.9. Especially, the increase of the offset voltage with respect to the cumulative dose is shown in the blown up part of the figure. Absolute value of offset voltage shift shows a linear dependence on cumulative electron doses with a degradation coefficient of 8.61 x 10-18 V.cm2/e and offset voltage before irradiation is given from the y-intercept value as 0.232 V (actual experimental value of 8 V0118et is 0.208 V). As shown in -Unexposect 1.20x10'5 ._-;- -------6.80x10'5 1" 11.6x 1015 /1' ,/' 6 16.4 x 1015/f / C-) 2 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VCE (V) FIGURE 6.9: Collector currentvoltage characteristics for a base current of 50 j.tA before and after different doses of electron irradiation. 178 0.40 Voffset Linear fit 0.38 0.36 U U 0.34 > 0.32 . 0.30 0.28 0.26 . = 861X1018fe + 0.232 0.24 0.22 I 0 2 I I I I 4 6 10 8 Electron Fluence I I 14 16 18 (1015 e/cm2) (V01 fset) of device as a function of cumulative VCE,(Sat) increases as the cumulative dose of the FIGURE 6.10: Offset voltage electron doses. Fig. 6.9, the saturation voltage 4e 12 irradiation increases. Also, the output conductance increases significantly after a dose of 11.6x10'5 cm2 whereas the degradation in 1c becomes noticeable after a dose of 6.80x 1015 cm2 The degradation of Ic as presented in Figs. 6.6, 6.7, and 6.8 is considered in the variation of commonemitter current gain (hfe = Ic/IB); currents are held constant in our measurement of Iy collector shows change with dose, h1e VCE since the base curves while the has been plotted as a function of 'B with respect to the cumulative dose levels of electron irradiation with VCE = 1.5 V in Fig. 6.11. The gain degradation (h1e) is maximum (> 50 %) at low base currents and decreases gradually to around 30 % at the highest collector currents. This DC current gain (3 = Ic/IB) of the transistor after electron irradiation 179 200 160 120 / / / 0 80 unexposed .. . 1.20x1015e/cm2 -----6.80x1015e/cm2 40 11.6x1015e/cm2 --------16.4x1015e/cm2 VCE = 1.5 V n I 100 80 60 40 20 0 FIGURE 6.11: Commonemitter current gain as a function of base current after different levels of electron irradiation at VCE=l.5 V. 170 8.5 160 8.0 150 0 ? lAr x 7.0 130 6.5 120 6.0 110 cc 0 2 4 6 8 10 12 14 16 18 Electron Fluence 4e (1015 e/ cm2) FIGURE 6.12: DC current gain (3) and reciprocal gain (1/13) with electron dose. is found to obey the following equation (6.1) which is known as MessengerSpratt relationship [70]. The gain degradation coefficient, K is given from the slope of the best fit line and found to be 1.7x 109cm2/e. Compared to the value from passivated devices fabricated with same material, the degradation coefficient of the unpassivated devices is found is larger than that of passivated one, which is found to be 3.7x Figs. 6.13 and 6.14 show 1c and 'B 1020cm2/e [28]. plots as function of VBE measured with the basecollector junction shorted (Gummel plots) for devices before irradiation and after a dose of 1.20x10'5 cm2, 11.6x10'5 cm2, and 16.4x10'5 cm2 as shown in the figures. It is evident from Fig. 6.13 that collector current is not affected by electron irradiation. From 1c VBE plot, it is seen that the collect current shows a constant leakage current for VBE < 0.4 V, a linear dependence for intermediate range of VBE (0.4 < VBE <0.8 V), where the current is increaing exponentially with ideality factor of almost 1, and finally saturates at the higher baseemitter voltage than 0.8 V. In this high current region nonideal behavior appears due to highlevel injection (nonnegligible minority carrier concentration) and possibly due to emitter and base resistances. However, through the entire VBE region, collector current does not seem to be affected up to the higher electron fluence. 'B VBE curve, on the other hand, shows a different behavior as shown in Fig. 6.14: At the low VBE regime (< 0.85 V), a small electron dose of 1.20 x 1015 e/cm2 'B shows a slight decrease with and an increase for higher electron doses. The decrease of base current for small dose (=1.20 x 1015 e/crn2) also correlates with the current gain increase seen in Fig. 6.11 for the same dose and 181 for 'B < 2OjiA. For VBE > 0.85 V, the base current progressively decrease with increasing electron doses. We believe that the increased recombination in the emitterbase junction space charge region with increase of electron cumulative fluence is responsible for this degradation. 0.1 unexposed 0.01 1 .20x1 015e/cm2 11.6x1015e/cm2 1 E-3 15 16.4x10 e/cm 2 1E-4 1 E-5 1 E-6 1E-7 / 0.0 0.2 0.4 0.8 0.6 1.0 VBE (V) FIGURE 6.13: Gummel plot for the collector current as a function of base emitter voltage before and after electron irradiation. Inverse Gummel plots are shown in Figs. 6.15 and two figures, while 'E gradually decreases with dose, 'B 6.16. As shown in these significantly increases (at the low voltage level) for smaller doses and reaches a constant value. The inverse mode gain 'B/lB with respect to the cumulative dose is plotted in Fig. 6.17 in which the gain degradation is explicitly shown with increasing dose. Diode characteristics of basecollector and baseemitter as a function of electron fluence are studied to check series resistance change after the irradiation. 182 0.1 unexposed 0.01 1.20x10'5e/cm2 1 E-3 11 .6x1 015e1cm2 16.4x1015e/cm2 .-. 1E-4 1E-5 1 E-6 1E-7 1P-R 0.4 0.9 0.8 0.7 0.6 0.5 1.0 VBE (V) FIGURE 6.14: Gummel plot for the base current as a function of baseemitter voltage before and after electron irradiation. 0.1 0.01 IE-3 1E-4 unexposed I E-5 1 .20x1 015e/cm2 I E-6 11.6x1015e/cm2 16.4x1015e/cm2 1 E-7 I P.R 0.0 0.2 0.4 0.6 0.8 1.0 VBC (V) FIGURE 6.15: Inverse gummel for the base current as a function of base collector voltage before and after electron irradiation. 183 1 E-4 1 E-5 1E-6 w unexposed ---- 1.20x1015e/cm2 /,;..;.' 1 E-7 ...... 11 .6x1 015e1cm2 ----- 16.4x1Oe/cm -I I .R I 0.2 0.4 I I 0.6 0.8 1.0 V8 (V) FIGURE 6.16: Inverse gummel for the emitter current as a function of base collector voltage before and after electron irradiation. 2.5 -1/IE 2.0 0 x 2.5 2.0 1.5 1.5 m 1.0 1.0 0 (.3 0.5 0.5 =1.0 v 0 2 4 6 8 DOSE 10 12 14 16 18 0.0 (x1015e/cm2) FIGURE 6.17: Gain (IE/IB) and reciprocal gain plots as a function of cumulative doses. (IB/IE) from inverse Gummel 184 By sweeping each junction with the terminal bias in the range of 0 2.0 V, forward biased currentvoltage of BC and BE diode characteristics are investigated as electron fluence is accumulated up to 16x 1015 e/cm2. Figs. 6.18 and 6.19 show the forward collector and emitter current, respectively. As shown in Fig. 6.18, collector resistance determined from the high voltage regime (see insert) seems to rather decrease as the electron fluence increases. On the other hand, Fig. 6.19 shows that the emitter resistance tends to increase gradually with increasing electron fluence. The knee voltage of BC diode in forward bias decreases as the electron dose increases whereas that of BE diode does not show any electron fluence dependence except the abnormal behavior of current at low voltage regime after the electron accumulate dose of 4.40 x 1015 e/cm2. We believe that the larger BC junction damage compared to BE junction damage is responsible for the offset voltage shift as electron fluence increases. 0.01 -Unexposed 4 4x1 015 elcm2 11.6x10'5e/cm2 16.4x1 015 e/cm2 1E-3 Lj 1E-4 1E-5 a I 0.0 0.2 0.4 0.6 0.8 1.0 VBC (V) FIGURE 6.18: Semilog plot for a basecollector diode I V characteristics for different cumulative electron fluences. The insert shows the linear I V plots. 185 0.01 unexposed l.20x1015e/cm2 1 E-3 '.--440x10'5e/cm2 6.80x10'5e/cm2 1 E-4 U- 1E-5 I E-6 1 E-7 r 7 V I Q 0.2 00 03 07 03 01 0 10 11 12 3 U / I 0.3 0.4 0.5 I I I 0.6 0.7 0.8 0.9 1.0 VBE (V) FIGURE 6.19: Semilog plot for baseemitter diode I V characteristics for different cumulative electron fluences. The insert shows the linear I V plots. To measure the breakdown voltage of the devices, a reverse bias from 5.0 V to 0 V are applied to basecollector diodes that are located at the most nearby transistors from ones that have been used for other previous characteristics. The results of the performances is plotted with respect to the cumulative dose of electron in Fig. 6.20: the breakdown voltage tends to decrease and correspondingly the leakage current increases with increasing dosage. The gradual increase of output conductance with increase in dose (seen in Fig. 6.9) is believed to be caused by the increased avalanche multiplication in the BC junction depletion region due to the decrease in the breakdown voltage. 6.4.2 Annealing Effect after Electron Irradiation Isochronal annealing on this electron irradiated device is performed. Starting with temperature of 200°C for 10 minutes of time duration in a forming gas 186 -2 -3 -4 0 -1 0.2 0.0 / -0.2 / I -0.4 I . / 'I 0 0 II .. I >< 1 .20x10'5e/cm2 . I .- -0.8 I j 4.40x1015e/cm2 6.80x1015e/cm2 -1.0 I . . -1.2 16.4x10'5e/cm2 / I 26.Oxl 015e/cm2 ! I -1.4 -4 -2 -3 0 -1 VBC (V) FIGURE 6.20: Diode characteristics of basecollector diodes for applied reverse bias after different levels of electron exposure. ambient, it was continued with a temperature step of 50°C up to 450°C. Time duration for each step of annealing was kept to be 10 minutes. Ic acteristics of the transistors and the I VCE char- V characteristics of the BC and BE diodes are mainly studied at different stages of annealing. As shown in Fig. 6.21, the transistor characteristics of Ic VCE covered immediately after annealing at 200°C. Dotted curves for 1c characteristics after the final cummulative electron dose, 26.Ox are reVCE 1015 e/crn2 , are also plotted to show the transistor action recovery after the annealing. The improvement of collector current of the device after the annealing at 200°C does not exceed the I level after a dose of 16.4x 1015 cm2. The collector current starts degrading when the annealing temperature is increased to 300° or more as shown in Fig. 6.22. The offset voltage did not show an increase after an- 187 2.0 accumulated dose: 26.0 x 1015 e/cm2 1.6 annealed at T = 2_- 1.2 0 301.tA 0.8 0.4 u,uIIuIIuII,IIIIIIlIIuIIIIIII', 0.0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 VCE (V) FIGURE 6.21: Collector currentvoltage characteristics after annealing at temperature TA = 200°C for 10 minutes. nealing at 300°C. After annealing at the highest temperature of 450°C, this device no longer showed transistor characteristics. The loss of transistor action is believed to be the degradation of contact metal at the high annealing temperature of 450°C. BC and BE junction diode characteristics are also studied with increasing annealing temperature and are shown in Figs. 6.23 and 6.24, respectively. Forward junction BE and BC diodes characteristics were not affected by electron irradiation through the entire range of electron fluence as shown in Figs. 6.18 and 6.19. Annealing of the devices up to 300°C did not also bring much change as shown in Figs. 6.23 and 6.24. After annealing at a temperature of 450°C, the knee voltage of BC diode in forward bias decreases whereas that of BE diode increases. By applying a reverse bias to BC and BE junctions, breakdown voltage changes with increase of annealing tempera- 0.07 0.06 IBisfrom5OpAwithastepoflOMA 0.05 0.04 0.03 Tneai=300°C I I 0.02 E 0.01 0.00 00 I 450 C I5 -0.01 -0.02 -0.03 -0.04 -0.05 .n 0.0 V (V) I I I I I I I I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VCE (V) FIGURE 6.22: Collector currentvoltage characteristics after annealing at temperature TA = 300°C (=450°C inserted figure) for 10 minutes ture was investigated. Inserted plots in both Figs. 6.23 and 6.24 show decrease of breakdown voltage as the temperature increases. The decrease of breakdown voltages are responsible for the increase of output conductance showing on the VCE characteristics. 6.5 Neutron Irradiation Effects in SHBT Devices The experimental results and analysis of neutron irradiation on unpassivated InP/InGaAs/InGaAs SHBT devices are presented in this section. Electrical DC characteristics of the devices before and after the irradiation and a qualitative analysis of the radiation induced degradation including comparison with passivated devices are presented in this section. Although four separate sample pieces cut from the same wafer were used for the different neutron fluences, 0.040 U 0.035 hi 0.030 v 0.025 - - U 26,lOok,, T-2 I" I II T=3ccc V T,=450C 0.020 vç u 0.015 Unexposed X 26x1015e/cm2 0 T=200°C 0.010 T,=30O°C 0.005 v T,1=450°C 0.000 _n fl(1 -15 I I -1.0 -0.5 I I I I VBC (V) FIGURE 6.23: Basecollector diode characteristics before and after isochronal annealing process. 0.04 U Unexposed 0.03 Y x 26x1 015 e/cm2 0 T = 200 °C = 300 °C 0.02 vT=45O°C U- Ji 0.01 0.00 _n (11 :20 I I -1.5 -1.0 -0.5 0.0 0.5 I I 1.0 1.5 2.0 2.5 VBE (V) FIGURE 6.24: Baseemitter diode characteristics before and after isochronal annealing process. 190 pre--irradiated characteristics (Ic and Gummel) showed same current VCE level within few percent. Hence, the absolute characteristics of the devices are plotted instead of normalizing with respect to unirradiated values. 6.5.1 Current Voltage Characteristics VCE characteristics of a typical device at different base The common emitter I VCE characteristics currents prior to irradiation are shown in Fig. 6.25. The I 11 Unexposed 10 9 50jA 8 7 4OA 6 <5 30*A Q4 20,A 3 2 10pA OAA 0 I 0.0 0.2 I 0.4 I I 0.6 0.8 1.2 1.0 V, I I 1.4 1.6 1.8 2.0 (V) FIGURE 6.25: Currentvoltage characteristics of a SHBT device before neutron irradiation measured at base current from 60 ,uA to 0 ,tA with a step of -10 A. of the same device after a dose of 6.48 x The Ic VCE 1014 n/cm2 are shown in Fig. 6.26. characteristics at a base current of 60 pA for different neutron doses are shown in Fig. 6.27. The following effects of neutron irradiation on this device characteristics are evident from Figs. 6.25 to 6.27: (1) The gradual reduction of I with neutron dose; (2) Increase in the output conductance; and 191 2.5 er a dose of 6.48x1 014(nlcm2) 2.0 1.5 =60I5O !10 0.0 lB -0.5 0.0 I 0.2 I I I I I 0.4 0.6 0.8 1.0 1.2 1.4 O1iA I I 1.6 1.8 2.0 VCE (V) FIGURE 6.26: Currentvoltage characteristics of a SHBT device after neutron irradiation with dose of 6.48 x 1014 n/cm2 measured at base current from 60 1aA to 0 jiA with a step of-lOjiA. II 10 9 00000 8 7 P 6 <5 E 0 q 00000000000000 o 0000000000 Q4 0 3 00 00 2 0 . - - AAA 0 -1 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 VCE (V) FIGURE 6.27: Currentvoltage characteristics of a SHBT device after neutron irradiation with four different doses at base current 'B = 60 jiA. 192 (3) Increase of Vojset. (the voltage at There is no significant change in V which I begins to saturate) after irradiation. Collectoremitter offset voltage shift to the positive direction after neutron irradiation is shown in the insert of Fig. 6.28. The change in VCE offset, doses in Fig. 6.28. /.Woiset is plotted as a function of neutron zVoffset shows an abrupt increase at a small dose and begins to saturate with increasing neutron fluences. 0.18 ...... 0.15 0.5 / / 0.12 D1: 0 0.3 > E 02 dlI / D4:b after /; I / / / / / / / /1 / / / / // I: / D4att o /1/ I . ----- oabet D3.5tt 0.09 before 1/1/ D2:bet --D2;ft > fi / /1 / / 0.06 vo 0.03 Mi 0.10 0.15 0.20 0.25 0.30 0.35 045 0.40 0.50 VCE o on 0 1 I I I I I 2 3 4 5 6 14 Neutron fluence (10 , 7 2 n i cm FIGURE 6.28: Collectoremitter offset voltage increase as a function of neutron dose. Inserted figure shows the absolute V0f fset values found from Fig. 6.27. The reciprocal DC gain(1//3) of the device at VCE 1.0 V as a function of dose is plotted in Fig. 6.29. The gain degradation as a function of radaition dose generally obeys the MessengerSpratt relation, which is given by (6.2) 193 where 3 = Ic/lB and c5e is electron fluence. From this figure, we can determine the gain degradation coefficient, (Kfi), from the slope of the linear line and it is found to be 1.4 x 10-16 cm2/n. 0.04 . -. 160 IE U 140 0.03 120 1 100 0.02 = 1 .4x1 O6(cm2/n) 80 60 0.01 40 VcE=1.OV 20 0.00 0.0 0.4 0.8 Neutron Dose 1.2 2.0 1.6 2.4 (x 1 014n/cm2) FIGURE 6.29: Inverse DC gain (i/) and gain (3) of the device at VCE = 1.0 V as a function of neutron dose. The Gummel plots of 'B as a function of VBE at VBC = 0 V for different exposures are shown in Fig. 6.30. Again, the Gummel plots for the unirradiated devices coming from the different wafer segments are slightly different but lie close to each other. In the low VBE regime (VBE < 0.7 V) the base current is clearly seen to increase with irradiation dose. The Gummel plots of 'c as a function of VBE at VBC = 0 V for different exposures are shown in Fig. 6.31. There is not much change in the collector current except for the largest dose for which the collector current for low VBE is seen to be significantly more than 194 100 10 0.1 < 0.01 W1E-3 filled symbols: unirradiated 1 E-4 --2.52x 1013 1 E-5 -0---- 7.20 x ---2.16x i0 o--6.48x io' 1 E-6 3 I I7 0.0 1.0 0.8 0.6 0.4 0.2 VBE (V) FIGURE 6.30: Gummel plots of different neutron irradiation dose. 'B as a function of VBE at VBC = OV for 100 10 0.1 E 0.01 0 Unexposed 1E-3 2.52x1 0'3(n/cm2) 7.20x1 013(nlcm2) 1 E-4 2.16x1014(nlcm2) 6.48x1 014(nlcm2) 1E-5 I 0.0 0.2 0.6 0.4 FIGURE 6.31: Gummel plots of 'c as a function of different neutron irradiation dose. 0.8 1.0 VBE (V) VBE at VBC = 0 V for 195 the unirradiated value. In the high VBE regime, the Gummel plots (of both 'B and I) for the irradiated devices are nearly coincident with those of the unirradiated devices except for the largest dose. This suggests that there is no significant increase in the series resistance after the lower doses. This is also consistent with the negligible change in VCE,Sat mentioned earlier. The base current dependence of low frequency gain for the different doses is shown in Fig. 6.32. It is seen that the gain curves for the different doses are parallel to each other, In other words, the degradation of gain for a given dose is constant for all the base currents. This suggests that the gain degradation is 200 180 U. 160 140 120 0 00000000 D0000DDOa0001 00000 -_ 100 0000000000000 2 0 80 000000000 60 40 20 GOOOOOOOOGOOGOG 0 20L 0.0 4 . 2.0x104 4.Oxl 6.Oxl 8.OxlO5 1.0x10 IB(A) FIGURE 6.32: Base current dependence of low frequency ac gain for different neutron irradiation dose. caused by the increase in the base current due to the additional recombination at the radiation induced defects in the neutral base region. 196 The increase in the output conductance of the devices after irradiation seen in Fig. 6.27 is believed to be caused by the avalanche multiplication in the collectorbase region due to a decrease in the BC junction breakdown voltage. This is also independently verified by the measurement of the breakdown voltage of separate diodes on the same wafer segment, as shown in Fig. 6.33. Output conductance of polyimide passivated devices, on the other hand, seemed to be not affected by neutron irradiation unlike passivated devices [53]. 0.0 -0.2 unexposed: Dl -0.6 unexposed: D2 unexposed: D3 -0.8 --0-- D1:2.52xl013 n/cm2 --0-- 02: 7.2Oxl 013 n/cm2 -1.0 --s-- D3: 2.l6xl 014 n/cm2 -1.2 -3.0 -2.5 -1.5 -2.0 -1.0 -0.5 0.0 VBC (V) FIGURE 6.33: BaseCollector breakdown voltage characteristics after three different neutron irradiation. 6.5.2 The I V Diode Characteristics Voffset of SHBTs has been a topic of great research interest. Many factors have been attributed to the cause of 17offset. Among these the most important 197 factor is the difference in knee voltages of the baseemitter and basecollector diodes. To investigate the origin of the shift in neutron irraditions, I of our devices after V0118et V characteristics of BE and BC diodes before and after irradiation were measured. The data are shown in Figs. 6.34 and 6.35. It is clearly seen that while there is negligible change in the knee voltage (at I = 1 mA) of the BE diode, there is a significant change of the same for the BC diode. This difference in the knee voltage shift of the two diodes is responsible for the observed 100 10 .6.48ou:n/cm2 0.1 0.01 1E-3 1 E-4 . ..S... 0 -Z0 -15 i ic 0.0 -10 .05 00 05 ID IS Z0 5 vIVI I 0.2 I I I 0.4 0.6 0.8 I 1.0 I 1.2 1.4 VBE (V) FIGURE 6.34: IV characteristics of baseemitter diode before and after neutron irradiation. 100 10 0.1 0.01 1 E-3 00.1 1 E-4 -2,0 .15 -tO 05 00 05 10 ao 15 05 V80() I 0.0 I I I I I 0.2 0.4 0.6 0.8 1.0 1.2 1.4 VBC (V) FIGURE 6.35: IV characteristics of basecollector diode before and after neutron irradiation. 6.6 Discussion Both electron and neutron irradiation in single heteroj unction bipolar transistors (SHBTs) show common degradation effects after each irradiation: i) decrease of collector current (Ia) at the constant base current (IB); ii) increase of output conductance (LIc/IVcE) in the active region; iii) increase of the offset voltage, Voffset. These three effects appeared on the devices after both electron and neutron irradiations. For the electron irradiated devices, saturation collectoremitter voltage, VCE, increased as the electron dose increased whereas this trend did not show in the neutron irradiated devices. Breakdown voltge decrease of reverse biased BC diode were seen from both irradiation results causing avalanche multiplication in the BC region, which is believed to be responsible for the increase of output conductance of the devices. 199 Forward biased BC and BE diode characteristics showed different irradiation effects: BC junction seemed to be affected by irradiation through knee voltage change whereas BE junction did not show noticeable change with increasing radiation doses. This characteristics appeared on the devices regardless of radiation particle types. Compared to polyimide passivated SHBT devices, unpassivated devices seemed to be vulnerable to both radiation particles, which was explicitly shown in output conductance increase and breakdown voltage decrease. Gain degradation coefficient of electron irradiated device for unpassivated devices was about 5 times larger than that of polyimide passivated device. 200 CHAPTER 7 CONCLUSION AND FUTURE WORK The primary purpose of this research was to study the radiationinduced effects in ITTV compound semiconductor materials and heterostructure devices. Electron and neutron irradiation effects in HEMT, HIGFET and SHBTs were studied and much of our attention was given to heterostructure field effect transistors, especially high electron mobility transistors (HEMTs) and heterostructure insulated gate FETs (HIGFETs) because of their capability of high speed device application. The major accomplishments and conclusions of this study are summarized below. 1. Prior to performing neutron and electron irradiation, the devices were fabricated by using typical lithography processing technique with MBE grown ITT-V compound semiconductor materials. The complete processing sequence of a A1GaAs/GaAs HEMT device fabrication has been developed and standardized for unpassivated devices. The mask sets for both passivated and unpassivated devices were designed with Mentor Graphics CAD software "Ic station". Although passivated devices were not used for this study, mask set and processing procedures of device fabrication were also developed for future study. Wet etch technique was employed to define mesa isolation. Metalization schemes were planned differently for ohmic contacts of source and drain and Schottky contacts of gate. Ni/AuGe/Au 201 layers were deposited in sequence starting with Ni for ohmic contact followed by annealing at 450°C for 10 minutes un forming gas atmosphere. For Schottky barriers, Ti was deposited prior to deposition of Au to enhance adhesion of Au to AIGaAs layer. 2. An analytical model of 2dimensional electron gas sheet concentration at the AIGaAs/GaAs interface was developed. Charge control model was also developed to study the 2DEG concentration as a function of applied gate bias voltage. We also developed the analytical model of current voltage characteristics of HFET. To establish a model for saturation drain current, ig and transconductance, g, we used the velocity saturation approximation for short gate devices ( 1 jtm or less) whereas pinchoff approximation was employed for long gate devices. The 2DEG sheet concentration was also simulated numerically with the help of the selfconsistent solution of SchrodingerPoisson equations. All these models were used to investigate radiation effects on device characteristics. 3. Neutron irradiation experiments were performed on AIGaAs/GaAs HEMT, A1GaAs/InGaAs/GaAs HIGFET, and Hall control samples. This experiments were conducted at OSU TRIGA "Marc II" 1 MW reactor operated at 100 kW in OSU. We chose four different neutron doses in the range of 1x io' to 5 x iO' n/cm2 (1 MeV GaAs equivalent) in this experiment. All devices showed gradual degradation with increase of neutron doses. For HEMT devices, threshold voltage increased by 0.25 V for the maximum neutron dose whereas HIGFET showed less than 0.03 V for the same dose. The threshold voltage degradation coefficients were found to be 4.62x 10_16 Vcm2 /n and 0.64x 1O_16 Vcm2/n for HEMT and HIGFET devices, respectively. Maximum transconductance degradation after neutron irradiation appeared to be more in HEMTs than in HIGFETs. Drain current degradation was also shown in both devices with larger effects in HEMTs. The difference in the magnitude of the threshold voltage shifts of these devices were explained to be due to the difference in sensitivity of the quantum well shapes to the radiationinduced defects in the channel region. In other words, squarewell potential in HIGFETs tends to be less affected by radiation than the triangular shape potential well in HEMTs. 4. Experimentally obtained 'D VDS characteristics showed good agreement with those of theoretical values. Mobility and resistance degradation obtained from the Hall effect measurement and TLM showed good agreement with those from the theoretical models. Transconductance between experiment and theory also showed good agreement in the linear regime. 2DEG sheet concentration and mobility were primarily affected by the radiationinduced acceptor concentration. We have examined the dependence of equilibrium 2DEG concentration on the various structure parameters of the device and have found follows: i) The increase of acceptor concentration in dopedA1GaAs layer did not affect on 2DEG concentration. Numerical as well as analytical models showed the same tendency except the fact that the degradation shown by the analytical model was a little more than that shown by the numerical model. We believe the numerical model is more accurate since the triangular potential approximation becomes less valid with increased acceptor concentration in the channel; ii) Regardless of the doping profile, either uniform or Sdoped, 203 acceptor concentration increase in A1GaAs layer did not contribute to the 2DEG concentration reduction. About same amount (45 %) of 2DEG concentration reduction took place when we introduced 1 x 10-16 cm3 acceptor concentration in GaAs layer. Aluminum composition of the AlGaAs layer, and doping concentration on A1GaAs layer did not contribute to the 2DEG concentration degradation with the additional radiation induced acceptor concentration in HEMT structure. We could conclude that increasing acceptor concentration in GaAs layer was a crucial cause of 2DEG reduction. 5. Numerical and analytical models showed increase in threshold voltage as acceptor concentration was increased. There was slight discrepancy on the threshold voltage shifts between these two models mainly because of the triangularshape potential assumption that we made for the analytical model. HIGFET device showed much less threshold voltage shift compared to that of HEMT. We also explained that this difference was due to the difference potential shapes of two devices, which have different sensitivity on increasing acceptor concentration. Comparison with experimental data showed good agreement. Again this simulation helped us to verify our conclusion that neutroninduced acceptorlike defects removed carriers from GaAs region of both devices causing threshold voltage increase, transconductance and drain current degradation along with the channel mobility degradation. From the theoretical models and experimental results from the neutron irradiation on HEMT devices and materials, we have found the pronounced radiationinduced effects on device perfomance and degradation mechanisms 204 due to the irradiation on materials. In Fig. 7.1, we have modeled the degradation mechanisms appearing on both HEMT and HIGFET devices. The induced [Radiation on HEMT Device] $ introduce defects tra s :' 'compensate with Th carriers ... '.. carrier removal I I increase scattering) .... senes resistance increase I i mobility de threshold voltage shift transconductance decrease Drain Current Degradation FIGURE 7.1: HEMT degradation simulated using radiation induced effects on model parameters. defects can either compensate the carriers that are already present in the device as a dopant or they can be scattered out from the interacted atoms. Corn- 205 pensation reduces carriers and increases series resistance whereas mobility can be degraded by scattering. These three major degradation mechanisms manifest as threshold voltage shift, transconductance degradation, and finally drain current degradation in HEMT electrical characteristics. Fig. 7.1 summarizes radiation effects starting with the material level leading to device performance degradation. Electron and neutron irradiation experiments were also performed on single heterostructure bipolar transistors (InP/InGaAs/InGaAs). Cumulative electron dose up to 26 x 1015 e/cm2 was used with several intermediate levels of doses whereas four different neutron fluences in the range of 1 x 1013 to 6.5 x 1014 n/cm2 were used for neutron radiation experiment. From both experiments, we found common degradation effects appearing currentvoltage characteristics with a gradual increase of the radiation dose: i) collector current (Ic) decreases at the constant base current (IB); ii) output conductance (LIc/VcE) increases in the active region; and iii) offset voltage, l7offset, increases. Breakdown voltages of reverse biased basecollector diode increased for both electron and neutron irradiated devices causing avalanche multiplication in the BC region. It seems to be responsible for output conductance increase of the devices. Unpassivated SHBTs were more sensitive to the radiation in terms of larger output conductance increase and also breakdown voltage decrease as compared to polyimide passivated SHBTs. Isochronal annealing experiment was performed on electron irradiated devices with temperatures up to 450°C and found a partial recovery on I VCE characteristics. Although several radiationinduced effects in some of 111V compound semi- 206 conductor materials and devices are investigated in the process of this study, a number of problems was questioned and left for future study. 1. This study focused only an unpassivated devices. This experimental techniques and the theoretical models can also apply to the passivated HEMT devices within a few modifications. We have already designed the entire mask set along with the optimized processing steps for this passivated HEMTs for future study. It will be useful to study radiation effects on passivated HEMT devices because we have seen the crucial role of passivation layer on SHBT devices. One can aso investigate the role of passivation layer on HEMT devices in terms of radiationhardness and vulnerability. In the process of dry etching for passivation layer, Si3 N4 or Si02 may be employed as well as polyimide for wet etching. 2. More study on metalization material as well as technique is required to reduce the series resistance, associated with ohmic contacts. 3. Analytical model for HIGFET devices which had squarewell shape potential profiles is required for more comprehensive understanding of radiation induced acceptor concentration effects on threshold voltage shift. Due to the different conduction band offset of two heterostructure interfaces (A1GaAs/InGaAs vs. InGaAs/GaAs), the square-well potential is not symmetric. Finding appropriate potential functions for an asymmetric squarepotential well might be crucial to develop a valid analytical model. 4. It is necessary to establish the fundamental level of interaction mechanism between electroninduced defects and Ill-V HFET devices. Although we performed electron irradiation on commercial HIGFET device, 207 which showed similar degradation as neutronirradiated devices, establishing appropriate analytical model is necessary to understand the degradation quantitatively. 5. Developing the detailed and more appropriate techniques to characterize of radiationinduced deeplevel traps or shallowtraps in materials are required for a better understanding the radiation induced defects. For shallow traps, we suggest photoluminescence technique whereas Deep Level Transient Spectroscopy (DLTS) technique is suggested for monitoring the deep level traps. 6. Radiation effects on other important device characteristics such as breakdown voltage and unity gain cutoff frequency (fT), can also be studied. 7. The entire experiment procedures and characterization techniques performed with neutron and electron sources can be also employed for irradiation study with other radioactive sources such as gamma, proton and alpha on HFET devices without much modification. BIBLIOGRAPHY [1] M.E. Hafizi, C.R. Crowell, and M.E. Grupen "The DC characteristics of GaAs/A1GaAs heterojunction bipolar transistors with application to device modeling" IEEE Transactions on Electron Devices Vol. 37, No. 10, pp 2121-2129, Oct. 1990. [2] Ju Sung Park and Arnost Neugroschel. "Parameter extraction for bipolar transistors" IEEE Transactions on Electron Devices Vol.36, No.1, pp 8895, Jan. 1989. [3] N. Chand, R. Fischer, and H. Morkoc "Collectoremitter offset voltage in AIGaAs/GaAs heterojunction bipolar transistors" Applied Physics Letters 47(3) pp 313-315,Aug. 1985. [4] Masashi Uematsu and Lazumi Wada "Recombinationenhanced impurity diffusion in Bedoped GaAs" Applied Physics Letters 58(18) pp 2015-2017, May 1991. [5] J.J.Liou, C.I.Huang, B.Bayraktaroglu, D.C. Williamson, and K.B. Parab. "Base and collector leakage currents of A1GaAs/GaAs heteroj unction bipolar transistors" Journal of Applied Physics 76(5) pp318'7-3l93, Sep. 1994. [6] Frank Stern and Sankar Das Sarma. "Electron energy levels in GaAs Gai_AlAs heterojunctions". In Physical Review B, Vol.30(no.2): pp 840-848, Jul 1984. [7] 5. Karmalkar and G. Ramesh "A simple yet comprehensive unified physical model of the 2D electron gas in deltadoped and uniformly dopid high electron mobility transistors" IEEE Transactions on Electron Devices Vol. 47, No.1, ppll-23, Jan. 2000. [8] A.Jorio, M. Parenteau, M. Aubin, C.Carlone, S.M. Khanna, and J.W. Gerdes Jr. "A mobility study of the radiation induced order effect in gallium arsenide" IEEE Transactions on Nuclear Science, Vol.47 No.6, ppl93'7-194A, Dec. 1994. [9] K.Lee, M.S.Shur, T.J. Timothy, J.Drummond, and H. Morkoc. "Current voltage and capacitancevoltage characteristics of modulationdoped field effect transistors" IEEE Transactions On Electron Devices Vol.ED-30, No.3, pp2O'7-2l2, Mar. 1983. [10] D. Delagebeaudeuf and N.T. Linh. "Metal(n) AIGaAsGaAs two dimensional electron gas FET" IEEE Transactions on Electron Devices, Vol. ED-29, No.6, pp955-96O, Jun. 1982. 209 [11] K.Lee, M.S.Shur, T.J. Timothy, J.Drummond, S.L. Su, W.G. Lyons, R.Fischer, and H. Morkoc. "Desigh and fabrication of high transconductance modulationdoped (Al, Ga)Ga/GaAs FETs" Vacuum Science Technology B Vol.1, No.2, pp186l89 1983. [121 L.Lee, M.S. Shur, T.J. Drummond, and H. Morkoc "Parasitic MESFET in A1GaAs/GaAs Modulation Doped FETs and MODFET Characterization" IEEE Transaction Electron Devices Vol.31, No.1, pp. 29-35, Jan. 1984. [13] W. Walukiewicz, H.E. Ruda, J. Lagowski, and H.C. Gatos. "Electron Mobility in modulationdoped heterostructures." Physical Review Vol. B.30, pp. 4571-4582, January 1984. [14] R.R. Daniels, R. Mactaggart, J.K. Abrokwah, O.N. Tuffe, M.Shur, J.Baek, and P. Jenkins. "Complementary heterostructure insulated gate FET circuits for high-speed, low power VLSI" In IEDM Tech. Dig., pp 448-449, 1986. [15] R.A. Kiehl, D.J. Frank, S.L. Wright, and J.H. Magerlein. "Device physics of quantum-well heterostructure MI3 SFET's" in Tech. Dig-mt. Electron Devices Meet.,pp 70-73, 1987. [16] R.R. Daniels, M.S. Shur, "Quantumwell pchannel AlGaAs/InGaAs/GaAs heterostructure insulated-gate field-effect transistors with very high transconductance" In IEEE Electron Device Letters, vol. 9, No.7 Jul 1988. [17] N.C. Cirillo, M.S. Shur, P.J. Vold, J.K. Abrokwah, O.N. Tufte. "Realization of nchannel and p-channel highmobility (Al, Ga)As/GaAs heterostructure insulation gate FET's on a planar wafer surface" In IEEE Electron Device Letters Vol. EDL-6 NO. 12, Dec.1985. [18] Herbert S. Bennett "Numerical simulation of neutron effects on bipolar transistors" IEEE Transaction on Nuclear Science vol. NS-34, No. 6, ppl372l375, Dec 1987. [19] J. Rolden, W.E. Ansley, J.D. Cressler, S.D. Clark, and NguyenNgoc. "Neutron radiation tolerance of advanced UHV/CVD SiGe HBTs." IEEE Transaction on Nuclear Science vol. 44, No. 6, pp.l965l9'73, Dec. 1997. [20] 5. Zhang, G. Niu, J.Cressler, S.D. Clark, and D. Ahlgren. "The effects of proton irradition in the RF performance of SiGe HBTs." IEEE Transaction on Nuclear Science vol. 46, No. 6, pp.l'7l6l'72l, Dec. 1997. [21] Y.Song, M.E. Kim, A.K.Oki, M.E. Hafizi, W.D. Murlin, J.B. Camou, and K.W. Kobayashi. "effects of neutron irradiation on GaAs/AlGaAs heterojunction bipolar transistors" IEEE Transaction on Nuclear Science vol. 36, No. 6, pp2l55-2l6O, Dec. 1989. 210 [22] G. A. Schrantz, N.W. van Vonno, W.A. Krull, M.A. Rao, S.I. Long, and H. Kroemer. "Neutron irradiation effects on AIGaAs/GaAs heterojunction bipolar transistors" IEEE Transaction on Nuclear Science vol. 35, No6, pp 1657-1661, Dec. 1988. [23] A.Sarkar, S.Subramanian, and S.M. Goodnick. "Effects of neutron irradiation io GaAs/A1GaAs heterojunction bipolar transistors." IEEE Transaction Electron Devices Nov. 2000. [24] S.B. Witmer, S.D. Mittleman, and S.J. Pearton. Radiation effects on InPbased heterojunction bipolar transistors, chapter6, pp195-228. InP HBTs: Growth, Processing and Applications. Artech House, 1995. [25] A. Bandyopadhyay, S. Subramanian, 5, Chandrasekhar, A. Dentai, and S.M. Goodnick. "Degradation of DC characteristics of InGaAs/InP single heterostructure bipolar transistors" IEEE Transaction on Electron Devices vol. 46, pp84O-849, 1999. [26] A. Shatalov, S. Subramanian, and A. Klein. "Correlation between nonionizing energy loss and threshold voltage shift in InP/InGaAs heterojunction bipolar transistors.". IEEE Transactions on Nuclear Science vol. 48. No.6, Dec. 2001. [27] A. Shatalov, S. Subramanian, S. Chandrasekhar, A. Dentai, and S.M. Goodnick. "Neutron irradiation effects in InP/InGaAs single heterojunction bipolar transistors". IEEE Transactions on Nuclear Science vol. 47. No.6, pp.2551-2556, 2000. [28] A. Shatalov, S. Subramanian, S. Chandrasekhar, A. Dentai, and S.M. Goodnick. "Electron irradiation effects in polymide passivated InP/InGaAs single heterojunction bipolar transistors". IEEE Transactions on Nuclear Science vol. 46. No.6, ppl7O8-l7l6, 1999. [29] A. Bandyopadhyay, S. Subramanian, 5, Chandrasekhar, A. Dentai, and S.M. Goodnick. "Degradation of InGaAs/InP double heterostructure bipolar transistors under electron irradiation" IEEE Transactions on Electron Devices vol. 46, pp85O-858, 1999. [30] R.J. Krantz, "High energy neutron irradiation effects in GaAs modulation doped field effect transistors" IEEE Transactions on Nuclear Science vol.35, No.62 pp1443-1448 Dec. 1988. [31] M. Papastamatiou, N. arpatzanis, G . J .Papaioannou, C. Papastergiou, and A. Christou. "neutron radiation effects in high electron mobility transistors" IEEE Transactions on Electron Devices vol. 44: pp 364-371, Mar. 1997. 211 [32] W. Jin, J. Zhou, Y. Huang and L Cai. "Quantum transport in neutron irradiated modulationdoped heterojunctions: I. Fast Neutrons." Physical Review B. Vol. 38 No. 18, ppl3O86-1O389, Dec. 1988. [33] B.K. Janousek, R.J. Krantz, W.L. Bloss, W.E. Yamada, S. Brown, R.L. Remke, and S. Witmer. "characteristics of GaAs Heterojunction FETs (HFETs) and Source Follower FET Logic (SFFL) Inverters Exposed to High Energy Neutrons" IEEE Transactions on Nuclear Science Vol. 36 No. 6, pp2223-2227, Dec. 1989. [34] R.J. Krantz and W.L. Bloss "The role of unintentional acceptor concentration on the threshold voltage of modulationdoped fieldeffect transistors" IEEE Transactions on Electron Devices vol. 30 No. 2, pp 451-453, Feb. 1989. [35] Michael J. O'Loughlin. "Radiation effects in high electron mobility transistors:total dose gamma irradiation" IEEE Transaction on Nuclear Science vol. NS-34, No. 6, ppl808l8ll, Dec. 1987 [36] B.K.Janousek, W.E. Yamada, and W.L. Bloss. "Neutron radiation effects in GaAs junction field effect transistors" IEEE Transaction on Nuclear Science vol. 35, No. 6, pp 1480-1486, Dec. 1988. [37] E.G. Stassinopoulos and J.P. Raymond. "The space radiation environment for electronics" In Proc. of the IEEE vol. 76, p. 1423, 1988. [38] K.G. Kerris. "Source considerations and testing techniques" In Ionizing Radiation Effects in MOS Devices and Circuits edited by T.P. Ma and P.V. Dressendorfer. Johns Wiley & Sons, New York, pp. 443-484.1989. [39] J. Schwank. "Basic mechanisms of radiation effects in the natural space 11-55, 1994. environment" In NSREC short course pp.'-1 [40] R.K. Freitag and D.B. Brown "Low dose rate effects on linear bipolar ICs: Experiments on the time dependence" In IEEE Trans. Nucl.Sci. vol. 34, no. 6, pp. 1220-1226, Dec. 1997. [41] K. A. Label [web page] "Single Event Effect Criticality Analysis" Feb. 15, http://RADHOME. gsfc. nasa. gov/radhome/papers/seecal. htm Ac1996 cessed on March 26, 2002. [42] A. HolmesSiedle and L. Adams. Handbook of Radiation Effects. Oxford university Press, 1993. [43] G.C. Messenger and M.S. Ash The Effects of Radiation on Electronic Systems pp 269-270. Van Nostrand Reinhold Company Inc., New York, 1986 212 [44] G.P. Summers, E.A. Burke, C.J. Dale, E.A. Wolicki, P.W. Marshall, and M.A. Gehihausen. "Correlation of particleinduced displacement damage in silicon" IEEE Transaction on Nuclear Science vol. 34, No6, pp 11341139 Dec. 1987. [45] G.P. Summers, E.A. Burke, M.A. Xapsos, C.J. Dale, P.W. Marshall, and E.L. Petersen. "Displacement damage in GaAs structures" IEEE Transaction on Nuclear Science vol. 35, No6, pp 1221-1226, Dec. 1988. [46] G.P. Summers. Displacement Damage: Mechanisms and Measurements, 1992 NSREC Short Course, Section IV. [47] G.P. Summers, E.A. Burke, P. Shapiro, S.R. Messenger, and R.J. Walters. "Damage correlations in semiconductors exposed to gamma, electron, and proton radiations" IEEE Transaction on Nuclear Science vol. 40, No6, pp 1372-1378, Dec. 1993. [48] T.F. Luera, J.G. Kelly, H.J. Stein, M.S. Lazo, C.E. Lee, and L.R. Dawson. "Neutron damage equivalent for silicon, silicon dioxide, and gallium arsenide" IEEE Transaction on Nuclear Science vol. 34, No6, pp 1557-1563 Dec. 1987. [49] P.J. Griffin, J.G. Kelly, T.F. Luera, A.L. Barry, and M.S. Lazo. "Neutron damage equivalece in GaAs" IEEE Transaction on Nuclear Science vol. 38, No6, pp 1216-1222 Dec. 1991. [50] M.A. Xapsos, G.P. Summers, C.C. Blatchley, C.W. Colerico, E.A. Burke, S.R. Messenger, and P. Shapiro. "Co6° gamma ray and electron displacement damage studies of semiconductors" IEEE Transaction on Nuclear Science vol. 41, No6, pp 1945-1949, Dec. 1994. [51] S.M. Khanna, A. Houdayer, A. Jorio, C. Carlone, M. Parenteau, and J.W. Gerdes Jr. "Nuclear radiation displacement damage prediction in galhum arsenide through low temperature photoluminescence measurements" IEEE Transaction on Nuclear Science vol. 43, No6, pp 2601-2608, Dec. 1996. [52] F.L. Vook Radiation Damage in Semiconductors. p. 51. Durod, Paris and Academic Press, New York, 1964 [53] A. Shatalov. "Radiation Effects in IllV Semiconductors and Heterojunction Bipolar Transistors" Ph.D. Thesis, Oregon State University, Corvallis, Oregon, 2000. [54] J.Lindhard, V. Nielsen, M. Scharif, and P.V. Thomsen. "Integral equations governing radiation effects" In Matematiskfysiske Meddelelser 33(10):142, 1963. 213 [55] D.G. Doran. "Neutron displacement cross section for stainless steel and tantalum based on a Lindhard model" In Nuclear Science and Engineering 49:130-144, 1972. [56] A. Sarkar. "Radiation Effects in Compound Semiconductor Heterostructure Devices" M.S. Thesis, Oregon State University, Corvallis, Oregon, 1998. [57] Michael Shur. GaAs Devices and Circuits. Plenum Press, 1987. [58] E.A. Burke, C.J. Dale, A.B. Campbell, G.P. Summers, W.J. Stapor, M.A. Xapsos, T. Palmer, and R. Zuleeg. "Energy dependence of protoninduced displacement damage in Gallium Arsenide." In IEEE Trans. Nucl.Sci. vol. 42, no. 6, pp. 1906-1913, Dec. 1987. [59] A.L. Barry, A.J. Houdayer, P.F. Hinrichsen, W.G. Letourneau, and J. Vincent. "The energy dependence of lifetime damage constants in GaAs LEDs for 1-500 MeV Protons." In IEEE Trans. Nucl.Sci. vol. 44, no. 6, pp. 2104-2107, Dec. 1995. [60] L.W. Aukerman and R.D. Graft. Physical Review vol. 127, p. 1576, 1962. [61] D.C. Look, in Semiconductors and Semimetals, edited by R.K. Willarson and A.C.Beer. Academic Press, New York. vol. 19, p. 93, 1983. [62] B. Ziebro, J.W. Hemsky, and D.C. Look. "Defect models in electron irradiated ntype GaAs" J.Appl. Phys. vol. 72, No.1, pp 78-81, 1992. [63] R.Zuleeg and K. Lehovec. "Radiation Effects in GaAs Junction Field Effect Transistors" IEEE Transaction on Nuclear Science vol. 27, No. 5, pp 13431354, 1980. [64] D.L. Rode. Physical Review vol. B2, p. 1012, 1970. [65] S. Sze. Physics of Semiconductor Device. John Wiley & Sons, 1981. [66] A.G. Chynoweth, W.L. Feldman, and R.A. Logan. "Excess tunnel current in silicon Esaki junctions" Physical Review vol.121 no.3, pp. 684-694, 1961. [67] M.J. Norgett, M.T. Robinson, and I.M. Torrens. "A proposed method of calculating displacement dose rates" In Nuclear Engineering and Design 33:50-54, 1975. [68] V. A. J. Van Lint. "Mechanism of Transient Radiation Effects " In Gulf Radiation Technology Doc. no. GA 8810, Aug. 1968. [69] M. Nawaz and G.U. Jensen "The Role of Unintentional Acceptor Density in GaAs/AlGaAs Based Quantum Well HEMTs" Solid State Electron Vol.41, No. 6, pp85l-855, June 1997. 214 [70] G. Messenger and J. Spratt. "The effects of irradation on germanium and silicon" Proceedings IRE 46:1038-1044, 1958. [71] Weimin Jin, Junming Zhou, Yi Huang, and Lihong Cai "Quantum transport in neutronirradiated modulationdoped heterojunctions. I.Fast neutrons" Physical Review B vol. 38, pp13086-13O89 Dec. 1988. [72] G.J.Papaioannou, M. Papastamatiou, and A. Christou. "He ion radiation effects in high electron mobility transistors" Journal of Applied Physics,78(5) pp 3066-3076, Sep 1995. [73] H.J. Stein "Electrical Studies of Low-Temperature Neutron and Electron irradiated Epitaxial ntype GaAs" Journal of Applied Physics vol. 40, no.13, pp 5300-5307, Dec. 1969. [74] G.M. Martin, E. Esteve, P. Langlade, and S. MakramEbeid "Kinetics of formation of the midgap donor EL2 in neutron irradiated GaAs materials" Journal of Applied Physics Vol.56, No.10, pp 2655-2657, Nov. 1984. [75] J.G. Williams, J.U. Patel, A.M. Ougouag, and S.Y. Yang. "Carrier removal and changes in electrical properties of neutron irradiated GaAs" Journal of Applied Physics Vol. 70, No. 9, pp 493 1-4937, Nov. 1991. [76] Frank Stern. "Quantum properties of surface spacecharge layers". In CRC Crit. Rev. Solid State Science, 499 1974. [77] Semiconductor Material and Device Characterization John Wiley & Sons, Inc., New York, 1990 [78] D. Schulte, S. Subramanian, L.Ungier, K. Bhattacharyya, and J.R. Arthur. "Mobility of twodimensional electron gas on lowtemperature MBE grown GaAs" Journal of Electronic Materials vol. 24, No. 4 pp359-363, 1995 [79] F. Au, I. Bahl, and A. Gupta. "Microwave and milimeterwave heterostructure Transistors and their applications." Artech House, Norwood, MA, 1989. [80] K. Inoue, H. Sakaki, J. Yoshino, and T. Hotta. "Selfconsistent calculation of electronic states in AIGaAs/GaAs/AIGaAs selectively doped double heteroj unction systems under electric fields." Journal of Applied Physics vol. 58, No. 1 pp42T7-428l, Dec. 1985. [81] Private conversation with S.Subramanian. Self-cocsistent calculation for multiwell coded by S.Subramanian. Apr. 2001. [82] F. Au and A. Gupta. "HEMTs and HBTs: Devices, Fabrications, and Circuits" Artech House, 1991. 215 [83] D. Sawdai, J.O. Plouchart, D. Pavlidis, A. Samelis, and K. Hong. "Power performance of InGaAs/InP single HBTs" IPRM Conference, 1996. [84] H.L. Stormer et al.. "Twodimensional electron gas at a semiconductor interface". In Solid State Commun, vol.29, pp 705-709, 1979. "Observation of twodimensional electrons in LPEgrown GaAs/AlGai_As heterojunction". In Appi. Phys. Lett., [85] D.C.Tsui and R.A. Logan. vol.35, pp 99-101, July 1979. [86] R. Dingle, et al.. "Electronic properties of the GaAsA1GaAs interface with applications to multiinterface heteroj unction superlattices". In Surface Sci., vol.98, pp 90-100, 1980. [87] G. Abstreiter. "Electronic properties of the two dimensional system at GaAs/AlGai_As interfaces". In Surface Sci., vol.98, pp 117-125, 1980.

AN ABSTRACT OF THE THESIS OF

Related documents

Products

Support

AN ABSTRACT OF THE THESIS OF

Related documents

Add this document to collection(s)

Add this document to saved

Suggest us how to improve StudyLib