Solid-State Electronics 44 (2000) 587±592
Experimental I±V characteristics of AlGaAs/GaAs and
GaInP/GaAs (D)HBTs with thin bases
Y.M. Hsin a,*, P.M. Asbeck b
a
Department of Electrical Engineering, National Central University, Chung-Li, 320, Taiwan
Department of Electrical and Computer Engineering, University of California, San Diego, La Jolla, CA 92093-0407, USA
b
Received 25 January 1999; received in revised form 20 October 1999
Abstract
N±p±n AlGaAs/GaAs and GaInP/GaAs (double) heterojunction bipolar transistors with thin base widths down
to 200 AÊ have been fabricated, and their collector and base current±voltage characteristics have been studied. The
experimental results show that the surface recombination current and the base bulk recombination current are lower
in 200 AÊ base AlGaAs/GaAs HBTs than in comparable devices with 500 AÊ base width. The experiment also showed
that the surface recombination currents and the sum of other base recombination currents (the base bulk
recombination current, the base-emitter junction space charge recombination current, and the base-to-emitter backinjected current) are signi®cantly lower in GaInP/GaAs (D)HBTs than that in comparable AlGaAs/GaAs HBTs.
For the thin base GaInP/GaAs (D)HBTs, the current gain measurements with di€erent emitter sizes demonstrates
high current gain, low emitter edge recombination and negligible emitter-size e€ect. 7 2000 Elsevier Science Ltd.
All rights reserved.
1. Introduction
Heterojunction bipolar transistors (HBTs) have
many advantages, including high current gain, low
base resistance, and a high cut-o€ frequency. For the
further improvement of HBT performance in high
speed, low power circuits, it is worthwhile to scale
down device dimensions (in both vertical and lateral
directions). One of the diculties in the lateral scaling
of HBTs is associated with recombination at the edge
of the emitter, which leads to reduced current gain in
small devices [1]. In this work, we study transistors in
two di€erent material systems (AlGaAs/GaAs and
* Corresponding author. Tel.: +886-3-422-7151-4468; fax:
+886-3-405-5830.
E-mail address: yhsin@ee.ncu.edu.tw (Y.M. Hsin).
GaInP/GaAs) with strongly scaled base layer thicknesses, down to 200 AÊ. In this regime, the minority
carrier across the base is in the quasi-ballistic regime,
rather than the di€usive regime. Our results show that
the current gain is enhanced in this regime, and that
the edge recombination component of base current is
signi®cantly suppressed in AlGaAs/GaAs HBTs. In
GaInP/GaAs (D)HBTs, the edge and recombination
components of base current are lower than in comparable AlGaAs/GaAs HBTs.
2. Material growth and fabrication
The device epitaxial structure was grown by low
pressure metalorganic chemical vapor deposition (LPMOCVD) on a 4 inch semi-insulating GaAs substrate.
For AlGaAs/GaAs HBTs, the aluminum composition
0038-1101/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 1 1 0 1 ( 9 9 ) 0 0 2 9 4 - 4
588
Y.M. Hsin, P.M. Asbeck / Solid-State Electronics 44 (2000) 587±592
of 25% at the emitter-base junction was not intentionally graded. Base layers of thicknesses of 500 and 200
AÊ were used, with carbon doping to provide p = 4 1019 cmÿ3. A conventional (non self-aligned) technology was used for device fabrication. Wet etches were
used for de®ning base and collector mesas. Electron
beam evaporation of Ti/Au was lifted o€ for de®ning
the emitter area. The Ti/Au emitter contacts were then
used as a mask for wet etching to etch down to the
base layer. The active device region was then de®ned
and the outside area was etched down to the subcollector layer. The principal issue in regard to device fabrication is to controllably etch through the emitter to
the thin base (which was carried out by a timed etch
for AlGaAs/GaAs HBTs and by selective etch for
GaInP/GaAs HBTs).
3. I±V characteristics and discussion
Representative Gummel plots for devices with base
thicknesses of 500 and 200 AÊ are shown in Fig. 1. The
collector current (IC) at a given VBE is almost identical
for the two base thicknesses, which is expected if the
minority carrier transport is ballistic rather than di€usive [2±4]. If the minority electron mean-free-path in
the HBT base is larger than the width of the base, electron transport is limited by the thermal velocity of
electrons rather than by conventional di€usive trans-
port. In this ballistic transport, the collector current
becomes JC=qn(0)2 VR instead of JC=qn(0)Dn/WB
di€usion transport, where n(0), VR=(KBT/2pm)1/2,
Dn, and WB are the electron density injected into the
base, the Richardson velocity, the electron di€usivity,
and the base width, respectively. It has been estimated
that the mean-free path for electrons in p-GaAs doped
at 4 1019 cmÿ3 is larger than 500 AÊ [5], so the present
devices should be within this quasi-ballistic regime.
The observed magnitude of IC against VBE in the
devices is in good accord with this expression, for a
value of VR=1.04 107 cm/s, at low VBE values (for
which the ideality factor is 1). At higher biases, the
ideality factor increases to 1.2, and the value of IS
drop, as expected from the e€ect of conduction band
discontinuity on minority carrier injection to the base;
current ¯ow across the base is still expected to process
by quasi-ballistic transport.
Fig. 2 shows the measured current gain (b=IC/IB)
against collector current density for di€erent emitter
size of AlGaAs/GaAs HBTs. The degradation in the
current gain with the decrease of emitter size is signi®cant in 500 AÊ base AlGaAs/GaAs HBTs, as shown in
Fig. 2(a). Figs. 3 and 4 show the measured current
gain (b=IC/IB) against collector current density for
di€erent emitter size of GaInP/GaAs HBTs and
DHBTs. The results in Figs. 2±4 indicate that the current gain is substantially larger for the 200 AÊ base
width devices and reaches 160 and 317 for AlGaAs/
Fig. 1. Gummel plots of thin base width AlGaAs/GaAs HBTs with an emitter area of 20 20 mm2.
Y.M. Hsin, P.M. Asbeck / Solid-State Electronics 44 (2000) 587±592
589
Fig. 2. DC current gain of (a) 500 AÊ base AlGaAs/GaAs HBTs and (b) 200 AÊ base AlGaAs/GaAs HBTs.
GaAs HBTs and GaInP/GaAs DHBTs, respectively.
The degradation in the current gain with a decrease of
emitter-base junction (emitter-size e€ect) is almost negligible for GaInP/GaAs HBTs with 500 and 200 AÊ
base widths according to Figs. 3 and 4. In general, the
relationship between the DC current gain (b ) and the
emitter dimensions can be expressed as in [6].
JBulk ‡ JBscr ‡ JBp † KBsurf PE
1
ˆ
1†
‡
b
JC
JC
AE
where JBulk (A/cm2), JBscr (A/cm2), and JBp (A/cm2)
are the base bulk recombination current density, the
base-emitter junction space charge recombination current density, and the base-to-emitter back-injected current density, respectively. KBsurf (A/cm) is the surface
recombination current density by the emitter periphery
(PE); and AE (mm2) is the emitter area. Due to the
di€erence in bandgap energy between emitter and
base, JBp is generally negligible in HBTs. Fig. 5 shows
the measured JC 1/b against PE/AE for all HBTs at
Fig. 3. DC current gain of 500 AÊ base GaInP/GaAs HBTs.
590
Y.M. Hsin, P.M. Asbeck / Solid-State Electronics 44 (2000) 587±592
JC=1 104 A/cm2. The intercepts of the curves in Fig.
5 with a y-axis, represent the sum of JBulk, JBscr, and
JBp and are summarized in Table 1. The sums of JBulk,
JBscr, and JBp in Table 1 seem to have the proportion
of 0WB and show the lower values in GaInP/GaAs
HBTs. The observed WB dependence of the base
recombination density is in accordance with expectations based on a quasi-ballistic transport model. The
lower values in the sum of JBulk, JBscr, and JBp for
GaInP/GaAs HBTs are mainly due to the larger
valence band discontinuity at the base-emitter junction
which could block holes from the base from entering
Fig. 4. DC current gain of (a) 500 AÊ base GaInP/GaAs DHBTs and (b) 200 AÊ base GaInP/GaAs DHBTs.
Y.M. Hsin, P.M. Asbeck / Solid-State Electronics 44 (2000) 587±592
591
Fig. 5. JC (1/b ) as a function of PE/AE for thin base width HBTs.
the emitter for recombination. The resulted current
gain of GaInP/GaAs HBTs is over 2 times higher than
that in comparable AlGaAs/GaAs HBTs.
From Eq. (1), the slopes of the curves in Fig. 5 represent KBsurf and are also summarized in Table 1. The
values of KBsurf for 500 and 200 AÊ base AlGaAs/GaAs
HBTs are 1.6 10ÿ6 and 2.8 10ÿ7 A/mm, respectively, showing a dramatic decrease for the thinner base
AlGaAs/GaAs HBTs. The surface recombination current density varies as W 2B for the thin base AlGaAs/
GaAs HBTs. We believe this observed W 2B dependence
of the periphery recombination is also in accordance
with the quasi-ballistic model. If we neglect the possible injection of carriers into channels at the surface of
the base, through saddlepoints at the emitter edge,
then base surface recombination is dominated by carriers that are injected into the base and subsequently
back-scattered in the direction of the base surface. At
this surface, the minority carriers have a ®nite probability of recombination (related to the surface recombination velocity). In the limit of thin bases, electrons
will typically undergo few scattering events, and we
may assume in the limit of thin bases, a single scattering event which redirects electrons to the exposed base
surface (outside of the intrinsic region of the device).
The ¯ux of carriers onto the exposed base surface is
then proportional to W 2B. One WB factor originates
from an increasing probability of electron scattering as
the carriers traverse increasing base thicknesses.
Another factor of WB arises since the width of the
region at the edge of the emitter that can contribute
electrons that will backscatter out of the emitter
increases. This observation in W 2B dependence of the
periphery recombination explains the increased surface
recombination current in the 500 AÊ base AlGaAs/
GaAs HBTs.
The values of KBsurf for 500 AÊ base GaInP/GaAs
HBTs, 500 and 200 AÊ base GaInP/GaAs DHBTs are
1.2 10ÿ7, 1.6 10ÿ8, and 2.0 10ÿ8 A/mm, respectively. The cause of the lower emitter edge recombination of the GaInP/GaAs HBTs is an issue of
considerable importance. We investigate that the di€er-
Table 1
y-intercepts and slopes of curves from Fig. 5
NBTs
WB (AÊ)
y-intercept (A/cm2): JBulk+JBscr+JBp
Slope (A/mm): KBsurf
AlGaAs/GaAs HBT
AlGaAs/GaAs HBT
InGaP/GaAs HBT
InGaP/GaAs DHBT
InGaP/GaAs DHBT
500
200
500
500
200
136
59
58
62
35
1.6 10ÿ6
2.8 10ÿ7
1.2 10ÿ7
1.6 10ÿ8
2.0 10ÿ8
592
Y.M. Hsin, P.M. Asbeck / Solid-State Electronics 44 (2000) 587±592
Fig. 6. Schematic cross-section of AlGaAs/GaAs and GaInP/GaAs HBTs.
ence stems from the fact that the device cross-sections
from AlGaAs/GaAs and GaInP/GaAs HBTs are not
identical due to the external base access during the wet
etching. Fig. 6 shows the schematic cross-sections for
both AlGaAs/GaAs and GaInP/GaAs HBTs. Due to
the selective wet etching between GaInP and GaAs,
the GaInP emitter edge (or the base-emitter junction)
is not exposed to the air as much as in the AlGaAs/
GaAs HBTs. In the AlGaAs/GaAs HBTs, the exposed
base in the wall causes more surface recombination
and thus results in higher surface recombination current density (KBsurf). Moreover, the KBsurfs of GaInP/
GaAs DHBTs are independent of base thickness and
smaller than that in the GaInP/GaAs HBTs. The
device cross-sections of both GaInP DHBTs and
HBTs are similar, thus the reduced emitter edge
recombination current could result from the di€erence
in the base and collector layers of DHBTs. The real
cause is unclear and under investigation. But this low
emitter edge recombination makes GaInP/GaAs HBTs
suitable for sub-micro device applications.
With thin base regions, the intrinsic base sheet resistance increases aggravating the potential problem of
emitter current crowding. Current crowding can alter
the relationships expressed in Eq. (1). We veri®ed that
in the regimes we reported current crowding did not
signi®cantly in¯uence the results.
4. Conclusion
This work shows that by decreasing the base thickness, the emitter edge recombination is suciently suppressed in AlGaAs/GaAs HBTs. This decrease in base
thickness increases DC current gain and ft, but carries
with it a penalty in higher base resistance. We believe
that this penalty will be very small if the extrinsic base
sheet resistance is decreased from the intrinsic value
through a technique such as epitaxial regrowth, ion implantation or di€usion. The current gain measurements
of GaInP/GaAs (D)HBTs with di€erent emitter sizes
demonstrate high current gain, low emitter edge
recombination and negligible emitter-size e€ect. The
excellent performance of GaInP/GaAs (D)HBTs make
them great candidates for sub-micro devices applications.
Acknowledgements
The authors would like to thank Kopin Corporation
for providing epitaxial materials.
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