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Temperature dependence of DC characteristics of NpN InP/GaAsSb/InP
double heterojunction bipolar transistors: an analytical study
Yuan Tian, Hong Wang *
School of Electrical and Electronics Engineering, Microelectronics Centre, Nanyang Technological University, Singapore 639798, Republic of Singapore
Received 28 June 2005; received in revised form 19 September 2005; accepted 23 September 2005
Available online 21 November 2005
Abstract
An analytic study of DC characteristics based on the drift-diffusion approach has been performed for the InP/GaAsSb DHBTs. The current
transport of InP/GaAsSb/InP DHBTs has been investigated focusing the device temperature dependence. Our simulation results show that, at
room temperature, the DC characteristics of the InP/GaAsSb/InP DHBTs similar to the conventional InP-based HBT using InGaAs as the base
layer although a type-II energy band alignment is presented in the InP/GaAsSb HBT. However, due to different mechanisms for the electron
injection from the emitter induced by the different conduction band alignments, the InP/GaAsSb HBTs may present a different temperature
dependent behavior in term of device current gain as compared to the conventional InP/InGaAs HBTs. Higher current gain could be achieved by
the InP/GaAsSb HBTs at elevated temperature.
q 2005 Elsevier Ltd. All rights reserved.
Keywords: InP/GaAsSb HBTs; Current mechanisms; Current gain; Temperature dependence
1. Introduction
InP/GaAsSb/InP double heterojunction bipolar transistors
(DHBTs) have been proposed and extensively studied [1,2] as a
new alternative for InP-based DHBTs. In comparison with the
conventional InP/InGaAs HBTs, the conduction band edge of
GaAs0.51Sb0.49 base is about 0.18 eV higher than that for InP
[3], which forms type-II staggered heterojunction. Using the
GaAsSb as the base layer results in several significant
advantages over the conventional InP/InGaAs/InP DHBT
scheme, such as elimination of current blocking effect due to
the staggered band line-up over InP, no hydrogen passivation
issue, and launching electrons from base into collector with
high initial velocity. In addition, the GaAsSb material has the
additional advantage that it can be easily p-doped with carbon
to levelsO1020 cmK3 [4]. Recently, some works have reported
the experimental results on DC characteristics of InP/GaAs0.51-Sb0.49/InP [5,6].
During the past few years, analytical and numerical analysis
have been intensively performed on AlGaAs/GaAs [7], InP/
InGaAs [8] HBTs leading to significant insight into structure
0026-2692/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.mejo.2005.09.004
* Corresponding author. Tel.: C65 67904358; fax: C65 67912687.
E-mail address: [email protected] (H. Wang).
design and physic mechanism on transistor characteristics.
However, although InP/GaAsSb/InP DHBTs with excellent
microwave performance have been demonstrated, detailed
analysis of structure design and material properties on the
device performance is still lacking. In this work, the current
transport mechanism in NpN InP/GaAs0.51Sb0.49/InP DHBT
and its impact on the device DC performance such as current
gain have been study. The temperature dependence of device
DC characteristics is evaluated. An analytical comparison of
the DC performance between the conventional InP/InGaAs/InP
DHBTs and the one using GaAsSb as the base is presented.
2. Device structure and physical model
The device layer structure used for simulation is given in
Table 1. The equilibrium band diagram for InP/InGaAs/InP
and InP/GaAsSb/InP DHBTs are sketched in Fig. 1. The major
difference between the two structures is the base-emitter (B–E)
energy band alignment. InP/InGaAs forms a type-I hetero-
junction (Fig. 1(a)) while InP/GaAsSb forms the type-II
staggered heterojunction (Fig. 1(b)). Therefore, the conduction
band spike at B–E junction interface seen in the InP/InGaAs is
not present in the InP/GaAsSb HBT.
To model the carrier transport in the devices, the one-
dimensional drift-diffusion formulation is applied with the
Possion’s equation. Compared to the conventional InP/InGaA-
s/InP DHBTs, the major difference for the InP/GaAsSb
Microelectronics Journal 37 (2006) 595–600
www.elsevier.com/locate/mejo
Table 1
DHBT device structure used in our calculation
Layer Doping (cmK3) Material Thickness (mm)
Emitter neZ3!1017 InP tEZ0.1
Base pbZ2!1019 GaAsSb or
InGaAs
tBZ0.05
Collector ncZ5!1016 InP tCZ0.4
Subcollector nscZ5!1018 InP tSCZ0.2
Y. Tian, H. Wang / Microelectronics Journal 37 (2006) 595–600596
structure is the absence of the conduction band spikes at the
emitter–base (E–B) and base–collector (B–C) heterojunction
interfaces. Therefore, the injection of electric crossing the E–B
heterojunction is purely determined by thermionic emission.
Also, injection of electron from the base into collector can be
achieved under zero electronic field conditions.
To calculate the electron and hole currents in InP/GaAsSb
HBT, various current components, which may contribute to the
overall current, are given as follows: (1) electron injection
current with thermionic emission from the emitter to the base
(JEnther); (2) the emitter bulk recombination current (JEp)
separating as Auger and radiative currents; (3) G–R current at
B–E heterojunction (JGR(BE)); (4) hole injection current with
thermionic emission from base to emitter (JBpther); (5) the base
bulk recombination current (JBr) separating as Auger and
radiative currents; (6) G–R current at B–C heterojunction
(JGR(BC)); (7) the collector bulk recombination current (JCp)
(a)
Emitter Base Collector
Eg(InP)
qV0pBEδp
∆Ev
qV0nBE
Eg(InGaAs)
Eg(InP)
∆EC
δn
(b)
Emitter Base Collector
Eg(InP)
qV0nBE
qV0pBE
∆Ev
Eg(GaAsSb)
δp
∆Ec
δn
Eg(InP)
Fig. 1. Equilibrium band diagram of InP/InGaAs/InP and InP/GaAsSb/InP
DHBTs. DEC: conduction band offset at the hetero-interface; DEV: valance
band offset at the hetero-interface; qV0BE: built-in potential at the B–E
hetero-junction; qV0nBE: built-in potential in the n-type material at the B–E
hetero-junction; qV0pBE: built-in potential in the p-type material at the B–E
hetero-junction; dn: fermi energy level in the n-type material; dp: fermi energy
level in the p-type material.
separating as Auger and radiative currents. By considering the
aforementioned current components, the currents in the
emitter, base and collector regions can thus be calculated by:
JE Z JEnther CJBpther CJGRðBEÞCJEp (1)
JB Z JBpther CJGRðBEÞCJEp CJBr CJGRðBCÞKJCp (2)
JC Z JEntherKJBrKJGRðBCÞCJCp (3)
The major mathematical treatments for some of the important
current components is summarized in below.
2.1. Thermionic emission at B–E heterojunction
Based on the thermionic current density expression for the
metal-semiconductor system, the electron or hole injection
current at B–E heterojunction interface is given by [9,10]. The
thermionic emission current includes two components: the
thermionic emission and thermionic field emission (tunneling),
and is given as:
Jther ZA � T2ð1CPtÞ exp KVbarrier
kT
� �exp
Va
kT
� �K1
� �; (4)
where Va is applied voltage at the hetero-interface and A* is the
effective Richardson’s constant as A*Z4pqk2 m*/h3 Pt is the
tunneling probability for electron to across the conduction band
spike by tunneling given by reference. In InP/GaAsSb, due to the
absence of the conduction band spike, only thermionic emission
is considered by setting PtZ0 during the simulation. Both of the
thermionic emission and thermionic field emission (tunneling)
are included for the calculation of the InP/InGaAs DHBTs.
Because of the conduction band spike, the thermionic field
emission in the InP/InGaAs structure becomes the dominant
contributor for the electron injection. Vbarrier is the barrier height
at the hetero-interface illustrated in Fig. 1. At the conduction
band, the barrier height for electrons is given by VbarrierZqV0nBECdn for an InP/InGaAs type-I hetero-interface and Vbarrier
ZqV0nBEC DEcj jCdn for an InP/GaAsSb type-II hetero-inter-
face. Besides the thermionic emission of electrons for the emitter
to the base at B–E junction, the holes in the base also can enter the
emitter by thermionic emission. For the holes, the barrier height is
given by VbarrierZqV0BECDEvCdp:
2.2. Bulk recombination in emitter, base and collector
The bulk recombination involving band–band transition
includes Auger and radiative recombination mechanisms. They
are considered as the minority recombination in the neutral
regions [11]. In the emitter and collector, the bulk recombina-
tion current is written as
Jjp Z qDj
LjðxÞ
n2ij
njexp
qVa
kT
� �K1
� �; (5)
where DjZkTuj/q and LjðxÞZffiffiffiffiffiffiffiffiffiffiffiffiffitðxÞDj
p. The subscript j is
referred to E or C for emitter and collector, respectively. Va is
the applied voltage at B–E or B–C junction in the equation.
0.0 0.1 0.2 0.3 0.4 0.5 0.610–19
10–15
10–11
10–7
10–3
101
InP/GaAsSb HBTVBC=–3 V
J B a
nd it
s co
mpo
nent
s (A
/cm
2 )
VBE (V)
JEp
JBpther
JGR(BE)JB(Auger)JB(Rad)JGR(BC)JB
Fig. 2. Base current, JB in an InP/GaAsSb DHBT and its current components
versus VBE calculated with TZ300 K. The surface recombination is not
considered in calculation.
Y. Tian, H. Wang / Microelectronics Journal 37 (2006) 595–600 597
While in base region, the bulk recombination current is
JrB ZqniBDB
LBðxÞpcothðWB=LBðxÞÞK1=sinhðWB=LBðxÞÞ� �
! expðqVBE=kTÞK1�
C expðqVBC=kTÞK1� � �
;
(6)
where ni is the intrinsic carrier concentration and VBC is the
voltage applied at B–C junction in the equation. t(x) is the
minority carrier recombination lifetime for Auger or radiative
mechanism [12,13]. WB is the quasi-neutral width. If the
thickness of the depletion region in the base at B–E and B–C
junctions are defined as WBL and WBR, WB is thus obtained by
WBZtBKWBLKWBR where tB is the base thickness.
2.3. SRH generation–recombination current (G–R)
at the heterojunction
The recombination current in a heterojunction under
forward bias can be separated is into two parts: one in the
wide band gap material with an ideality factor close to 2 and
the other in the narrow band gap material with an ideality factor
close to 1. Therefore, the E–B current in the HBT structure
under normal operation condition is given by
JGRðEÞZqniEWE
2tSRHðEÞexp
qVBE
2kT
� �; (7)
JGRðBLÞZ 1:7qn2iBWBL
tSRHðBÞpexp
qVBE
kT
� �(8)
The G–R current presented in the reverse biased B–C junction
can be simplified as
JGRðBRÞZqniBWBR
2tSRHðBÞ(9)
JGRðCÞZqniCWC
2tSRHðCÞ(10)
where WE and WC are the thickness of depletion region in the
emitter region at B–E junction and the one in the collector
region at B–C junction. tSRH is SRH recombination lifetime
defined as tSRHZ1=ðVthsNfÞ. VthZ(3 kT/m*)1/2 is the thermal
saturation velocity, s is capture cross section in the unit of cm2,
and Nf is the trap density in the unit of cmK3.
The DC current gain of the device can be calculated by
comparing the overall collector and base currents based on the
above analytic modes. Detailed results are discussed in next
section.
Fig. 3. Comparison of Gummel plot and current gain b (inset) at TZ300 K
between InP/InGaAs and InP/GaAsSb DHBTs.
3. Results and discussion
Fig. 2 shows the JB and its components in InP/GaAsSb/InP
DHBT as a function of VBE at TZ300 K without considering
surface recombination. The voltage at B–C junction is set to
K3 V. The base current consists of the hole (minority carrier)
recombination current in the emitter region (JEp), the thermionic
emission current of holes from the base to the emitter (JBphter),
the Shcottkey-Read-Hall (SRH) generation–recombination
(GR) current at both B–E junction [JGR(BE)] and B–C junction
[JGR(BC)], and the electron bulk recombination current in the
base region which includes Auger [JB(Auger)] and radiative
[JB(Rad)] recombination mechanisms. Note that the value of
JGR(BC) shown in the figure is negative. The GR recombination
lifetime is tSRHZ10K7 s that is corresponding to a defect
density of about 1014 cmK3. Although the base current consists
of the above six components, it ismainly determined by theG–R
current at B–C junction in the low VBE region and Auger
recombination in the highVBE region as seen in Fig. 2.Due to the
much larger band gap of the emittermaterial, InP (EgZ1.35 eV)
as compared to the GaAs0.51Sb0.49 (0.72 eV) base, the bulk
recombination mechanism in the emitter (JEp) is negligible. In
addition, the contribution of hole injection current (JBpther) from
the base to the emitter is very limited as compared to the other
currents, confirming the benefit of blocking the hole injection
using a relative larger DEv between InP and GaAsSb. Fig. 3 and
its inset show the calculated Gummel plots DC current gain, b
for an InP/GaAsSb/InP DHBT. The data calculated based on a
referenced InP/InGaAs DHBT are also plotted for comparison.
0.0 0.2 0.4 0.6 0.810–5
10–3
10–1
101
103
105
T=100 K
200 K
300 K
400 K
JB
, Jsu
rf, a
nd J
B(A
uger
) (A
/cm
2 )
VBE (V)
JB
Jsurf
JB(Auger)
Fig. 5. Temperature-dependent JB, JB(Auger) and Jsurf versus VBE in an
InP/GaAsSb DHBT.
Y. Tian, H. Wang / Microelectronics Journal 37 (2006) 595–600598
JB and JC in InP/GaAsSb/InP DHBT are slightly higher than
those in InP/InGaAs/InP DHBT. This is found be mainly due to
the difference of the band gap between the GaAsSb and InGaAs
(0.75 eV) bases. Notice that, the current blocking effect at B–C
junction for the InP/InGaAs DHBT is ignored during the
calculation assuming that the conduction band spike at B–C
junction interface could be smoothen out by properly designing
the device structure using different approaches such as graded
interface [14] or composite collector [15], etc.
To study the effect of surface recombination on the device
performance, the base current and its surface recombination
components were calculated using different surface recombi-
nation velocities (vsurf). Fig. 4(a) shows the JB and Jsurf versus
VBE for InP/GaAsSb/InP DHBT as a function of vsurf. The
current gain b versus the collector current JC for different vsurfare plotted in Fig. 4(b). Both calculation were made at TZ300 K. As we can see in Fig. 4(a), the increase of surface
recombination velocity vsurf results in the increase of the base
current in low VBE region, which in turn causes the decreases of
b at low JC indicated in Fig. 4(b). Although the surface
recombination velocity for GaAsSb is less well documented,
our results indicate that the surface recombination effect has to
be account for if the vsurf for GaAsSb is higher than 10 cm/s.
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.810-9
10-7
10–5
10–3
10–1
101
103VBC=–3 V
vSurf=0 cm/s
vSurf=1 cm/s
vSurf=10 cm/s
vSurf=100 cm/s
J B a
nd J
Surf
(A
/cm
2 )
VBE (V)
JSurf
JB
(a)
10–3 10–1 101 103 105100
101
102
103
VBC=–3 V
Cur
rent
Gai
n,b
JC (A/cm2)
vSurf
=0vSurf=1 cm/sv
Surf=10 cm/s
vSurf
=100 cm/s
(b)
Fig. 4. JB and Jsurf versus VBE (a) and b versus JC (b) as a function of vsurf at TZ300 K in an InP/GaAsSb DHBT.
From the previous calculation, we have shown that, at room
temperature, the Auger recombination in the base region,
JB(Auger), as well as surface recombination, Jsurf, may play the
dominant roles in determining the base current and thus the DC
gain. In the later part, wewill focus on these two important current
components to study the effect of junction temperature on the
device performance. Fig. 5 shows JB, Jsurf and JB(Auger) in InP/
GaAsSb/InP DHBT versus VBE calculated at different tempera-
tures in the range of 100–400 K. vsurf is set to 10 cm/s during the
calculation. As the temperature is lowered from 400 to 100 K, the
increase of GaAsSb band gap reduces the Auger recombination.
Also, the reduction of intrinsic carrier concentration ofGaAsSb in
conjunction with the increase of band gap reduces the surface
recombination.Combining these twoaspects presents a significant
reduction of the base current with decrease of the temperature.
Notice that, with the decease of temperature, the surface
recombination tends to more contribution to the total base current
as compared to the Auger recombination. This is consistent with
the experimental observation on the InP/InGaAs DHBTs. More
attention has to be paid on the surface effect if the devices are
required to be used in the low temperature environments.
The Gummel plots of InP/GaAsSb/InP DHBT at different
temperatures are given in Fig. 6. Besides the decrease of
0.0 0.2 0.4 0.6 0.810–8
10–6
10–4
10–2
100
102
104
106
400 K
300 K 200 KT=100 K
Cur
rent
(A
/cm
2 )
VBE (V)
JB
JC
Fig. 6. Gummel plots of an InP/GaAsSb DHBT at different temperatures.
Fig. 7. b versus JC for (a) InP/GaAsSb and (b) InP/InGaAs DHBTs calculated in the temperature range from 100 to 450 K with a step of 50 K. Insert: comparison of
the temperature dependence of b between InP/GaAsSb and InP/InGaAs DHBTs at JCZ105 A/cm2.
Y. Tian, H. Wang / Microelectronics Journal 37 (2006) 595–600 599
the base current with the decrease of the temperature, the
collector currents are also reduced. This could be attributed to
the increase of GaAsSb band gap at the low temperature,
which causes the increase of the conduction band disconti-
nuity at InP/GaAsSb B–E junction. The electron thermionic
emission form the emitter into the base is thus limited,
reducing the collector current. As the temperature decreases
from 400 to 100 K, the base–emitter turn-on voltage is
increased more than 0.4 V. Similar results are obtained in
referenced InP/InGaAs/InP DHBT which was conformed by
the experiments [16,17].
The b versus Jc at different temperatures for InP/GaAsS-
b/InP and InP/InGaAs/InP DHBTs are compared in Fig. 7.
The temperature is varied from 100 to 450 K with a step of
50 K. The temperature dependences of b at JcZ105 A/cm2 for
these two devices are re-plotted as the inset of Fig. 7. It can
be seen that the b of InP/GaAsSb DHBT is more sensitive to
temperature. For the given structures, the DHBT with a
GaAsSb base show a smaller DC gain at low temperature as
compared to the referenced InP/InGaAs HBT. However, with
the increase of temperature, the b for the InP/GaAsSb HBT is
increased almost monotonously, while b for the InP/InGaAs
HBT shows a saturation behavior at high temperature. A
higher b is observed on the InP/GaAsSb HBT if the
temperature is higher than 350 K. The different trend for the
temperature dependence of b obtained in the two structures is
due to the difference of electron injection from the emitter. In
InP/GaAsSb structure, the electron injection from emitter to
base is solely determined by thermionic emission. The emitter
current injection and thereby the collector current is related to
the Eq. (4) with PtZ0. By setting the Pt to 0, plotting
the temperature-dependent Jther gives a monotonously
increasing trend with the increase of temperature. Therefore,
the b for InP/GaAsSb HBT shows a more sensitive (positive)
temperature-dependence. However, in InP/InGaAs HBTs, the
thermionic-field emission plays a more important role than
thermionic emission in determining the electron injection.
Therefore, the temperature dependence of tunneling prob-
ability Pt should provide an important contribution and
significantly affect the temperature dependence of the
collector current. As shown in reference , Pt reduces with
the increase of the temperature and tends to saturate at high
temperature region. This is believed to be the major course of
the saturation of b at elevated temperatures observed in
referenced InP/InGaAs device.
4. Conclusions
An analytic study of DC characteristics based on the drift-
diffusion approach has been performed for the InP/GaAsSb
DHBTs. The current transport of InP/GaAsSb/InP DHBTs has
been investigated focusing the device temperature dependence.
Our simulation results show that, at room temperature, the DC
characteristics of the InP/GaAsSb/InP DHBTs similar to the
conventional InP-based HBT using InGaAs as the base layer
although a type-II energy band alignment is presented in the
InP/GaAsSb HBT. However, due to different mechanisms for
the electron injection from the emitter induced by the different
conduction band alignments, the InP/GaAsSb HBTs may
present a different temperature dependent behavior in terms of
device current gain. Higher current gain could be achieved by
Y. Tian, H. Wang / Microelectronics Journal 37 (2006) 595–600600
the InP/GaAsSb HBTs at elevated temperature, which is
important for device applications.
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