Microelectronics Reliability xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Microelectronics Reliability
journal homepage: www.elsevier.com/locate/microrel
The model parameter extraction and simulation for the effects of gamma
irradiation on the DC characteristics of InGaP/GaAs single heterojunction
bipolar transistors
Jincan Zhang ⇑, Yuming Zhang, Hongliang Lu, Yimen Zhang, Shi Yang
School of Microelectronics, Xidian University, Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices of China, Xi’an 710071, PR China
a r t i c l e
i n f o
Article history:
Received 15 November 2011
Received in revised form 25 June 2012
Accepted 9 July 2012
Available online xxxx
a b s t r a c t
In this article, we report the effect of gamma irradiation on the DC characteristics of InGaP/GaAs single
heterojunction bipolar transistors (SHBTs) based on the simulation with the extracted model parameters
from experiment data before irradiation, after irradiation and after annealing. A simplified Vertical Bipolar Inter-Company (VBIC) static model is proposed to study the operational mechanism and the DC characteristics of SHBTs. The results show that the defects induced by irradiation are responsible for the
changes on the DC characteristics of the devices.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Heterojunction bipolar transistors (HBTs) have superior electrical performance over conventional silicon bipolar junction transistors (BJTs) for high-speed/high-frequency applications. HBTs based
on InGaP/GaAs heterojunctions as a major contender are of
increasing interest since many of its features are attractive, such
as higher valence band offset to suppress the back injection of
holes into the emitter, higher etch selectivity between InGaP and
GaAs, lower surface recombination velocity, absence of DX centers
that plague the Al-based systems, and better long-term reliability
as compared with the AlGaAs/GaAs HBTs [1]. Hence, InGaP/GaAs
HBTs are very attractive candidates for the applications in radiation-rich environment such as nuclear reactor, high-energy particle accelerators, artificial satellites. Hence, the issues of reliability
of these devices in high-energy radiation environment should be
paid much attention.
Most of the radiation studies on HBTs reported so far have
mainly focused on experimental results on the radiation induced
changes in the measured electrical characteristics of the devices
(e.g., excess base current, current gain, etc.) [2–5]. However, to
our knowledge, there is not much published information available
on modeling the electrical characteristics of HBTs subjected to
high-energy radiations [6,7], and SPICE-like Gumml–Poon model
was used in their studies. The VBIC model describes the electrical
characteristics of HBTs more accurately than the SPICE Gumml–
Poon (SGP) model [8], because there are some modeling enhancements of VBIC over SGP (e.g., decoupling of base and collector
⇑ Corresponding author. Address: Mail Box No. #397, Xidian University, No. 2
South TaiBai Road, Xi’an, Shaanxi, PR China. Tel.: +86 29 88201824.
E-mail address: zjc850126@163.com (J. Zhang).
currents, quasi-saturation modeling, improved temperature depen
dence modeling, etc.) [9]. And there is very little progress has been
made in modeling the annealing characteristics of HBTs, which is
now studied in this work.
In this article, a simplified VBIC static model is proposed to describe the DC characteristics of InGaP/GaAs SHBTs. The simplified
VBIC static model is used to extract device model parameters before irradiation, after irradiation and after annealing. Then, the extracted model parameters are used to study the radiation induced
degradation of the device performance.
2. Experiment and analysis
The InGaP/GaAs SHBTs used in this study are commercial products from WIN Semiconductors Corp. The device structure is very
similar to that of a conventional AlGaAs/GaAs SHBTs except the AlGaAs emitter layer replaced by an InGaP layer. More details of the
device fabrication can be found in Ref. [5]. In the InGaP/GaAs
SHBTs, the base–emitter is a heterojunction and the device is passivated with SiN.
2.1. Experiment
Irradiation of devices, without biased, was performed in a
‘‘Gamma-Cell’’ with a Co60 source providing a dose rate of about
50 rad(Si)/s [1 rad(Si) = 0.94 rad(GaAs)], and irradiation time of
55 h, equivalent to a gamma total dose of 10 Mrad(Si) used for
the test samples. On-wafer DC measurement of the samples were
made with an HP4142 Semiconductor Analyzer at room temperature (T = 300 K) before irradiation, after irradiation and after
annealing. Scattering parameters (S-parameters) were measured
using an HP8510C vector network analyzer from 100 MHz to
0026-2714/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.microrel.2012.07.020
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
2
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
40 GHz, with a wide bias current range based on circuit applications. Delay time between irradiation and remote testing was
approximately 45 min. These samples were self-annealed at a period of room temperature (T = 300 K) for about 700 h to observe the
changes of device after annealing.
2.2. Analysis
Embedding well-recognized VBIC model into ADS (Advanced
Design System), the commercially available device/circuit simulation package, the simulation is implemented In order to study
the physical mechanisms of the degradation of device performance
and the equivalent network of VBIC is given in Ref. [9].
However, specifics of the HBTs make it possible to consider a
simplified VBIC static model, as shown in Fig. 1. In this simplified
VBIC static model, the following points are considered. (1) There is
no parasitic pnp transistor in npn HBTs [10], therefore the parameters to describe parasitic transistor can be eliminated. (2) The
extrinsic base–emitter current IBEX can be neglected comparing with
intrinsic base–emitter current. (3) Since the avalanche multiplication has not happened during testing, the weak avalanche current
can be ignored. (4) In HBTs, both early voltages and knee currents
for the cases in forward and reverse operations can be considered
to be infinite [11], therefore the normalized base charge qb tends
to be 1. (5) The quasi-saturation region does not appear in properly
designed HBTs for usual applications [11], so the model parameters
except the intrinsic collector resistance RCI used for expressing quasi-saturation region are not considered. However one can optimize
RCI to fit the quasi-saturation region data if necessary.
2.2.1. Forward-mode Gummel plot
In the measurement of forward-mode Gummel plot, the base
current IBE and collector current ICC are measured when VBC is fixed
at zero while the base–emitter junction is in forward-bias. The
presence of the relatively large valence band discontinuity at the
base–emitter heterointerface leads to an effective suppression of
the hole injection current from the base region into the emitter.
Thus, the base current is mostly determined by the recombination
of (1) in the bulk and along the periphery of the base–emitter space
charge region (BE-SCR) and (2) in the bulk and at the surface of the
neutral base region (NBR) [12]. Then most of electrons injected
from emitter are collected by the base–collector junction as
collector current.
The base current is followed by:
IBE ¼ IBEi þ IBEn
V BE IE RE IB RB
1
¼ IBEI exp
N EI V tv
V BE IE RE IB RB
1
þ IBEN exp
NEN V tv
Fig. 1. Simplified VBIC static model.
where VBEi = VBE IERE IBRB since The base–emitter junction voltage VBEi is different from the terminal voltage VBE due to the voltage
drops across the parasitic series resistances RE and RB, and Vtv = kT/q
is the thermal voltage. As can be seen from Eq. (1), the base current
includes a component IBEi, formed by the NBR recombination and
modeled with saturation current IBEI and ideality factor NEI 1,
and a component IBEn caused by the BE-SCR recombination and
modeled with saturation current IBEN and ideality factor NEN 2.
The collector current is expressed as:
V BE IE RE IB RB
ICC ¼ IS exp
1
N F V tv
ð2Þ
where IS is the transport saturation current and NF is the forward
ideality factor.
2.2.2. Inverse-mode Gummel plot
In inverse mode, the base current IBC and emitter current IEC are
measured by varying the bias voltage VBC across the base–collector
poles with VBE = 0. The hole current injected from the heavily
doped base into the collector is the dominant component flowing
the base–collector homojunction. The relatively low electron current is injected from collector to the base to form the base current
IBC and the emitter current IEC, which is only a small fraction.
Hence, it is reasonable to assume IEC IBC.
The emitter current IEC is followed by:
V BC þ IC RC IB RB
IEC ¼ IS exp
1
N R V tv
ð3Þ
where the relation VBCi = VBC + ICRC IBRB is used similar to Eq. (1),
NR is the reverse ideality factor. And RC is the parasitic series resistance of collector region.
The base current consists of two main components: (1) recombination of electron-hole pair in the bulk of the base–collector
space charge region (BC-SCR) and along its periphery and (2)
recombination of injected holes in the bulk of neutral collector region (NCR) and at the surface [7]. The base current can be expressed similarly to Eq. (1):
IBC ¼ IBCi þ IBCn
V BC þ IC RC IB RB
1
¼ IBCI exp
N CI V tv
V BC þ IC RC IB RB
1
þ IBCN exp
NCN V tv
ð4Þ
where IBCi is modeled with saturation current IBCI and ideality factor
NCI 1, and IBCn modeled with saturation current IBCN and ideality
ð1Þ
Table 1
Values of model parameters before irradiation, after irradiation and after annealing of
10 Mrad(Si).
Parameters
Pre-irradiation
Post-irradiation
Post-annealing
IS (A)
NF
IBEI (A)
NEI
IBEN (A)
NEN
IBCI (A)
NCI
IBCN (A)
NCN
NR
RE (X)
RB (X)
RC (X)
RTH (X)
1.84 1025
1.02
1.489 1026
1.073
1.77 1015
3.033
1.495 1020
1.24
5.055 1014
2.084
1.004
3.486
10.1
3.94
1400
7.439 1025
1.042
3.254 1026
1.084
3 1014
3.306
4.485 1020
1.265
7.824 1014
2.105
1.026
3.486
10.1
4.04
1400
9.13 1025
1.047
5.132 1026
1.095
8.02 1014
3.473
2.242 1019
1.309
6.041 1011
2.897
1.03
3.486
10.1
4.94
1400
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
3
Fig. 2. Measured and modeled forward Gummel plots (a) before irradiation, (b) after irradiation of 10 Mrad(Si), and (c) after annealing of 10 Mrad(Si).
factor NCN 2, which represent the recombination currents in the
NCR and the BC-SCR, respectively.
2.2.3. DC common-emitter collector current–voltage (IC–VCE)
characteristics
In most practical application HBTs are used as current amplifiers in the common-emitter configuration. In the saturation mode,
both base–collector junction and base–emitter junction in
forward-biased, it can be obtained from Fig. 1 that the collector
current IC can be written as:
IC ICC IBC
ð5Þ
with
V CEðsatÞ ¼ V BE V BC
ð6Þ
To obtain Eq. (5), the relation IEC IBC has been made in the inverse
mode.
In the active region, IC is expected to be constant with respect to
the increase of VCE. However, our measurements of IC–VCE characteristics show a significant decrease of IC with VCE. It is believed
that the phenomenon is caused by the self-heating effect modeled
with the thermal R network as shown on the right of Fig. 1.
3. Modeled results and discussions
Extraction of the forward Gummel current parameters is as follows. The IBEN and NEN can be determined from the intercept and
the slope of log(IBE)–VBE plot in the region of low VBE, and then
the values of IBEI and NEI are easily obtained by fitting the log(IBE)–VBE curve in the high injection region. The IS and NF are found
the same way from the measurement of ICC.
For extraction of the reverse Gummel current parameters, similar method with the forward parameter extraction, IBCI, IBCN, NCI
and NCN can be extracted from the measured IBC, and NR from the
measured IEC.
To extract resistor parameters, the RE is obtained with the flyback technique, in which the emitter is grounded and the base is
stimulated with a current in strong saturation. The open circuit
collector voltage is measured. The emitter resistance is taken as
the slope of the linear segment of the curve. The RC is achieved with
the similar method to RE. The RB is extracted from cold S-parameter
data, by extrapolating S11 on a circle of constant impedance to the
intersection with the real axis in the Smith Chart, the value associated with the intersection being equal to the sum of RB and RE. The
RTH is determined by optimizing the fit to high current data in
which output curve shows obvious self heating.
The obtained parameter values are listed in Table 1 for pre-irradiation, post-irradiation and post-annealing of 10 Mrad(Si). It
seems that parameters do not change much among the pre-irradiation, post-irradiation and post-annealing. Because the most susceptible transistor materials to be sensitive to total dose effect
are insulators, the SiN insulator instead of oxides in the GaAs HBTs
does not show serious degradation to total dose effect [13].
3.1. Forward-mode Gummel plot
Fig. 2a–c shows the measured Gummel plots in the forward
mode and the model predictions for pre-irradiation, post-irradiation and post-annealing of 10 Mrad(Si), respectively. Excellent
agreement between the experimental and the modeled in the wide
bias range. However, at low bias, the modeled collector current is
smaller than the experimental data. It is believed that the extra
component of the collector current is simply the leakage current
originated from the electron and hole leakage through the
dielectric-layer (e.g., polyimide, nitride, etc.) interface at the emitter–base and base–collector peripheries, as well as through the n+subcollector/semi-insulating substrate interface [14]. Similar phenomena can be observed between modeled and experimental base
current. However, one can optimize the fit of forward Gummel
plots with the values of current gain (beta = ICC/IBE) measured as
the norm. We believe that such an approach is very effective to
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
4
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
Fig. 3. Measured and modeled beta (a) before irradiation, (b) after irradiation of 10 Mrad(Si), and (c) after annealing of 10 Mrad(Si).
Fig. 4. Forward Gummel plots for the (a) base current and (b) collector current before irradiation, after irradiation and after annealing of 10 Mrad(Si).
understand the physical mechanism responsible for the observed
radiation induced changes in forward Gummel plots. The comparisons between modeled and experimental beta are shown in Fig. 3.
Fig. 4a and b shows the plots of IBE and ICC before irradiation,
after irradiation and after annealing of 10 Mrad(Si), respectively.
Degradation of beta is demonstrated in Fig. 5. It can be seen from
Fig. 4, there is a significant increase of the base current at low
VBE after irradiation and further increase after annealing, which
in turn causes the degradation of beta. In Table 1, the small changes
of IBEI and NEI imply that the recombination due to radiation induced traps in the NBR is not very significant. On the other hand,
the large change in IBEN and NEN suggests that radiation induced defects in base–emitter junction mostly appear in the BE-SCR. So, it
can be concluded that the degradation of beta is mainly due to
the irradiation induced defects in BE-SCR.
There are two possible recombination mechanisms in the BESCR to be consistent with the measured base current ideality factor
NEN > 2. One mechanism is trap-assisted tunneling due to gamma
radiation induced traps. A second possible mechanism is the
Fig. 5. Current gain (beta) for pre-irradiation, post-irradiation and post-annealing
of 10 Mrad(Si).
recombination from a nonuniform distribution of Shockley–
Read–Hall centers within the BE-SCR.
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
5
Fig. 6. Measured and modeled inverse Gummel plots (a) before irradiation, (b) after irradiation of 10 Mrad(Si), and (c) after annealing of 10 Mrad(Si).
Fig. 7. Inverse Gummel plots for the (a) base current and (b) emitter current before irradiation, after irradiation and after annealing of 10 Mrad(Si).
3.2. Inverse-mode Gummel plot
The measured Gummel plots and simulation results in the inverse mode are shown in Fig. 6a–c for pre-irradiation, post-irradiation and post-annealing of 10 Mrad(Si), respectively. The good
agreement between the modeled and the measured data shows
the accuracy of the extracted parameters.
The plots of IBC and IEC before irradiation, after irradiation and
after annealing of 10 Mrad(Si) are shown in Fig. 7a and b, respectively. In Table 1, it can be seen that the relatively smaller changes
in the base–collector junction parameters (IBCI, NCI, IBCN and NCN)
after irradiation imply that gamma irradiation has not affected
base–collector junction immediately. However, the relatively
larger changes in these parameters after annealing show that radiation induced defects in base–collector junction may mainly consist of slow interface states which become fast interface states
with increased annealing time, leading to the degradation of
base–collector junction. The model parameters extracted also
show that the changes in IBCI and NCI are small compared with
the changes in IBCN and NCN, respectively, which suggests that
radiation induced defects in the BC-SCR play a dominant role in
the degradation of the devices, and then we can conclude that
the increase steady of IBC at low VBC, as shown in Fig. 7, is mainly
due to the radiation induced damage in the BC-SCR.
3.3. DC common-emitter collector current–voltage (IC–VCE)
characteristics
Fig. 8a–c shows the measured and modeled IC–VCE characteristics at the base current of 10 lA, 20 lA, 30 lA, 40 lA and 50 lA
before irradiation, after irradiation and after annealing of
10 Mrad(Si), respectively. To further validate the applicability of
the proposed simplified VBIC static model, we have compared
modeled with measured VBE corresponding to the IC–VCE characteristics, as shown in Fig. 9. The excellent agreement between the
modeled and the measured results shows that the presented static
model makes the simulation robust and effective.
Fig. 10 shows the IC–VCE characteristics of SHBT before irradiation, after irradiation and after annealing of 10 Mrad(Si). One major
effect is evident from Fig. 10 that the collector–emitter saturation
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
6
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
Fig. 8. Measured and modeled IC–VCE characteristics (a) before irradiation, (b) after irradiation of 10 Mrad(Si), and (c) after annealing of 10 Mrad(Si).
Fig. 9. Measured and modeled VBE corresponding to the IC–VCE characteristics (a) before irradiation, (b) after irradiation of 10 Mrad(Si), and (c) after annealing of 10 Mrad(Si).
voltage VCE(sat) increases after annealing. The possible reason is that
the gamma-irradiation induces slow interface states, which are located in the insulating passivation layer to be far way from device
active region, to make charge exchange with semiconductor in a
long time, that in turn causes the degradation of VCE(sat) with increased annealing time.
From the analysis above, it can be obtained that ICC and IBC are
associated with VBE and VBC, respectively. VCE(sat) may be given
graphically by the voltage horizontal separation between ICC and
IBC at the desired values of the base current and collector current,
e.g., IB = 10 lA and IC = 0.5 mA, as illustrated in Fig. 11, where
two horizontal lines, i.e., line A and line B, are drawn at the values
of VBE and VBC which are respectively determined from Figs. 9 and
8. From Fig. 11, we conclude that the increase of IBC after annealing
is responsible for the decrease of VBC for a fixed IC, that in turn
causes the observed increase of VCE(sat). In other words, the increase
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020
J. Zhang et al. / Microelectronics Reliability xxx (2012) xxx–xxx
7
and increase in the base currents from inverse Gummel in postannealing. The changes of model parameters show that the radiation induced defects are mainly located in the BE-SCR and the BCSCR, and also show that the defects in the base–collector junction
may mainly consist of slow interface states.
Acknowledgment
This work is supported by the National Basic Research Program
of China with Grant No. 2010CB327505.
References
Fig. 10. IC–VCE characteristics for pre-irradiation, post-irradiation and post-annealing of 10 Mrad(Si).
Fig. 11. Measured IBC and ICC plots before irradiation, after irradiation and after
annealing of 10 Mrad(Si).
of VCE(sat) is mainly caused by the irradiation induced defects in the
BC-SCR.
4. Conclusions
We have studied the effects of gamma irradiation on the DC characteristics of InGaP/GaAs SHBTs with a proposed simplified VBIC
static model. The parameters of the model are extracted from the
measured results. This study shows that the main degradations
are reduction of current gain after irradiation and further degradation after annealing, significant increase of VCE(sat) in post-annealing
[1] Vuppala S, Li CS, Zwicknagl P, Subramanian S. Neutron, proton, and electron
irradiation effects in InGaP/GaAs single heterojunction bipolar transistors. IEEE
Trans Nucl Sci 2003;50(6):1846.
[2] Díez S, Lozano M, Pellegrini G, Campabadal F, Mandic I, Knoll D, et al. Proton
radiation damage on SiGe: C HBTs and additivity of ionization and
displacement effects. IEEE Trans Nucl Sci 2009;56(4):1931–6.
[3] Phillips SD, Sutton AK, Appaswamy A, et al. Impact of deep trench isolation on
advanced SiGe HBT reliability in radiation environments. In: IEEE CFP09RPSCDR 47th annual international reliability physics symposium. Montreal; 2009.
p. 157–64.
[4] Ricet JS, Ullan M, Brooijmans G, Cressler JD, Damianit D, Dier S, et al.
Performance of the SiGe HBT 8HP and 8WL technologies after high dose/
fluence radiation exposure. In: IEEE nuclear science symposium conference
record. Guildford; 2008. p. 2206–10.
[5] Zhang SM, Niu GF, Cressler JD, et al. A Comparison of the effects of gamma
irradiation on SiGe HBT and GaAs HBT technologies. IEEE Trans Nucl Sci
2000;47(6):2521–7.
[6] Bandyopadhyay A, Subramanian S, Chandrasekhar S, Dentai AG, Goodnick SM.
Degradation of InGaAs/InP double heterojunction bipolar transistors under
electron irradiation. IEEE Trans Electron Dev 1999;46(5):850–8.
[7] Bandyopadhyay A, Subramanian S, Chandrasekhar S, Dentai AG, Goodnick SM.
Degradation of DC characteristics of InGaAs/InP single heterojunction bipolar
transistors under electron irradiation. IEEE Trans Electron Dev
1999;46(5):840–9.
[8] Cao XC, McMacken J, Stiles K, Layman P, et al. Comparison of the new VBIC and
conventional Gummel–Poon bipolar transistor models. IEEE Trans Electron
Dev 2000;47(2):427–33.
[9] <http://www.designers-guide.org/VBIC/>, 2011.
[10] Cherepko SV, Hwang JCM. VBIC model applicability and extraction procedure
for InGaP/GaAs HBT. In: Proceedings of APMC. Taipei; 2001. p. 716–21.
[11] Liu W. In: Handbook of III–V heterojunction bipolar transistors. John Wiley &
Sons; 1998.
[12] Liou JJ. Calculation of the base current components and determination of their
relative importance in AlGaAs/GaAs and InAlAs/InGaAs heterojunction bipolar
transistors. J Appl Phys 1991;69(5):3328–34.
[13] Weatherford TR. Radiation effects in high speed III–V integrated circuits. Int J
High Speed Electron Syst 2003;13(1):277–92.
[14] Liou JJ, Huang CI, Bayraktaroglu B, Williamson DC, Parab KB. Base and collector
leakage currents of AlGaAs/GaAs heterojunction bipolar transistors. J Appl
Phys 1994;76(5):3187–93.
Please cite this article in press as: Zhang J et al. The model parameter extraction and simulation for the effects of gamma irradiation on the DC characteristics of InGaP/GaAs single heterojunction bipolar transistors. Microelectron Reliab (2012), http://dx.doi.org/10.1016/j.microrel.2012.07.020