Comprehensive study of InGaP–Alx Ga1À x As–GaAs composite-emitter
heterojunction bipolar transistors with different thickness
of Alx Ga1À x As graded layers
Shiou-Ying Chenga)
Department of Electronic Engineering, National Ilan University, No. 1, Sec. 1, Shen-Lung Road, 1-Lan,
Taiwan 26041, Republic of China
Chun-Yuan Chen, Jing-Yuh Chen, Hung-Ming Chuang, Chih-Hung Yen,
and Wen-Chau Liu
Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University,
1 University Road, Tainan, Taiwan 70101, Republic of China
共Received 16 October 2003; accepted 26 April 2004; published 30 June 2004兲
The characteristics of InGaP–Alx Ga1⫺x As–GaAs composite-emitter heterojunction bipolar
transistors 共CEHBTs兲 with different thickness of Alx Ga1⫺x As graded layer are comprehensively
studied and demonstrated. The thickness of the graded Alx Ga1⫺x As layer is an important factor to
affect device performances. In this work, it is found that CEHBTs with 70 Å⬃100 Å Alx Ga1⫺x As
graded layers exhibit better properties due to the absence of potential spike. For comparison,
experimentally, two practical CEHBTs with 0 and 100 Å Alx Ga1⫺x As graded layers are fabricated.
Generally, good agreements between experimental results and theoretical simulations are found. It
is, therefore, concluded that the CEHBT with an appropriate thickness of the Alx Ga1⫺x As graded
layer offers the promise for analog, digital, and microwave device applications, especially for lower
operation voltage and lower power consumption circuits. © 2004 American Vacuum Society.
关DOI: 10.1116/1.1763890兴
I. INTRODUCTION
Today, heterojunction bipolar transistors 共HBTs兲 have
been commercially used for power amplifier in mobile
phones due to their better microwave performances. Many
researches, such as transfer substrate method, air-bridge
technology, emitter edge thinning, InP–GaAsSb–InP and
AlGaN–GaN heterostructures, tunneling emitter bipolar
transistors 共TEBTs兲, and double heterostructure-emitter bipolar transistors 共DHEBTs兲, etc.,1– 6 have been reported to fabricate high-performance HBTs. For GaAs-based HBTs, the
AlGaAs–GaAs and InGaP–GaAs material systems are usually employed.7 However, for HBTs, the existence of
conduction-band discontinuity (⌬E C ) substantially deteriorates the device performance. Although the typical magnitude of ⌬E C at In0.49Ga0.51P–GaAs heterointerface is smaller
than that of Alx Ga1⫺x As–GaAs heterointerface, the undesired potential spike ⌬V C is still observed. In order to reduce
the ⌬E C , the device structures including the employments of
共1兲 thick undoped spacer, 共2兲 planar-doping sheet, and 共3兲 the
compositionally graded layer at emitter-base 共E-B兲 heterojunctions have been reported.8,9 Nevertheless, some relative
drawbacks of these works are presented. These disadvantages and limitations are 共1兲 increased bulk recombination
and the recombination within the thick undoped spacer; 共2兲
more difficult to control the accurate doping level; and 共3兲
lattice match of InGaP–GaAs material system. It is known
that the magnitude of ⌬E C is close to zero at
In0.49Ga0.51P–Al0.11Ga0.89As heterointerface.10 In this work,
a兲
Author to whom correspondence should be addressed; electronic mail:
sycheng@niu.edu.tu
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J. Vac. Sci. Technol. B 22„4…, JulÕAug 2004
comprehensively theoretical and experimental studies of
InGaP–Alx Ga1⫺x As–GaAs composite-emitter heterojunction bipolar transistors 共CEHBTs兲 with different thickness of
Alx Ga1⫺x As graded layers are implemented and
demonstrated.11 The studied devices can present lower
turn-on voltage, lower offset voltage, lower saturation voltage and uniform current gain by means of the use of an
appropriate Alx Ga1⫺x As graded layer at lower bias region.
Thus, the studied devices are suitable to be operated at lowpower communication system. The basic structure of the
studied devices is similar to conventional single heterojunction bipolar transistors 共SHBTs兲 except the insertion of an
Alx Ga1⫺x As graded layer between base and emitter layer.
The insertion of Alx Ga1⫺x As graded layer is used to continuously smooth and suppress the ⌬E C at E-B hetrojunction and
improve device performances. It is known that, from simulated and experimental results, the thickness of Alx Ga1⫺x As
graded layer plays an important role on affecting device
properties. So, a proper thickness of the Alx Ga1⫺x As graded
layer is essential to acquire good device performance for
analog 共lower offset voltage and uniform current gain兲 and
digital 共lower turn-on and lower saturation voltage兲 applications. The lower operational voltage could reduce the power
dissipation and increase the battery recharge period of some
electronic products 共e.g., mobile phones and PDA兲. In addition, the uniform current gain could reduce the additional
complexity effort for circuit design.
II. THEORETICAL CONSIDERATIONS
The used CEHBT device structure in the theoretical
analysis is depicted in Fig. 1. The corresponding
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FIG. 1. Used device structure in this study.
Alx Ga1⫺x As graded layer thicknesses of studied devices A,
B, C, D, E, F, and G are presented in Table I. The device
structure includes a 5000 Å n ⫹ -GaAs subcollector layer
(n ⫹ ⫽5⫻1018 cm⫺3 ), a 3000 Å n ⫺ -GaAs collector layer
(n ⫺ ⫽5⫻1016 cm⫺3 ), an 800 Å p ⫹ -GaAs base layer (p ⫹
⫽4⫻1019 cm⫺3 ), a tÅ n-Alx Ga1⫺x As (x⫽0 – 0.11) graded
layer (n ⫺ ⫽5⫻1017 cm⫺3 ), a 700 tÅ n-In0.49Ga0.51P emitter
layer (n⫽5⫻1017 cm⫺3 ), a 500 Å n ⫹ -GaAs cap layer (n
⫽5⫻1018 cm⫺3 ). As revealed in Table I, the thickness values of Alx Ga1⫺x As graded layers selected for the study are 0,
30, 50, 70, 100, 200, and 300 Å for devices A, B, C, D, E, F,
and G, respectively. For comparison, the total thickness of
Alx Ga1⫺x As graded layer and InGaP emitter layer of all
studied devices is fixed at 700 Å. Presumably, the studied
devices are similar to a conventional InGaP–GaAs HBT except the use of Alx Ga1⫺x As graded layers with different
thickness. An important aspect of this study is to find the
appropriate thickness of the Alx Ga1⫺x As graded layer so as
to continuously smooth the ⌬E C of E-B hetrojunction.
In this work, a two-dimensional semiconductor simulation
package ATLAS was used to analyze energy band diagrams
and device characteristics.12,13 Other important physical
mechanisms, such as Fermi–Dirac statistics, Shockley–
Read–Hall 共SRH兲 recombination, field-dependent mobility,
surface recombination, Auger recombination, Boltzmann statistics, trap-assisted tunneling, and bandgap narrowing effect
were also included in this work. For better accuracy, the total
triangle unit number up to 19 264 and total grid points up to
9250 were used in the simulations. In the theoretical analysis, the base-emitter 共B-E兲 and base-collector 共B-C兲 junction
areas of studied devices are fixed at 1⫻1 ␮ m2 and
1⫻1.25 ␮ m2 , respectively.
TABLE I. Used Alx Ga1⫺x As graded layer thicknesses of studied devices.
Device Device Device Device Device Device Device
A
B
C
D
E
F
G
Thickness
共tÅ兲 of
Alx Ga1⫺x As
graded layer
0Å
30 Å
50 Å
70 Å
100 Å
200 Å
J. Vac. Sci. Technol. B, Vol. 22, No. 4, JulÕAug 2004
300 Å
FIG. 2. Complete energy band diagrams of studied devices 共a兲 at thermal
equilibrium and 共b兲 under applied voltages of V CE ⫽0 V and V BE ⫽1.4 V.
The insets show the enlarged energy band diagrams of studied devices near
the B/E junction.
III. EXPERIMENT
For comparison, two practical devices, denoted as devices
A1 and E1, were fabricated. The structures of devices A1 and
E1 are the same as devices A and E, respectively. In other
words, the values of Alx Ga1⫺x As graded layer thickness t
used in devices A1 and E1 are 0 and 100 Å, respectively. The
practical devices A1 and E1 were grown by a metal organic
chemical vapor deposition 共MOCVD兲 system on 共100兲 oriented GaAs structures. After the epitaxial growth, the conventional photolithography, wet chemical etching, thermal
evaporation and lift-off techniques were used to fabricate
mesa-type devices. AuGeNi–Au emitter contacts were alloyed at 300 °C for 30 s. Then the n ⫹ -GaAs cap layer was
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FIG. 3. Distributions of 共a兲 electron concentration, 共b兲 hole concentration, and 共c兲 recombination rate of studied devices at V BE ⫽1.4 V.
etched by a solution of 6 H3 PO4 :2 H2 O:100 H2 O2 and the
InGaP emitter layer was then selective etched in an HClbased solution. Low-resistance AuZn metals were used as
base contacts. After the etching to subcollector and the mesa
isolation of devices, AuGeNi–Au metals were then deposited
for collector contacts. The B-E junction area of studied devices is 40⫻40 ␮ m2 .
IV. RESULTS AND DISCUSSION
The complete- and enlarged-energy band diagrams near
the B-E junction of studied devices with different thickness t
of Alx Ga1⫺x As graded layers at thermal equilibrium are
shown in the inset and main parts of Fig. 2共a兲, respectively. It
is clearly seen that the ⌬E C is smoothed out when an
Alx Ga1⫺x As graded layer is inserted between B-E junction.
JVST B - Microelectronics and Nanometer Structures
In addition, the location of valance band discontinuity
(⌬E V ) is found to shift toward emitter side with the increase
of t. Under the forward-active biased condition (V BE
⫽1.4 V), the complete- and enlarged-energy band diagrams
near the B-E junction are shown in inset and main parts of
Fig. 2共b兲, respectively. As the thickness of Alx Ga1⫺x As
graded layer t is reduced below 70 Å 共devices A, B, C, and
D兲, the effective potential spike starts to appear. This transition of energy bands is mainly caused by the 共i兲 different
dielectric constant between InGaP and Alx Ga1⫺x As graded
layer and 共ii兲 strongly applied V BE voltage 共electric field兲.
This undesired potential spike would certainly modulate the
carrier transport mechanism. So, the t value should be carefully considered.
Figures 3共a兲–3共c兲 show the distributions of electron con-
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FIG. 4. 共a兲 Offset voltage as a function of base current for simulated devices. Collector current as a function of collector-emitter voltage V CE for 共b兲 device
A1 and 共c兲 device E1.
centration, hole concentration, and recombination rate, respectively. The applied bias voltage is kept at V BE ⫽1.4 V.
The use of Alx Ga1⫺x As graded layer making the potential
spike at the B-E junction smooth can improve the electron
injection rate, so that the electron concentration is increased
once the t is increased, as illustrated in Fig. 3共a兲. Due to the
presence of Alx Ga1⫺x As graded layer, the distribution of
hole concentration is shifted toward emitter side as revealed
in Fig. 3共b兲. Therefore, from Fig. 3共c兲, the recombination rate
is increased with increasing the Alx Ga1⫺x As graded layer
thickness t. This shows that the use of thick Alx Ga1⫺x As
graded layer is not a good choice for high-performance
HBTs. Therefore, a comprehensive study is needed to find an
appropriate value of t.
J. Vac. Sci. Technol. B, Vol. 22, No. 4, JulÕAug 2004
The common-emitter current–voltage 共I–V兲 characteristics of studied devices have been simulated. Due to the use
of Alx Ga1⫺x As graded layers, the collector-emitter offset
voltages of studied devices can be reduced. Figure 4共a兲
shows the offset voltage ⌬V CE versus the base current for
studied devices. In general, ⌬V CE is a function of the potential spike ⌬V C at E-B junction, collector area A C , emitter
area A E , and emitter resistance R E . Generally, the ⌬V CE is
defined as the collector-emitter voltage at which the collector
current reaches zero. Significantly, it is found that the ⌬V CE
is reduced when the t value is increased. This is clearly understandable. As the t is increased, the potential spike appeared at B-E junction is decreased which causes the suppression of ⌬V CE . Experimentally, the collector current IC
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FIG. 5. 共a兲 Base and 共b兲 collector current ideality factor as a function of
base-emitter voltage V BE .
as a function of collector-emitter voltage V CE near origin of
devices A1 and E1 are illustrated in Figs. 4共b兲 and 4共c兲,
respectively. Clearly, the device A1 (t⫽0) exhibits a larger
⌬V CE than device E1 (t⫽100 Å). This result is the same as
simulated data except the relatively larger values of ⌬V CE
found in practical devices. The larger ⌬V CE is mainly caused
by the larger area ratio between collector and emitter and
large Ohmic contact resistance for devices A1 and E1.
Figure 5共a兲 shows the relationship between base current
ideality factor (n B ) and base-emitter voltage (V BE ) for studied devices. The experimental results of devices A1 and E1,
marked by symbols of 䉭 and 䊐, are also illustrated in Fig.
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5共a兲 for comparison. At lower V BE bias region, two distinct
variations related to t are found. n B is decreased with the
increase of t before V BE ⫽1.05 V for devices A, B, C, and D
(t⭐70 Å). On the contrary, for devices E, F, and G (t
⭓100 Å), the n B is increased with the increase of t before
V BE ⫽1.05 V. This remarkable phenomenon can be explained as follows. As the thicknesses of Alx Ga1⫺x As graded
layers are increased, both of the 1-KT base bulk recombination current (IB,BULK) and 2-KT space charge region recombination current (IB,SCR) are increased. The increment of
1-KT base bulk recombination current (IB,BULK) is mainly
due to the disappearance of the ⌬V C . On the other hand, the
increment of the 2-KT space charge region recombination
current (IB,SCR) is caused by the shiftness of the ⌬E V location toward to the emitter side. For those devices with thinner Alx Ga1⫺x As graded layers 共devices A, B, C, and D兲, the
potential spikes become smooth with the increase of t. Therefore, the contribution of the 2-KT space charge region recombination current (IB,SCR) compared to the base bulk current is nearly negligible. Therefore, electrons are relatively
easy to emit across the E/B heterojunction and toward the
p ⫹ -GaAs base region. As a result, the 1-KT base bulk recombination current (IB,BULK) is more pronounced by the
increase of t. For those devices with thicker Alx Ga1⫺x As
graded layers 共devices E, F, and G兲, on the other hand, more
holes are accumulated and restricted within the Alx Ga1⫺x As
graded layer. Hence, the recombination probability of electron and hole is surely increased. So, the 2-KT space charge
region recombination current (IB,SCR) becomes a major portion of total base current as the t value is increased from 100
to 300 Å. In addition, the dissimilar trends of n B with increasing V BE are found for studied devices. For first case, the
devices with thinner Alx Ga1⫺x As layers 共devices B, C, and
D兲 exhibit a weakly increasing trend of n B at V BE ⭓1.15 V.
Due to the increase of V BE , the potential spike is gradually
appeared at B-E junction. This situation indeed enhances the
contribution from 2kT-IB,SCR and therefore increases the n B .
For second case, the devices with thicker Alx Ga1⫺x As
graded layers 共devices F and G兲 exhibit an apparent decreasing trend of n B at V BE ⭓1 V. As mentioned above, the potential spike of devices F and G are completely suppressed
despite the introduction of V BE . Consequently, both of the
magnitudes of electrons injecting from emitter and the
1 kT-IB,BULK are increased while V BE is increased. This certainly decreases the n B value. For experimental results, the
devices A1 and E1 also exhibit similar variation trends at
lower bias region (1 V⭐V BE ⭐1.25 V). The experimental
results show the accuracy of theoretical simulations except at
higher bias region (V BE ⭓1.25 V). Under this bias regime
(V BE ⬎1.25 V), the relatively large base current ideality factor n B is principally caused by the layer series resistance
which is originated from the nonoptimized Ohmic contacts
of practical devices A1 and E1.
Figure 5共b兲 shows the collector current ideality factor
(n C ) as a function of V BE for studied devices. For devices E,
F, and G (t⭓100 Å), the n C values are very close to unity at
V BE ⭐1.3 V and are increased at high series resistance region
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Cheng et al.: Comprehensive study of InGaP–Alx Ga1À x As–GaAs
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dc current gains than device A 共conventional abrupt-HBT兲
are found at J C ⭐0.01 A/cm2 . In contrast, the devices with
thicker Alx Ga1⫺x As graded layer 共devices F and G兲 show
lower values of dc current gain than device A under extensive J C regime of 10⫺6 ⭐J C ⭐104 A/cm2 . The relatively
smaller current gains are mainly caused by the additional
larger IB,SCR . As stated above, the excess IB,SCR is due to
Alx Ga1⫺x As graded layer being too thick. It can be seen
from Fig. 6, that the experimental results are consistent with
theoretical simulations at the collector current density region
of 10⫺6 ⭐J C ⭐103 A/cm2 .
V. CONCLUSION
FIG. 6. Common-emitter
density J C .
dc
current
gain
vs
collector
current
(V BE ⬎1.3 V). It indicates that electrons diffusing from
emitter to base and drifting from base to collector dominate
the collector current transport due to the entire elimination of
potential spike. This viewpoint is coincident with Fig. 2. For
devices A, B, C, and D (t⭐70 Å), in contrast, the nonunity
values 共about 1.05–1.1兲 of n C imply that the tunneling current component acts as part of the collector current. The
tunneling currents for those devices are resulted from the
tunneling through the triangular-like potential barrier at E/B
junction. As illustrated in the inset of Fig. 5共b兲, the obvious
potential spikes are found for devices A, B, C, and D. Moreover, the tunneling probability is mainly dominated by the
barrier height and thickness of triangular-like potential barrier. Thus, the tunneling current is relatively pronounced for
devices A, B, C, and D. On the other hand, experimentally,
the practical devices A1 and E1 exhibit similar results to
simulations over the entire applied V BE bias regime.
Figure 6 shows the common-emitter dc current gain versus collector current density (J C ). The enlarged curves
within middle collector current regime (0.1⬍J C
⬍100 A/cm2 ) are illustrated in the inset of Fig. 6. Based on
the use of appropriate thickness of Alx Ga1⫺x As graded layer,
the absence of potential spike at B-E junction considerably
improves the dc current gain. For those devices with thinner
Alx Ga1⫺x As graded layers 共devices B, C, and D兲, the larger
J. Vac. Sci. Technol. B, Vol. 22, No. 4, JulÕAug 2004
The dc characteristics of InGaP–Alx Ga1⫺x As–GaAs CEHBTs with different thickness of Alx Ga1⫺x As graded layers
are comprehensively studied. The use of Alx Ga1⫺x As graded
layer can effectively reduce the offset voltage. Simulated and
experimental results reveal that the studied devices B, C, D,
E, and E1 exhibit lower base current ideality factor n B and
the studied devices D, E, E1, F, and G express lower collector current ideality factor n C . At lower collector current density, the studied devices B, C, D, E, and E1 have higher dc
current gains than the conventional HBTs 共device A and A1兲.
Generally, good agreements between theoretical simulations
and experimental results are obtained. Based on this work,
the appropriate thickness of the Alx Ga1⫺x As graded layer is
about 70–100 Å. Consequentially, the use of an optimum
Alx Ga1⫺x As graded layer can provide the promise for HBT
applications.
ACKNOWLEDGMENT
Part of this work was supported by the National Science
Council of the Republic of China under Contract No. NSC
92-2218-E197-003 and NSC 92-2218-E006-032.
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