Reliability Study of Silicon Germanium HBT
Yogesh Kumar Verma and Dr. R.K. Chauhan
Department of Electronics and Communication Engineering
Madan Mohan Malaviya Engineering College
Gorakhpur-273010, U.P, INDIA
Abstract: In this paper, a thorough study of the potential
of SiGe HBT to extreme environment conditions is
done. The extreme environment conditions affect the
reliability of SiGe HBT. The reliability issues of SiGe
HBT are classified into two categories, extrinsic and
intrinsic. The extrinsic reliability issues are very low
temperature conditions, very high temperature
conditions, space, high vibration environment, low
pressure environment, and caustic or chemically
corrosive environment. Very low temperatures mean
temperatures below 70K or even 4.2K. Very high
temperatures mean temperatures up to 473K or 573K.
In this research paper the effects of very high
temperature conditions and the radiation hardness of
SiGe HBT has been examined using SILVACO-ATLAS
Device Simulator. The ionizing radiation is varied from
Pre radiation to 1 Mrad (Si) total dose.
Keywords-: Silicon Germanium HBT,
extreme environment, radiation hardness.
in
various
applications
as
radar
systems,
communications, THz sensing, Imaging and
communications, analog applications, electronic
warfare and the most important domain of “mixedsignal” ICs. The cross-sectional view of SiGe HBT et
al [2] is shown in figure 1.
reliability,
I. INTRODUCTION
Silicon Germanium Heterojunction Bipolar Transistors
are vertical transport devices. The overall performance
of Silicon Germanium HBT is more closely tied to the
vertical profile and the resultant Ge profiles used. The
performance is more conveniently defined by the
transistor-level maximum small-signal frequency
response. According to Sir J.D. Cressler, et al [1],
commercially there exists four generations of SiGe
HBT technology. These generations are classified as
first generation, second generation, third generation,
and fourth generation. First generation comprises of
peak unity gain cut-off frequency of 50 GHz, followed
by 100-120 GHz of second generation SiGe HBT
technology. Third generation has peak unity-gain cut
off frequency of 200 GHz range. Fourth generation also
called as research level has record peak unity-gain
cutoff frequency of 350 GHz. SiGe HBT technologies
exists
in
BiCMOS
implementation
(SiGe
HBT+SiCMOS). This BiCMOS implementation of
SiGe HBT provides the optimum advantages of its
potential strength and makes it an excellent contender
Figure 1: Cross-sectional view of SiGe HBT
II. RELIABILITY STUDY OF SiGe HBT
The reliability of a system is defined as the ability of
the system to perform successfully in both expected
and unexpected circumstances. The major reliability
issues of SiGe HBTs are classified into two types:
intrinsic and extrinsic. The intrinsic reliability issues
are generated in the device itself. The extrinsic
reliability issues are caused due to external factors.
The extrinsic reliability issues of SiGe HBT are:
1. Very Low operating Temperature.
2. Very High operating Temperature.
3. Space.
4. High Vibration Environment.
5. Low Pressure Environment.
6. Caustic or chemically corrosive Environment.
The intrinsic reliability issues of SiGe HBT are:
1. Self Heating Effects.
There are different reliability issues for different devices.
Consider a capacitor, its main reliability issues are
operating temperature, ripple current, inrush current and
operating voltage. In the same fashion for SiGe HBT,
temperature and radiation effects are the solely key
concerns. According to Sir J. D. Cressler, et al [1],
reliability issues of SiGe HBTs are low temperature
conditions, high temperature conditions and radiation rich
environment. It is proposed that SiGe HBT can
successfully operate under all these three extreme
environment conditions. Sir J. D. Cressler et al [1] also
proposed that reliability issues for mixed circuit
applications of SiGe HBTs are thermal effects; impact
ionization induced bias point instabilities, and operating
voltages. Electronics operating in harsh surroundings
falling outside the domain of conventional circuit
specifications
is
called
“extreme
environment
electronics”. SiGe HBT has ability to operate in extreme
environment. Extreme environment is the important
profitable corner of the market for electronics. Extreme
environment includes very low operating temperatures,
very high operating temperatures, and radiation rich
environment. Very low temperatures mean temperatures
below 70K or even 4.2K. Very high temperatures mean
temperatures up to 473K or 573K. The unique band gap
engineered feature of SiGe HBT offers great potential to
make it operate successfully without need of any
modification. This ultimately makes Silicon Germanium
HBT economically strong at both IC and system level.
This unique feature of SiGe HBT to operate successfully
outside the domain of conventional commercial
specifications maintains its reliability.
profile with optimum total Ge concentration of 14 % at
300 K is used in this work. After 14 % Ge concentration
there is no significant reduction in base transit time. This
optimized profile makes it possible to improve the high
frequency performance of the device. Figure 2 shows the
structure of SiGe HBT with base having triangular Ge
profile with optimum total Ge concentration of 14 % at
300 K. The base transit time at 14 % Ge concentration is
0.42 pS at room temperature. The structure of SiGe HBT
having triangular Ge profile with optimum total Ge
concentration of 14 % at 300 K is prepared by SILVACO
ATLAS Device Simulator et al [3]. The effects of
extrinsic reliability issues: very low temperature
conditions are analyzed on Fourth Generation SiGe HBT.
Figure 2: Structure of SiGe HBT
III. RESULTS AND DISCUSSION
Effects of Extrinsic reliability issues on SiGe HBTs
After the introduction of Ge into the base of Si BJT, the
potential barrier to injection of electrons from emitter into
the base is decreased. Intuitively, this will yield
exponentially more electron injection for the same applied
emitter to base voltage translating into higher collector
current and hence higher current gain in the device. The
base current remains unchanged. The introduction of Ge
into the base of Si BJT generates an accelerating electric
field. This accelerating electric field from graded base
band gap decreases base transit time. Minimum is the
device base transit time, better is the device. In SiGe
HBTs having triangular Ge profile, after 14 % Ge
concentration at room temperature base transit time does
not reduce significantly. It has been analyzed that only 0.1
pS reduction in base transit time occurs when % Ge
concentration is increased by 10 %. The triangular Ge
The extrinsic reliability issues of SiGe HBTs are very
low operating temperature, very high operating
temperature, space, high vibration environment, low
pressure environment and caustic or chemically
corrosive environment.
A. VERY HIGH TEMPERATURE OPERATION OF SiGe
HBTs
In Si BJT, current gain is positive temperature
coefficient of temperature. The current gain of SiGe
HBT does indeed have an opposite temperature
dependence. The changes in β between 300 K and 473 K
are maximum 33 %, and clearly are not cause for alarm
for any realistic circuit. The problem of thermal-runaway
in high power Si BJTs is the result of the positive
temperature coefficient of β. The fact that SiGe HBTs
naturally have a negative temperature coefficient of β
suggests that this might present interesting opportunities
for power amplifiers. SiGe HBTs work just fine to 125˚C
has at present generally been accepted. The applicability
of SiGe HBTs to emerging extreme environment
applications at considerably high temperatures (say to
300 degree Celsius) has only very recently begun to be
seriously contemplated. At present, the device
technologies deployed for such high temperature
applications typically include SOI CMOS, GaAs, SiC,
and GaN, all of which are expensive and in some cases
immature.
KT components in device equations provide their
favorable impact only on cooling. When temperature is
increased, there is degradation in both DC and AC
properties of SiGe HBTs. The degree of degradation is
quite acceptable up to 398 K.
Table 2
Change in Parameters
Increment in temperature from 300 K to 473 K
Peak 𝑓𝑇 (GHz)
Decreases by 29 %
Peak 𝑓𝑚𝑎𝑥 (GHz)
Decreases by 23 %
β
Decreases by 25 %
Table 2 summarizes the variation in Peak β, Peak 𝑓𝑚𝑎𝑥
and Peak 𝑓𝑇 of fourth generation SiGe HBT having
triangular Ge profile with optimum total Ge
concentration of 14 % after very high temperature
operation. The peak cut off frequency of SiGe HBT
decreases by 29 %, when the temperature of the device is
increased from 300 K upto 473 K. The maximum
oscillation frequency of SiGe HBT decreases by 23 %,
when temperature of device is increased from 300 K upto
473 K. The current gain decreases by 25 %, when
temperature of device is increased from 300 K upto 473
K. Thus, SiGe HBTs suffers degradation in its
performance parameters when temperature is increased.
But, the degradation is within the limits and the device
can adequately perform for practical circuit applications.
B. RADIATION HARDNESS OF SiGe HBTs
I. Variation in β with ionizing radiation
In figure 1, the variation in current gain β with
ionizing radiation is observed using SILVACOATLAS Device Simulator. The ionizing radiation is
varied from Pre radiation to 1 Mrad (Si) total dose. It
can be noticed that the Peak value of current Gain β
degrades by 16.28% after 1 Mrad (Si) gamma total
dose. The main reason of degradation of current gain
β with ionizing radiation is the generation of
radiation-induced generation-recombination centers
in the low bias region of SiGe HBT.
The current gain of SiGe HBT is given by equation
(1) et al [1],
𝜂ϒ∆(𝐸𝑔,𝐺𝑒(𝑔𝑟𝑎𝑑𝑒) )/𝐾𝑇 𝑒 (∆𝐸𝑔,𝐺𝑒(0))/𝐾𝑇
𝛽𝑆𝑖𝐺𝑒
=(
)
𝛽𝑆𝑖
1 − 𝑒 −(∆𝐸𝑔,𝐺𝑒(𝑔𝑟𝑎𝑑𝑒))/𝐾𝑇
From above equation, it can also be noticed that the
thermal energy (KT), in equation of Current Gain is
inherently arranged not intentionally designed in a
(1)
manner to improve the Current Gain of SiGe HBT
with cooling.
Variation in β with ionizing radiation
900
800
700
600
500
400
300
200
100
0
0
100
200
300
400
500
600
700
800
900
1000
Figure 1: Variation in Current Gain (along Y axis) with ionizing radiation (along X axis)
Variation in cut off frequency (GHz) with ionizing
radiation
400
350
300
250
200
150
100
50
0
0
100
200
300
400
500
600
700
800
900
Figure 2: Variation in Cutoff frequency (along Y axis) with ionizing radiation variation (along X axis)
1000
II. Variation in 𝒇𝑻 (GHz) with ionizing radiation
III. Variation in 𝒇𝒎𝒂𝒙 (GHz) with ionizing radiation
In figure 2, the variation in Cut off frequency with
ionizing radiation is observed using SILVACOATLAS Device Simulator. The ionizing radiation is
varied from Pre radiation to 1 Mrad (Si) total dose. It
can be noticed that the Cuttoff frequency degrades by
4.175 % after 1 Mrad (Si) gamma total dose
irradiation. The degradation in Cutoff frequency is
due to the increment in the recombination rate per
carrier after irradiation. So it causes the reduction in
cut off frequency of the device.
In figure 3, the variation in Maximum Oscillation
frequency with ionizing radiation is observed. The
ionizing radiation is varied from Pre radiation to 1
Mrad (Si) total dose. The degradation in Peak
Maximum oscillation frequency after irradiation is
due to two factors. First factor is the increment in the
recombination rate per carrier after irradiation and
second factor is the degradation of Peak Cutoff
frequency up to 1 Mrad (Si) total dose Gamma
irradiation.
Variation in maximum oscillation frequency (GHz) with
ionizing radiation
400
350
300
250
200
150
100
50
0
0
100
200
300
400
500
600
700
800
900
1000
Figure 3: Variation in Maximum Oscillation Frequency (along Y axis) with ionizing radiation (along X axis) variation from Pre
radiation to 1Mrad (Si) total dose gamma irradiation
IV. Variation in Base Transit Time τ with ionizing radiation
In figure 4, the variation in Base Transit Time with
ionizing radiation is observed using SILVACOATLAS Device Simulator. The ionizing radiation is
varied from Pre radiation to 1 Mrad (Si) total dose
gamma irradiation.
The Base Transit Time is given by equation (2) et al
[1],
𝜏𝑏,𝑆𝑖𝐺𝑒 2
𝐾𝑇
𝐾𝑇
=
[1 −
[1 − 𝑒 (−∆𝐸𝑔,𝐺𝑒(𝑔𝑟𝑎𝑑𝑒))/𝐾𝑇 ]]
𝜏𝑏,𝑆𝑖
𝜂 ∆𝐸𝑔,𝐺𝑒(𝑔𝑟𝑎𝑑𝑒)
∆𝐸𝑔,𝐺𝑒(𝑔𝑟𝑎𝑑𝑒)
From equation (8), it can be noticed that the thermal
energy (KT), in equation of Base Transit Time is
inherently arranged not intentionally designed in a
(2)
manner to improve the Base Transit Time of Silicon
Germanium Heterojunction Bipolar Transistor with
cooling.
Variation in τ (pS)with ionizing radiation
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
100
200
300
400
500
600
700
800
900
1000
Figure 4: Variation in τ (along Y axis) with ionizing radiation (along X axis) variation from Pre radiation to 1Mrad (Si) total dose
gamma irradiation
In figure 4, the variation in Base Transit Time with
ionizing radiation is observed. The ionizing radiation
is varied from Pre radiation to 1 Mrad (Si) total dose.
It can be noticed that the in SiGe HBT Base Transit
Time increases by 7.1% after 1 Mrad (Si) gamma
total dose. Base Transit Time of SiGe HBT is
calculated by the y-intercept of the plot between
and
1
𝐼𝐶
. The y-intercept increases after 1 Mrad (Si)
total dose Gamma irradiation because after 1 Mrad
(Si) total dose Gamma irradiation 𝑓𝑇 decreases
which in turn increases the term
particular value of
1
1
𝐼𝐶
1
2𝛱𝑓𝑇
.
2𝛱𝑓𝑇
Summary of effect of Gamma Irradiation on various parameters of SiGe HBT at 300 K
Table 3
Gamma Irradiation
Peak value of β
𝑓𝑇 (GHz)
𝑓𝑚𝑎𝑥 (GHz)
τ (pS)
Pre-Rad
827
309
343
0.42
100 Krad (Si)
820
297
325.85
0.44
1 Mrad (Si)
692.36
296.09
300.67
0.45
for each
The radiation hardness of SiGe HBT after exposure
to ionizing radiation has been examined. Table 3
shows that in SiGe HBT Base Transit Time increases
by 7.1% up to 1 Mrad (Si) Gamma total dose. Cuttoff
frequency degrades by 4.175% up to 1 Mrad (Si)
Gamma total dose. Maximum Oscillation Frequency
degrades by 12.34% up to 1 Mrad (Si) Gamma total
dose. Peak value of current Gain degrades by 16.28%
up to 1 Mrad (Si) Gamma total dose.
IV. CONCLUSION
Extreme environment includes operation to very low
temperatures, operation at very high temperatures and
operation in radiation rich environments. The unique
band gap engineered features of SiGe HBTs offer
great potential to simultaneously satisfy all three
extreme environment applications, potentially with
little or no process modification. This ultimately
provides compelling cost advantages at the IC and
system level. The effects of very high temperature on
the performance of SiGe HBT is examined. It has
been observed that performance parameters of SiGe
HBT gets degraded with working limit at very high
temperatures, and therefore the device can adequately
perform for practical circuit applications. It has been
observed that SiGe HBTs can successfully operate up
to 398 K.
The radiation hardness of SiGe HBT has also been
examined. In SiGe HBT Base Transit Time increases
by 7.1% up to 1 Mrad (Si) gamma total dose. Cuttoff
frequency degrades by 4.175% up to 1 Mrad (Si)
gamma total dose. Maximum Oscillation Frequency
degrades by 12.34% up to 1 Mrad (Si) gamma total
dose. Peak value of current Gain degrades by 16.28%
up to 1 Mrad (Si) gamma total dose. Thus unique
feature of SiGe HBT to operate successfully outside
the
domain
of
conventional
commercial
specifications maintains its reliability.
[5]. Ankit Kashyap and R.K. Chauhan , “Profile
Design Optimization of SiGe Heterojunction Bipolar
Transistors for High Speed Applications”, Journal of
Computational and Therotical Nanoscience, Volume
5, 2238-2242, 2008.
[6]. Ankit Kashyap and R.K. Chauhan, “Effect of the
Ge Profile design on the performance of an
n-p-n
SiGe HBT-based analog circuit”, Microelectronics
journal, MEJ: 2554, (2008).
[7]. Bhaskar Banerjee, Sunitha Venkatraman, ChangHo Lee, and Jay Laskar, “Broadband Noise Modeling
of SiGe HBT under Cryogenic Temperatures”, IEEE
Radio Frequency Integrated Circuits Symposium,
pages pp 765-768, 2007.
[8]. J. Kuchenbecker, M. Borgarino, L. Bary, G.
Cibiel, O. Llopis, J. G. Tartarin, S. Kovacic, J.L.
Roux, and R. Plana, “Reliability Investigation in
SiGe HBTs”, IEEE, pages pp 131-pp 134, 2001.
[9]. V.S. Patri and M. Jagadesh Kumar, “Novel Geprofile design for high-speed SiGe HBTs: modeling
and analysis”, IEEE Proc-Circuits Devices Syst.,
VOL. 146, NO. 5, pages pp 291-pp 296, October
1999.
[10]. John D. Cressler, “Silicon-Germanium as an
Enabling Technology for Extreme Environment
Electronics”, IEEE Transactions on Device and
Materials Reliability, VOL. 10, NO. 4, pages pp 437pp 448, December 2010.
[11]. Yogesh Kumar Verma and Dr. R.K.Chauhan,
"Study of Radiation and Temperature Effects on
Devices with Si and SiGe Base" ISSN: 2249-1945,
GJCAT, Vol2 (1), 2012, pp 917-pp 920.
[12]. Yogesh Kumar Verma and Dr. R.K. Chauhan,
"Reliability issues of SiGe HBTs", National
Conference, Advances in Computer Communication
and Embedded Systems, pp 248-pp 251, 2012.
AUTHORS
V. REFERENCES
[1]. John D. Cressler “On the potential of SiGe HBTs
for Extreme Environment Electronics”, Proceedings
of the IEEE, VOL93, No.9, pages pp 1559-1582,
SEPTEMBER 2005.
[2]. Peter Ashburn, “SiGe Heterojunction Bipolar
Transistors”, John Wiley & Sons Publication, 2003.
[3]. ATLAS User’s Manual Device Simulation
Software, SILVACO International, 2004.
[4].
Frank Larin,
“Radiation Effects
in
Semiconductor Devices”, John Wiley & Sons
Publication.
Er. Yogesh Kumar Verma born on 25 June 1988
received his B.Tech. Degree in Electronics and
Communication Engineering from Sharda Group of
Institutions, Mathura in 2009 and M.Tech. Degree in
Electronics and Communication Engineering with
specialization in Digital Systems from Madan Mohan
Malaviya Engineering College, Gorakhpur in 2012.
His research interest includes Device modeling in
general and Modeling and Simulation of HBT based
devices in particular. His area of interest also
includes analysis of Mixed Signal Circuits and
Integrated Circuits. He is currently working as
Assistant
Professor
in
Electronics
and
Communication Engineering Department in Noida
Institute of Engineering and Technology, Greater
Noida.
Contact: 07503697453
R. K. Chauhan was born in Dehradoon, India in
1967. He received the B.Tech. degree in Electronics
& Communication Engineering, from G.B.P.U.A.T
Pantnagar, in 1989 and M.E. in Control &
Instrumentation, from MNNIT-Allahabad in 1993
and Ph.D in Electronics Engineering, from IT-BHU,
Varanasi, INDIA in 2002. He joined the department
of ECE, Madan Mohan Malviya Engineering
College, Gorakhpur, India as a lecturer, in 1993, as
an Assistant Professor since 2002 and thereafter as an
Associate Professor since Jan, 2006 to till date in the
same institute. He also worked as a Professor in
Department of ECE, Faculty of Technology, Addis
Ababa University, Ethiopia between 2003 to 2005.
He is reviewer of Microelectronics Journal, CSP etc.
His research interests include device modeling and
simulation of MOS, CMOS and HBT based circuits.
He was selected as one of top 100 Engineers of 2010
by International Biographical Centre, Cambridge,
England.
Ph: +91-9235500556