Low Frequency Noise Characterization of AlGaN/GaN High Electron
Mobility Transistors
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in
the Graduate School of The Ohio State University
By
Ningjiao ZHANG
Graduate Program in Electrical and Computer Science Engineering
The Ohio State University
2013
Master's Examination Committee:
Professor Wu Lu, Advisor
Professor Siddharth Rajan
Copyright by
Ningjiao ZHANG
2013
Abstract
Low frequency noise performance is an important aspect of semiconductor transistors
evaluation. It is also a macroscopical method of defect spectroscopy. Characterization of
low frequency noise performance at different bias conditions can help to locate defects of
the devices together with other materials and device characterization techniques.
In this work, low frequency noise of non-passivated and passivated AlGaN/GaN high
electron mobility transistors (HEMTs) at different bias conditions are carefully measured
and studied. By studying band-diagram, defects locations and time constant of the
devices, comparing and analyzing meausurement results, we have the following major
findings: (1) At the subthreshold region, surface defects trapping and detrapping process
dominates the low frequency noise for non-passivated devices while process of defects
in AlGaN layer dominates for passivated devices; (2) Contributions of defects at different
locations including surface, AlGaN layer, and GaN layer with different time constants
have large impact on the frequency exponent of noise power density spectrum based on
drain bias low frequency noise spectra; (3) By doing gate bias dependent measurement
and related fitting, the results show that carrier number flucuation dominates at small gate
bias (near pinch-off) and mobility flucuation becomes more and more dominant as gate
bias increases. If the gate bias is large enough that both channel resistance and noise
induced by channel resistance are negligible compared to the access region ressistance,
low frequency noise power density is independent of gate bias.
ii
Dedication
This document is dedicated to my family.
iii
Acknowledgments
This study is finished under the meticulous guidance of my advisor, Professor Wu Lu.
During the time doing research with him, not only did I have better understanding of
semiconductor physics fundamentals, learn from his research experience and perceive
effective methods to approcah research objective, I also learnt positive attitude and strong
passion for work. I appreciate these life long influences from him.
I would like to thank Hyeong Nam Kim, the formal student in group who I nearly did
not have chance to meet. He made part of the devices I used in this study. I’d like to
thank Ye Shao, Yuji Wang and Xiangxiang Fang who had daily discussion with me.
They offered valueable sugguestions to my study.
Also, I would like to express my thankfullness to all teachers and classmates who
helped me gain knowledge and strengthen understanding of the field. Especially to
Professor Siddharth Rajan, Professor Patrick Roblin and Professor Waleed Khalil who
enlightened me directly related to this study.
iv
Vita
2007 to 2011 ..................................................B.S. Electronic Science and Technology,
Tianjin University
2011 to present ..............................................Graduate Student, Department of
Electrical and Computer Engineering, the
Ohio State University
Fields of Study
Major Field: Electrical and Computer Engineering
v
Table of Contents
Abstract ............................................................................................................................... ii
Dedication .......................................................................................................................... iii
Acknowledgments.............................................................................................................. iv
Vita...................................................................................................................................... v
Table of Contents ............................................................................................................... vi
List of Tables ................................................................................................................... viii
List of Figures .................................................................................................................... ix
Chapter 1 Introduction ........................................................................................................ 1
1.1
AlGaN/GaN HEMTs ............................................................................................ 1
1.2
Noise Theory ........................................................................................................ 2
1.2.1
Thermal Noise (Johnson noise, Nyquist noise) ............................................ 3
1.2.2
Shot Noise ..................................................................................................... 3
1.2.3
Generation-Recombination noise (g-r noise, burst-noise or popcorn-noise) 4
1.2.4
1/f noise(flicker noise, pink noise)................................................................ 5
1.3
Device Noise Model ............................................................................................. 9
1.4
Research Motivations and Objectives ................................................................ 11
vi
1.5
Organization of Thesis ....................................................................................... 12
Chapter 2 Measurement Setup and Device Structure ....................................................... 14
2.1.
Low Frequency Noise Setup .............................................................................. 14
2.2.
Device Structure and Model ............................................................................... 16
2.2.1
Device Structure.......................................................................................... 16
2.2.2
Device Noise Model ................................................................................... 18
Chapter 3 Results and Discussion ..................................................................................... 21
3.1.
Unpassivated Devices ....................................................................................... 21
3.1.1
Low Frequency Noise in Sub-threshold Region ......................................... 21
3.1.2
Drain Bias Dependence Measurement of Low Frequency Noise ............... 26
3.1.3
Gate Bias Dependence Measurement of Low Frequency Noise ................ 30
3.2.
Passivated Devices ............................................................................................. 34
3.2.1
Low Frequency Noise Performance in Sub-threshold Region ................... 34
3.2.2
Drain Bias Dependence Measurement of Low Frequency Noise ............... 36
3.2.3
Gate Bias Dependence Measurement of Low Frequency Noise ................ 38
3.3.
Channel Resistance and Access Resistance ....................................................... 40
Chapter 4 Conclusion and Future Work ........................................................................... 43
Reference .......................................................................................................................... 45
vii
List of Tables
Table 1 Comparison of GaN, Si, GaAs, and SiC material properties ................................ 1
Table 2 Non-passivated device fitting equations and parameters for gate dependence PSD
........................................................................................................................................... 33
Table 3 Passivated device fitting equations and parameters for gate dependence PSD ... 40
viii
List of Figures
Figure 1 Equivalent schematic of thermal noise source .................................................... 3
Figure 2 Noise schematic of diode................................................................................... 10
Figure 3 Noise schematic of BJT ..................................................................................... 10
Figure 4 Noise equivalent schematic of the SPICE2 MOS level2 and level3 transistor
model................................................................................................................................. 11
Figure 5 Low frequency noise measurement setup.......................................................... 15
Figure 6 Low pass filter schematic .................................................................................. 16
Figure 7 Epitaxial structure of the devices in this work .................................................. 17
Figure 8 Band-diagram of the devices in this work ......................................................... 17
Figure 9 HEMT device small signal model on a cross section of HEMT [12] ................ 19
Figure 10 Device noise equivalent circuit ....................................................................... 19
Figure 11 Ids- Vgs of test device 1 ................................................................................... 23
Figure 12 Low frequency noise in sub-threshold region for non-passivated device ........ 24
Figure 13 Trapping and detrapping process for on state and off state .............................. 25
Figure 14 Time constant relations of traps in different locations ..................................... 26
Figure 15 Ids- Vgs of test device 2 ................................................................................... 27
Figure 16 Ids-Vds of test device 2 ................................................................................... 28
Figure 17 Drain bias dependence measurements(Vd=0.2V) ........................................... 29
Figure 18 γ extraction for drain bias dependence measurement ....................................... 30
ix
Figure 19 Low frequency noise measurement at Vd=0.2V ............................................. 31
Figure 20 Low frequency noise measurement at Vd=0.6V ............................................. 32
Figure 21 Low frequency noise versus Vgs-Vt ............................................................... 33
Figure 22 Ids- Vgs of test device 4 ................................................................................... 35
Figure 23 Low frequency noise in sub-threshold region for passivated device ............... 36
Figure 24 Ids-Vds for device 4 ......................................................................................... 37
Figure 25 Drain bias dependence measurements (Vg=-3.7V) .......................................... 38
Figure 26 low frequency noise of gate bias dependence .................................................. 39
Figure 27 Low frequency noise versus Vgs-Vt ................................................................ 39
x
Chapter 1 Introduction
1.1
AlGaN/GaN HEMTs
Aluminum Gallium Nitride/Gallium Nitride (AlGaN/GaN) high electron mobility
transistors (HEMTs) have been studied widely because of their promising applications in
the microwave field as high frequency, high power and high-temperature amplifiers
amplifiers and power switching in power electronics. Other technologies of high power
amplifiers also include Si-LDMOS (lateral-diffused MOS) and bipolar transistors, GaAs
MESFETs, GaAs (or GaAs/InGaP) heterojunction bipolar transistors (HBTs), SiC
(Silicon Carbide) MESFETs. Table 1[1] below shows the comparison of critical material
parameters for Si, GaAs, SiC and GaN.
Table 1 Comparison of GaN, Si, GaAs, and SiC material properties[1]
1
From Table 1, compared to other commonly used materials, GaN has a wide bandgap, high mobility, high breakdown field, and high saturation velocity (~ 3 × 107 cm/s).
Also, AlGaN/GaN heterostructures have a high density of two-dimensional electron gas
(2DEG) due to strong piezoelectric effects. Johnson’s figure-of-merits and Baliga’s
figure of merit which are normalized to Si for comparision clearly show that GaN
materials have the superiority in high power, high frequency, and power switiching
applications.
1.2
Noise Theory
Phase noise and noise figure are mostly used parameters to qualify the noise
performance of modern communication circuits. It limits the selectivity of the system and
introduces error. As the modulation becomes more and more complex, phase noise
becomes critical because it decides the minimum channel bandwidth, the smallest signal
can be detected correctly.
Knowing the source mechanism of noise and characterizing and predicting noise are
very important for circuit design. Overall, the noise can be introduced by external noise
sources and by fundamental physical processes. External sources are for example crosstalk between adjacent circuits, electrostatic and electromagnetic coupling from AC power
lines, vibration etc. These disturbances can often be eliminated by shielding, filtering, and
change of layout. Fundamental physical sources cannot be eliminated, but it is however
possible to reduce them by proper designs of devices and circuits.[2] Noise comes from
fundamental physical sources can be modeled as several parts below.
2
1.2.1
Thermal Noise (Johnson noise, Nyquist noise)
Thermal noise is the noise associated with the thermal random motion of charge
carriers. The direct current has no influence on the thermal noise since the electron drift
velocity is much less than the electron thermal velocity. Thermal noise can be represented
by a current noise source parallel to resistance R or a voltage noise source in series to
resistance R (Figure1). It can be expressed by equations below:
(1)
Where k is the Boltzmann’s constant and ∆f is the bandwidth in Hertz. From the
above equations, thermal noise is proportional to the absolute temperature and
independent of frequency. So it is also called white noise sources.
Figure 1 Equivalent schematic of thermal noise source
1.2.2
Shot Noise
3
Shot noise is associated to the direct current flowing across a potential barrier like a
PN-Junction. It is caused by the random fluctuation of the electric current due to the
discrete nature of the electronic charge (electrons and holes). Theses electrons or holes
introduce individual current impulses. Since the motion of electrons and holes is
statistical, assuming of individual, rectangular current impulses of the width τ for every
charge component, power density spectrum can be expressed as[3]:
(2)
Since τ is very small, the commonly used equation is simplified as:
(3)
Where q is the electronic charge and I is the flowing current crossing the potential
barrier. Shot noise is independent of frequency and increases proportionally to the
flowing current crossing the potential barrier.
1.2.3
Generation-recombination noise (g-r noise, burst-noise or popcorn-noise)
Generation-recombination noise is from traps that randomly capture and emit carriers
acting as generation-recombination centers. These activities cause fluctuation in the
number of carriers available for current transport. The trapped charge can also induce
fluctuations in the mobility, electric field and barrier height. Traps involved in g-r noise
are located in the forbidden band-gap and they exist due to the presence of various
defects or impurities in the semiconductor or at its surfaces and interfaces. Though there
4
have not been found exact physical expression, empirically, the power spectral density of
g-r noise follows equations below:
(4)
Where KB, AB, and FB are modeling parameters need to be extracted. A single
trapping/detrapping process leads to random telegraph signal (RTS) noise.
1.2.4
1/f noise(flicker noise, pink noise)
The study of 1/f noise is the major part of this report. Although discovered very early
and nearly everywhere like in biology, astronomy, fluid dynamics and optical systems,
until today, the source of the 1/f noise is still not clear. Though the physical mechanisms
and models are never stopped to be proposed, currently, common approaches are still
using measurement results to fit various empirical or hybrid models. The simple one of
them is shown below:
(5)
Where KF is a constant associated with the device, known as flicker noise coefficient.
I is the direct current flow, AF is the current exponent with a value between 0.5 and 2,
and γ s a flicker noise frequency exponent parameter with a value close to 1. [4][5][6]
5
Currently, there are several theories which are verified by different groups and
different devices and even for same devices but different operation regions. Two
important one are McWhorther’s model[7] and Hooge’s model[8].
1) McWhorther’s Model (trapping-detrapping model)
Carrier number fluctuation theory is also known as the trapping-detrapping model,
proposed by McWhorther. But these fluctuations can also induce fluctuation of the
channel mobility of remaining carriers in channel since the traps act as coulomb
scattering sites when they capture carriers.
At weak inversion region (sub-threshold) in a MOSFET[9],
(6)
Where Cox is the oxide capacitance per unit area, Cd is the depletion capacitance, Cinv
is the inversion capacitance which is much smaller than (Cox+Cd) in the subthreshold
region, SΔNt is oxide traps, using SRH model, it can be expressed as
(7)
At strong inversion including both linear and saturation regions,
(8)
Where Svg is noise power spectrum density (PSD) of gate voltage,
6
Since what we measure is the noise PSD of drain current SId, and what we used later
for characterization is SId/Id2, we need to derive expressions for SId in terms of Id. The
relation between Svg and SId is
(9)
Where gm is the transconductance of devices which has different expressions in
different operation regions
In Sub-threshold region,
(10)
In the linear region,
(11)
In the saturation region,
(12)
Where Leff is effective gate length. So, SId/Id2 in different region has relations below:
In the sub-threshold region,
is independent of Vgs.
In the linear region,
(13)
7
In the saturation region,
(14)
Where Id (drain current) has the following expressions in different drain bias regions
when the device is operated at on-state:
(15)
When drain bias is larger than Vgs-Vt,
(16)
2) Hooge’s Model (mobility fluctuation model)
Hooge thinks the flicker noise is attributed to mobility fluctuation. It is a result of the
fluctuation in bulk mobility based on Hooge’s empirical relation for the PSD of flicker
noise
(17)
Where α is Hooge’s empirical parameter proved to be between 10-7 to 104, N is total
number of carriers.
(18)
8
Obviously, in both linear and saturation regions,
(19)
1.3
Device Noise Model
When the noise behavior of circuits is modeled, all circuit components need to have
noise models included. All lossy components will exhibit thermal noise, corresponding to
the temperature (TEMP). Semiconductor devices will additionally also exhibit 1/f
noise.[10]
The noise of resistor is simply modeled by white noise as describe above. The
equations and equivalent schematic are shown as Equation (1) and Figure 1.
Ideal inductors, capacitors, and transmission lines are considered lossless components
which are noise free.
The main noise source of diodes is shot noise. Parasitic resistor also contributes 1/f
noise and thermal noise. The noise source of a diode can be expressed by:
(20)
The equivalent schematic is shown in Figure 2 below.
9
Figure 2 Noise schematic of diode
For bipolar transistors, each physical resistor RB, RC, RE is in parallel with a thermal
noise source and noise sources in parallel with the Base-Emitter contact inb, the CollectorEmitter contact inc, ars the combinations of 1/f noise and shot noise.
The equivalent schematic is shown in Figure 3 below
Figure 3 Noise schematic of BJT
For MOSFETs, take the SPICE2 model [11] of the UCB University of California,
Berkeley for exmaple, Figure 4 shows noise equivalent schematic. The resistors RD and
10
RS are associated with thermal noise, channel noise is descibed by thermal noise and 1/f
noise
Figure 4 Noise equivalent schematic of the SPICE2 MOS level2 and level3 transistor
model
The noise models of other kinds of transistors are omitted in this thesis. the low
frequency noise model for AlGaN/GaN HEMTs will be studied in next chapter.
1.4
Research Motivations and Objectives
11
Although AlGaN/GaN HEMTs demonstrate attracting performance in theory, in
reality, these devices demonstrate good performance initially but start to degrade in a
short period of time because of immature processes. It is important to investigate the
mechanisms which cause degradation and affect short term and long term reliability so
that people can optimize device structures and develop processes to make better and more
reliable devices.
For solid state devices, one of the primary cause of degradation is generation and
migration of defects in critical locations inside devices. Defects can be characterized by
techniques such as pulsed I-V, charge-pumping, and deep level transient spectroscopy
(DLTS) [12]. Compared to these techniques, low frequency noise measurement is a
macroscopical method of defect spectroscopy. Defects near Fermi level affect charge
transport in the device hence it is in noise spectra. By changing bias conditions, Fermi
level changes and noise spectra can be used to study is defects contributed to the noise
performance at different locations with different energies.
The objective of this work is to study the low frequency noise of AlGaN/GaN
HEMTs. To achieve this goal, we have first built a measurement setup with low noise
components. Then precise measuremts at different bias conditions have been performed.
Finally, contributions of defects to noise spectra from various locations, mechanisms are
analyzed in this thesis.
1.5
Organization of This Thesis
12
In this chapter, background information about devices and noise theory is introduced.
Formulas of two different low frequency noise theories which will be used later are
presented. Motivation, objective and the research approach are discussed and outlined.
In chapter 2, the measurement setup and device structure will be introduced. It is
proved that it is critical for successful measurements to have a measurement setup with
high capability to restrict unrelated noise from the environment, powler supplies, etc.
Chapter 3 is the major part of the thesis. The detailed mesurement results will be
presented. Analysis of the results will be discussed in detail
explanations and
mechanisms of the studies devices will be proposed.
Chapter 4 summarizes the work finished and future work will be proposed.
13
Chapter 2 Measurement Setup and Device Structure
2.1.
Low Frequency Noise Setup
When measuring the noise of a transistor, noise is usually separated into two parts by
frequency. The first part is low frequency part. Typically from several Hz up to several
MHz, in this range noise is 1/f noise and generation/recombination noise which have
certain relationships with frequency. So, noise power density spectrum is measured to
characterize the property of devices. The second range is up to the maximum operation
range of device, GHz range is pretty common for modern transistors. In this range,
thermal noise dominates and it is less dependent of frequency. Noise Figure NF, is used
to characterize the noise performance of devices in this range.
To measure the noise power spectrum in the low frequency range, a setup is built
shown Figure 5 below
14
Figure 5 Low frequency noise measurement setup
The low frequency noise setup shown in Figure 5 consists of battery power supply to
offer gate bias without AC harmonics, a custom-made low pass filter with very low cut
off frequency to filter the noise effect from gate to restrict gate noise. This is important
because gate noise will have a very severe effect after amplification. There is a tradeoff
of cutoff frequency and the number of components in the low pass filter. Very low cutoff
frequency is needed since the major noise of DC supply is in low frequency range.
However, each actual component in the low pass filter introduces noise. Figure 5 below
shows the customer-made low-pass filter. It follows the design of Agilent toolkit. The
cutoff frequency is about 1 Hz. All components are chosen with low noise materials. A
low-noise current lock-in amplifier SR570 is used to convert current signal to wanted
voltage signal. The gain of lock-in amplifier can be adjusted by changing the sensitivity
of the amplifier. Apart from amplifying AC noise signal, the SR570 lock-in amplifier can
15
also bias the drain of DUT (device under test) using the battery inside; it can compensate
for drain DC current with an ultra-low noise DC current source. A bandpass filter in
SR570 is adoped to filter unwanted frequency range for better sensitivity. Agilent’s
precision spectrum analyzer E4440A is used to measure noise spectrum. The working
frequency span of E4440A is 3Hz-26.5GHz. The displayed average noise level (DANL)
of E4440A is -110 dBm which is the minimum we can measure.
Figure 6 Low pass filter schematic
2.2.
Device Structure and Model
2.2.1
Device Structure
The devices used were grown by metal-organic chemical vapor deposition (MOCVD)
on SiC substrate. The epitaxial structures are shown in Figure 6. Figure 7 shows the
coresponding energy band-diagram. A thin AlN interlayer between the AlGaN barrier
and the GaN channel is grown to simultaneously improve the sheet charge density and
mobility in order to get higher power density [13]. Device processing began with
Ti/Al/Ni/Au electron beam evaporated source and drain contacts. These were annealed at
16
870◦C for 30 s in a rapid thermal annealing (RTA). Device isolation was achieved using
reactive ion etching (RIE) in Cl2 plasma. Ni/Au gates were electron beam evaporated
with a gate length of 0.25 µm. The gate width is 50 µm
AlGaN: 23nm
AlN: 1nm
GaN Buffer layer
Si Substrate
Figure 7 Epitaxial structure of the devices in this work
Figure 8 Band-diagram of the devices in this work
17
The original plan was to passivate the sample after first step measurement to study the
passivation effect. Another passivated sample was used at last because it was not possible
to passivate the formal sample at that time. The passivated device has the identical
structure except that a 200 nm SiN surface passivation layer deposited by plasmaenhanced chemical vapor deposition (PECVD). By passivation, the surface defects would
be supressed. In HEMT structure, the surface defects and dislocations are believed to
serve as trapping centers and affect the device performance via leakage current and low
frequency noise.
2.2.2
Device Noise Model
Figure 8 shows the small signal model on a cross section of HEMT[14]. Noise
performance is the combined contribution of all noise sources from different components.
The equivalent circuit including noise source is shown in Figure 9
18
Figure 9 HEMT device small signal model on a cross section of HEMT [12]
Figure 10 Device noise equivalent circuit model[12]
In Figure 9, the noise source expressions are shown below:
19
=
=
=
The extrinsic components are considered only generating thermal noise.
=
,
=
The intrinsic components are considered as major 1/f source and their correlation is
neglected in this model.
2.2.3
Extraction Process of 1/f Noise
Compared to model equation (5), current PSD SId corresponds to normalize
by Δf
with a unit of A2/Hz, if set Δf=1 Hz:
(21)
Step1: γ extraction
Log conversion of Equation (21)
(22)
Apply a regression curve fitting, ‘-slope’ of the fit line is taken as γ.
Step2: AF and KF extraction
At 1 Hz measure point,
,
(23)
Apply linear regression, get linear function like y=a+bx, then, AF=b, KF=10a to fit to
the measured low frequency noise spectrum.
20
Chapter 3 Results and Discussion
3.1.
Non-passivated Devices
3.1.1
Low Frequency Noise in the Sub-threshold Region
Figure 11 shows the transfer characteristics of test device 1. There are various
definitions of threshold voltage.[15] Here we adopt the method based on the subthreshold Ids-Vgs characteristics. Threshold voltage Vt is defined as the gate bias point Vgs
at which drain current Ids equals to 50nA×W/L. In our case, Ids =10µA, Vt=4.3V.
Figure 12 shows the measured noise performance in the sub-threshold region. Gate
bias varies from -4.6V to -4V with a 0.1 V step while keep the drain bias is kept at 0.2V.
For all measurements below, noise power density is normalized by drain current. The
measurement data is fitted using equation below:
y=axγ
(24)
Where y stands for normalized current Sid/Id2 which has unit of 1/Hz, x stands for
frequency, a is the fitting power density level and γ is the exponent of frequency.
Figure 13 shows several major trapping and detrapping processes of AlGaN/GaN
HEMTs at on state and off state[16]. Traps are mainly located at three regions: surface of
AlGaN layer(TP1); GaN buffer(TP2) and AlGaN barrier layer(TP3). From Figure 14[15],
TP2 has the smallest time constant and traps in the AlGaN layer has the largest time
constant. Also, from literature [15], defects in the interface of GaN buffer and AlGaN
21
interlayer have fixed time constant because the energy level is a certain value.
Differently, the time constant of surface traps, TP1, decreases as the drain bias increases.
From Figure 11, at low gate bias, for example, - 4.6 V, γ is around - 2. As the gate
bias increases, γ decreases. When channel is on, γ stays the same. This is because, at low
gate bias, channel is turned off and there are very few electrons in the channel, noise is
mainly from surface trapping and detrapping process. As the gate bias increases, the sheet
carrier concentration increases and trapping and detrapping process of TP2 increaess fast.
Since TP2 has a shorter time constant, its traping and detrapping process contributes to
higer frequency range and γ decreases. When channel is totally on, the trapping and
detrapping process for traps of all locations maintains at a fixed level so that γ maintains
as a constant value.
22
Figure 11 Ids- Vgs of test device 1
23
LFN in sub threshold region non-passivated devices
1.00E-06
Vg=-4.6V
1.00E-07
Vg=-4.5V
Si/I^2 (1/Hz)
Vg=-4.4V
1.00E-08
Vg=-4.3V
Vg=-4.2V
1.00E-09
y = 2E-06x-1.11
Vg=-4.1V
Vg=-4V
1.00E-10
1.00E-11
y = 2E-06x-2.055
1.00E-12
1E+0
1E+1
1E+2
1E+3
1E+4
Frequency (Hz)
Figure 12 Low frequency noise in sub-threshold region for non-passivated device
24
TP1
TP3
TP3
TP3
TP1
TP3
Figure 13 Trapping and detrapping process for on state and off state [15]
25
TP3
Figure 14 Time constant relations of traps in different locations [15]
3.1.2
Drain Bias Dependence Measurement of Low Frequency Noise
Figure 15 and Figure 16 show DC family I-V and transfer characteristics of Device 2
that to be used for drain bias dependence low frequency noise measurement. For noise
measurements in this device, the gate is biased in the linear region and the drain bias is
from 0.05 V to 5 V.
Figure 17 shows the drain bias dependence measurement results of noise power
density. To make plot easier to read, Figure 18 selects several measurement points to fit.
From Figure 17, the γ value decreases as the drain bias increases. In other words, with the
increasing of drain bias, the noise power density of high frequency part increases. That
means there are more short time constant trapping and detrapping taking place at a high
26
drain bias or electric field between drian and gate. From Figure 14, it is easily found that
this is because as the drain bias increasing, TP2 with a shorter time constant trapping and
detrapping process increases pretty fast and time constant of TP1 becomes shorter, too.
Figure 15 Ids- Vgs of test device 2
27
Figure 16 Ids-Vds of test device 2
28
Figure 17 Drain bias dependence measurements(Vd=0.2V)
29
Drain bias dependence of low frequency
noise
1.00E-07
1.00E-08
D0.1
D0.15
Si/I^2
1.00E-09
D0.2
y = 7E-08x-1.052
1.00E-10
D0.3
D0.4
1.00E-11
0D0.5
D0.7
y = 3E-07x-1.59
1.00E-12
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
Frequency (f)
Figure 18 γ extraction for drain bias dependence measurement
3.1.3
Gate Bias Dependence Measurement of Low Frequency Noise
Devices used for gate bias dependence measurement have the same structure and very
similar DC characteristics as Device 2. Refer to Figure 15 to see the bias region used for
the measurement. Here the biasing conditions for low frequency noise measurements are:
The drain bias are 0.2 V and 0.6 V while the gate bias varies from - 3.9 V to - 3.5 V with
a 1V step. Considering Vt is -4.3V, the gate overdrive voltage (Vgs-Vt) varies from 0.4V
to 0.8V.
30
Figure 19 and Figure 20 show the gate bias dependence measurement plots with a
drain bias of 0.2V and 0.6V. From these two figures, at a certain drain bias, as gate bias
increases, the sheet carrier density increases and Coulomb interaction between scattering
centers and channel carriers reduces due to high carrier concentration screening
effects[17]. As a result, reduced noise level is observed.
Figure 19 Low frequency noise measurement at Vd=0.2V
31
Figure 20 Low frequency noise measurement at Vd=0.6V
To further study the gate dependence trend, Figure 20 shows the normalized noise
power density versus Vgs-Vt when the drain bias is fixed at 0.2 V. Table 2 shows the
fitting equation and parameters using Origin [18] :
y=axb
(25)
where y stands for normalized current Sid/Id2 which has a unit of 1/Hz., x stands for
the gate overdrive voltage Vgs-Vt, a is the fitting parameter and γ is the exponent of
overdrive gate voltage.
32
Figure 21 Low frequency noise versus the gate overdrive voltage Vgs-Vt
Table 2 Non-passivated device fitting equations and parameters for gate dependence PSD
To make the trend line clearer for analysis, Figure 21 uses a log scale for
both x-axis and y-axis. The fitting parameter ‘b’ becomes the slope of the fitting
line. From the fitting results, two distinct regions of Vgs-Vt dependence are
observed. When Vgs-Vt is relative small, noise power density is proportional to
Vg-1.5, while when Vgs-Vt is larger, it is proportional to Vg-4.3. These
33
dependences will be discussed in the third part of this chapter together with
passivated device measurement results.
3.2.
Passivated Devices
3.2.1
Low Frequency Noise Performance in Sub-threshold Region
Similar to the non-passivated devices study, Figure 22 shows the transfer
characteristic of test device 4. Here Vt is about 4.7V.
Figure 23 shows the measured noise performance for the sub-threshold region. The
gate bias varies from – 5 V to - 4.5 V with a 0.1 V step while keeping the drain bias at 5
V. Noise power density is normalized by drain current. From the measurement plot and
extrapolation, γ is a constant (about -1.2) for all gate bias points. This is because surface
traps are passivated and noise from trapping and detrapping process of those TP2 traps in
the GaN buffer with a fixed time constant dominate.
34
Figure 22 Ids- Vgs transfer characteristics of test device 4
35
LFN sub-threshold region-Passivated device
1.00E-03
1.00E-04
y = 0.0029x-1.232
Si/I^2 (1/Hz)
1.00E-05
G-5V
1.00E-06
G-4.7V
1.00E-07
G-4.9V
1.00E-08
G-4.8V
y=
1.00E-09
5E-07x-1.232
G-4.6V
G-4.5V
1.00E-10
1.00E-11
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
Frequency (Hz)
Figure 23 Low frequency noise spectra in sub-threshold region for the passivated device
3.2.2
Drain Bias Dependence Measurement of Low Frequency Noise
Figure 24 shows the DC characteristics of device 4 for the drain bias dependence
measurement. Figure 25 shows the drain bias measurement results from 0.05V to 5V.
From Figure 25, the noise power density at all bias conditions nearly overlap. Different
from the non-passivated case (Figure 18) surface traps (TP1) are passivated and TP2
dominates, so γ is fixed.
36
Figure 24 Family Ids-Vds characteristics for device 4
LFN Drain bias varies for passivated
devices(1)
Si/I^2 (1/Hz)
1.00E-07
1.00E+00
1.00E-08
1.00E+01
1.00E+02
1.00E+03
1.00E+04
D0.1
1.00E-09
D0.2
1.00E-10
y = 1E-07x-1.467
D0.5
1.00E-11
1.00E-12
D0.3
Frequency (Hz)
37
LFN Drain bias varies for passivated
devices(2)
1.00E-06
Si/I^2 (1/Hz)
1.00E-07
D0.7
D1
1.00E-08
D1.5
1.00E-09
D2
y = 3E-07x-1.382
D2.5
1.00E-10
D3
1.00E-11
1.00E+00
1.00E+01
1.00E+02
1.00E+03
1.00E+04
D4
Frequency (Hz)
Figure 25 Drain bias dependence measurements (Vg=-3.7V). The drian bias ranges from
0.1 V to 4 V.
3.2.3
Gate Bias Dependence Measurement of Low Frequency Noise
Figure 26 shows the gate bias dependence low frequency noise measurement results
for passivated device 5. The Drain bias is 0.1 V while the gate bias varies from -4.7V to 1 V. Considering Vt is -4.7V, Vgs-Vt varies from 0 V to 3.7 V.
From Figure 26, at a certain drain bias, as the gate bias increases, normalized noise
power density decreases. Similar to the non-passivated approach, Figure 27 shows a log
scale fitting plot and table 3 shows the fitting equation and fitted parameters.
38
LFN varies with gate bias for device5
1.00E+00
G-4.7N
Si/I^2 (1/Hz)
1.00E-02
y=
0.0075x-1.277
1.00E-04
G-4.6N
1.00E-06
G-4.55N
1.00E-08
nG-4.5N
1.00E-10
1.00E-12
1.00E+00
G-4.65N
G-4.45N
1.00E+01
1.00E+02
1.00E+03
1.00E+04
Frequency (Hz)
G-4.4N
G-4.3N
Figure 26 Low frequency noise of gate bias dependence
Figure 27 Low frequency noise versus gate overdrive voltage Vgs-Vt
39
Table 3 Passivated device fitting equations and parameters for gate dependence PSD
3.3.
Channel Resistance and Access Resistance
In general, the flicker noises in FET devices are caused by contact resistance, gate
leakage current, and bulk resistance. The total flicker noise density is constructed by the
SRch and SRs noises, which are induced from the channel resistance Rch and series
resistance Rs between the source and drain electrodes. Based on Hooger’s mobility
flucuation theory, normalized noise spectral density can be expressed as[19]
(26)
SRs and Rs is independent of Vgs-Vt, N is proportional to Vg, Rch is proportional to
Vg-1. Here, we are trying to see the Vgb dependence to diagnose the measured devices.
Use Hooge’s relation as in Chapter 1. When Vgs is close to the pinch off voltage, Rch
>>Rs and SRch dominates,
(27)
Si/I2 is proportional to Vg-1.
40
As Vgs increase, Rch decreases. When Rch <<Rs, but noise from series noise should be
very small, if SRch still dominates,
(28)
Si/I2 is proportional to Vg-3.
As Vgs continue increase, Rch is negligible and SRs is the major noise source,
(29)
then Si/I2 is independent of Vg.[20]
From our results, the first fitting line for non-passivated devices, the exponent of gate
overdrive voltage (parameter b in Table 2) is -1.5 and for passivated device b is around -1
as shown in Table 3. For Non-passivated device, there is lack of high gate bias region
data. For passivated device, the flat region is clear. In the transition region (second fitting
line in Figure 27), the b value in both devices is about -4.2. This is similar to in earlier
papers but the reason why it is lower than -3 was not expained specifically [21]. Here we
propose the following mechansim. Notice that those derivations for Vgs-Vt dependence
are based on Hooge equation which only considers mobility fluctuation. Taking noise
source from carrier number flucation including surface and interlayer defects trapping
and detrapping effect into account, at the near-pinch off region, the sheet carrier
concentration is low and carrier number flucuation could dominate. From the derivation
in Chapter 1, in the subthreshold region, noise power density is independent of gate bias
41
while in the linear region it is proportional to (Vgs-Vt)-2. So, the exponent parameter b
should have a value between 0 to -2 near this region. As Vgs-Vt increases, the sheet
carrier concentration increases and trapping and detraping process reduces very fast and
finally mobility flucuation dominates in this region. So, it is not surprising to see a sharp
fall down in this region. And finally, only mobility flucuation needs to be considered and
access resistance and noise introduced by access region is dominant, the noise power
density is independent of gate bias then.
42
Chapter 4 Conclusion and Future Work
Low frequency noise is the major contributor to the phase noise which is an important
symbol to qualify active devices and effect device performance. The property of low
frequency noise also contains information related to device structure and defects. It is a
very useful tool to diagnose and find the way to improve manufacturing process.
In this work, low frequency noise performance for passivated and non-passivated
GaN HEMT devices under different conditions is studied. First, in sub-threshold region,
for non-passivated device, noise introduced by surface defects with a relative large time
constant dominates and results a higher γ induced by slower trapping and detrapping
process. For passivated devices, surface defects are passivated. At a relative high drain
bias, defects in buffer layer are the major source of trapping and detrapping process.
Since trapping and detrapping time constant of buffer layer defects is independent of gate
bias, no obvious γ variation is observed. Second, in the linear region and saturation
region, for non-passivated device, the trapping and detrapping time constant of defects
inside the AlGaN barrier layer and at the surface close to the gate edge surface is
dependent on drain bias. Also, because of stronger band-bending with a higher drain bias,
noise from GaN buffer defects increases and thus time constant is shorten and γ
decreases. For passivated devices, surface defects are passivated and as a result no
obvious γ variation is observed. At last, the gate dependence of low frequency noise is
43
studied for both kinds of devices. Normalized noise power density reduces as the sheet
carrier concentration increases because of electrical screening effect. For the noise power
density versus overdrive gate voltage results, first slope (near the pinch off region) of log
plot fitting results is believed to be the case that carrier number flucuation dominates. The
sharp slope is the combination results of carrier number flucuation and mobility
flucuation. The flat part (only have data for passivated device) is the region where
channel effect is negligible compared to access region hence the normalized noise density
is independent of gate bias.
This study is far from completion as using low frequency noise to guide improvement
of fabrication process. First, direct comparison of same device under similar condition is
needed for passivation effect. More detailed data under weak and strong bias for both
drain and gate will be helpful to understand the effect of surface state. Second,
temperature dependence measurements [22] need to be performed to study γ to confirm
the defect location. For gate bias dependence relation, more points near pinch-off is
needed since several previous studies [23],[24],[25] show that fitting lines changed at
overdrive voltage around 0.1V.
44
Reference
[1]U.K. Mishra, P. P., Y.F. Wu "AlGaN/GaN HEMTs: An overview of device operation
and applications."
[2 ]Zanolla, N. "Characterization and Modeling of Low-Frequency Noise in Mosfets.",
2008.
[3 ]B.Schiek, H.-J.Siweris, Rauschen in Hochfrequenzschaltungen, Hüthig Buch Verlag,
Heidelberg, 1990.
[4 ]A. Balandin, “Low Flicker-Noise GaN/AlGaN Heterostructure Field-Effect
Transistors for Microwave communications,”Microwave Theory and Techniques, IEEE
Transactions on , vol. 47, no. 8, pp. 1413–1417, 1999.
[5 ]A. Balandin, Ed., Noise and Fluctuations Control in Electronic Devices. Steven-son
Ranch, CA: American Scientific Publishers, 2002.
[6 ]S. Hsu, P. Valizadeh, D. Pavlidis, J. Moon, M. Micovic, D. Wong, and T. Hus-sain,
“Characterization and Analysis of Gate and Drain Low-Frequency Noise in AlGaN/GaN
HEMTs,” in High Performance Devices, 2002. Proceedings. IEEE Lester Eastman
Conference on, 2002, pp. 453–460.
[7 ]A. L. McWhorter, “Semiconductor Surface Physics,” R. H. Kinston, Ed. Philadelphia: Univ. of Pennsylvania Press, 1956, pp. 207–228
[8 ]F. N. Hooge, T. G. M. Kleinpenning, and L. K. J. Vandamme, “Experimental studies
on 1 /f noise,” Rep. Prog. Phys. , vol. 44, no. 5, pp. 479–532, May 1981.
[9 ]Noise Modeling in MOSFET and Bipolar Devices, Silvaco material
[10 ]Noise Modeling for semiconductors, modeling toolkits NOISE.DOC | 22.04.02
Franz Sischka
[11]G. Massobrio, P. Antognietti, „Semiconductor Device Modeling with SPICE“,
McGraw-Hill, New York, 1993
[12 ]HEMANT P. RAO, “Advanced Device Reliability Study Of GaN HEMTs Using
Low Frequency Noise Specscopy” 2012
[13 ]L. Shen, S. H., B. Moran, R. Coffie, N.-Q. Zhang, D. Buttari, I. P. Smorchkova, S.
Keller, S. P. DenBaars, and U. K. Mishra "AlGaN/AlN/GaN High-Power Microwave
HEMT." 2001.
45
[14 ]Sanabria, C.. Noise of AlGaN/GaN HEMTs and Oscillators. 2006
[15 ]Yuan Taur and Tak H. Ning, Fundamentals of Modern VLSI Devices, Cambridge
Univerisity Press. 1998, 2009
[16 ]Alamo, J. J. a. J. A. d"A Current-Transient Methodology for Trap Analysis for GaN
High Electron Mobility transistors." IEEE TRANSACTIONS ON ELECTRON
DEVICES 58. 2011.
[17 ]Shawn.S.H.Hsu et al "Characterization and analysis of gate and drain low frequency
noise in AlGaN/GaN HEMTs." IEEE 6. 2002.
[18 ]Origin 8, 1991-2007 Originlab Corporation
[19 ]J. M. Peransin, P. Vignaud, D. Rigaud, and L. K. J. Vandamme, IEEE Trans.
Electron Devices , 37, 2250 1990
[20 ]Vandamme, L. K. J. "Noise as a Diagnostic Tool for Quality and Reliability of
Electronic Devices." IEEE TRANSACTIONS ON ELECTRON DEVICES 4. 1994.
[21 ]Eastman, A. V. V. a. L. F. "Effect of the Surface and Barrier Defects on the
AlGaN/GaN HEMT Low-Frequency." IEEE ELECTRON DEVICE LETTERS 24. 2003.
[22 ]Roy, T., et al "Temperature-dependence and microscopic origin of low frequency 1/f
noise in GaN/AlGaN high electron mobility transistors." Applied Physics Letters 99(20):
203501. . 2011.
[23 ]Rao, H. and G. Bosman "Simultaneous low-frequency noise characterization of gate
and drain currents in AlGaN/GaN high electron mobility transistors." Journal of Applied
Physics 106(10): 103712. 2009.
[24 ]Roy, T., et al. "1/f Noise in GaN HEMTs grown under Ga-rich, N-rich, and NH3rich conditions." Microelectronics Reliability 51(2): 212-216. 2011.
[25] Lee, C.-T., et al. "Flicker Noises of AlGaN/GaN Metal-Oxide-Semiconductor High
Electron Mobility Transistors." Journal of The Electrochemical Society 157(7): H734.
2010.
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