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Potentially high-performance carbon-doped
GalnP/GaAs hetero’unction bipolar
transistors with diderent compositional
base gradings
J.-I. Song, C. Caneau, W.-P. Hong and K.B. Chough
Indcxing f e w : Heterojwtctwn bipolar transistors, Gallium indium
The characteristics of carbondoped GaInPIGaAs heterojunction
bipolar transistors (HBTs) with a compositionally-graded base are
reported. The characteristin of HBTs with three different
0.1, 0 -. 0.2,0
linearly-gradd AI,Ga,.& bases ( x = 0
0.3) arc compared with those of an HBT without basc grading.
Nearly idcal transistor characteristics were okrved for x values
up to 0.2, indicating possible high-speed o p t i o n of graded base
GaInP/GaAs HBTs.
collect or
Fig. 1 Schematic energy band diagram of heterojunction bipolar transistors with compositional base grading and without compositwnol base
with composition base
_ _ _ _ without compositional ciin&ing
Innohetion: GaInFVGaAs HBTs have attracted increasing attention as an alternative to AIGaAdGaAs HBTs. GaInP/GaAs HBTs
with high-performance DC characteristics such as nearly ideal I-V
characteristics [l] have been reported. Recrently, there have been
several reports on their microwave performance [2, 31 which
approaches that of well-established AIGaAdGaAs HBTs.
Compositionally-graded bases have been used in AlGaAdGaAs
HBTs to improve the microwave performance of the device by
exploiting their reduced base transit times due to the gradinginduced quasi-electric field for electrons in the base [4, 51. An ultrahighf, e x d i g 170GHz has been reported with an AIGaAd
GaAs HBT using a graded Al,Ga,&s base [6]. Whereas it has
been relatively easy to compositionally grade the base in AIGaAd
GaAs HBTs, there has been no report of using the graded bandgap base structure in GaInF’/GaAs HBTs primarily because of the
difficulty in achieving good GaInP/AI,Ga,Js heterointerface
characteristics. In this Letter, we first report the characteristics of
carbondoped base GaInPIGaAs HBTs using a carbon-doped linearly-graded AI,Ga,_Ps base grown by MOCVD.The characteristics of GaInP/GaAs HBTs having three different grading
0.2, and 0
compositions in the AI,Ga,.& base (x = 0 0.1,O
-c 0.3) have been investigated and compared with those of a
GaInP/GaAs HBT without base grading.
Results: Typical emitter-basejunction characteristics of the HBTs
are shown in Fig. 2. Our base-line HBT structure without base
grading (structure D) has an emitter-& junction ideality factor
of 1.00 and consequently ahnost flat current gain for more than
seven orders of magnitude change of the collector current. The
ideality factors of structures A and B were 1.01 and 1.04, respectively, indicating that the interface quality of the GaInP/AlzGa,.
& emitter-base heterojunction with x up to 0.2 is as good as that
of the GaInP/GaAs without base grading. The signifcant increase
in the emitter-base junction ideality factor of structure C (1.16) is
attributed to the increased recombination of electrons at the emitter-base heterointerface induced by the electron potential spike,
because the AE, of Ab,G%,AS/GaAs (- 0.23eV) [q is larger than
that of GaInP/GaAs reported (0.19 - 0.22eV) [8, 91.
Experiment: The HBT structures were grown on <loo> semi-insulating G a A s substrates by MOCVD in the following sequence:
5000A GaAs (Si = 10i9/cm3)subcollector, 5 w A GaAs (Si = 2 x
collector, 5ooA graded AI,Ga,.& base, 800A GaInP (S
= 5 x 10L’/cm3)emitter, 2 2 ~ A
GaInP (Si = 1019/cm3)
and lO133.A
GaAs (Si = 2 x 1019/cm3)emitter cap, l50A graded AlxGai_& ( x
= 0 to 0.7, Si = 1019/cm3)and l50A Iq,,G%& (Si = 10i9/cm3)
layers for non-alloyed emitter ohmic contact. The wafers were
grown at a reactor pressure of 76 torr, using trimethylindium, trimethylgalhum, arsine, and phosphine as main reagents and SizH6,
H2S, and CCl, as doping sources. The GaJn,J’ ( x = 0.52) is lattice matched to GaAs ( M u c 0.1%) and has a bandgap of E, =
1.9eV as measured by photoluminescence. The compositional
grading in the base was linear with x = 0.0 to 0.1, 0.0 to 0.2, and
0.0 to 0.3 for structures A, B, and C, respectively. At the same
time, a structure without base grading (structure D) was grown for
the purpose of comparison with the graded HBTs. Schematic
energy band diagrams of the structure with base grading and without base grading are shown in Fig. 1. The base was doped with
carbon using CCl, as a dopant to produce a nominal hole concentration of l(yalcm3. Large area HBTs having emitter and base
areas of 6.4 x l&’cmz and 1.6 x IPcm,, respectively, were fabricated using a standard mesa etching technique. The emitter mesa
was defmed by ion-millig the emitter cap layer and 0 . 2 of
~ the
GaInP emitter and subsequent selective wet-chemical etching of
GaInP against AI,Ga,.& using an HC1:H3P0, (1:3) solution. NU
AuGe/Ag/Au and T W A u were used for n- and ptype ohmic
contacts, respectively. Collector and base ohmic contacts were
alloyed at 400°C for 15s in a rapid thermal annealer. A nonalloyed ohmic contact was employed for the emitter ohmic metallisation.
The parameters of the transistors are listed in Table 1. Estimated quasi-electric fields induced by the hear compositional
grading in the base are 2.7 x lWV/cm, 5.4 x l(rV/cm, and 8.0 x
l(rV/cm for structures A, B, and C, respectively. Owing to the
high quasi-electric fields and very thin base of the structures, nonequilibrium electron transport is expected across the base layer
[lo]. In HBT structures with a very thin, highly doped base, the
common-emitter current gain can be expressed as fi = T”/T& where
rnis the electron Lifetime in the p-type. base and T~ is the base transit time [Ill. As the base Fading is increased, a higher current
gain is expected because the base transit time will be shorter under
the influence of higher quasi-electric field. Contrary to this supposition, a slight decrease in the current gain is observed as the grading is increased. It is also noted that a d e c r e a d t h e base sheet
No. 7
Vol. 30
FI% 2 Typicp‘ emitter-base
H Ts with dflerent base gr&s
- emitter voltage, V
ction characteristics ‘of GaInP/GaAs
The curves Shown are, from left to right, for structure D, A,’B, C. The
emtter area of the transistors is 6.4 x l&’cm2
resistance is observed as the composition (x value) at the emitterbase junction is increased despite the lower hole mobility in
due to alloy scattering. It is ascribed to the increased
incorporation of carbon as the aluminum mole fraction is
increased, resulting in higher bole concentration in the base. At a
doping concentration of 1P/cm3,the primary process determining the electron lifetime in p-type GaAs base is nonradiative
recombination, where the electron lifetime can be expressed using
the equation IrC. = c p 2 + 4,where cI and cz are constants and p
is the hole concentration in the base, respectively [12]. It is
believed that in the graded base structures, the decrease in the
electron lifetime due to the increased hole concentration was more
dramatic than the decrease in the base transit time, resulting in
reduced current gains.
DEFOUR, M., and OMNES, F.: ‘Conduction- and valence-band offsets
in GaAdGaInP single quantum wells grown by metalorganic
chemical vapor deposition’, Appl. Phys. Lett., 1990, 56, pp. 833835
Table 1: Parameters of GaInP/GaAs HBTs with different base
and mIGHT.S,L,: ‘Energy band alignment in
GaAs:(Al,Ga)As heterostructures: The dependence on alloy
composition’,J. Appl. Phys., 1986,59, pp. 2W209
HOPKINSON. M., and CLAXTON, P.A.: ‘Conduction-band discontinuity
in InGaP/GaAs measured using both current-voltage and
photoemission methods’, Appl .Phys. Lert., 1992,60, pp. 474-476
10 MALONEY,T.I., and FRN, 1.: “Transient and steady-state electron
transport properties og GaAs and InP’, J. Appl. Phys., 1977, 48,
pp. 781
11 RITTER, D., HAM, R.A., FEYGENSON,A., and PANISH, M.B.: ‘Diffusive
base transport in narrow base InP/GaInAs heterojunction bipolar
transistors’, Appl. Phys. Lett., 1991, 59, pp. 3431-3433
CHANG. w., YANG. L.w., and WRIGHT, P.D.: ‘Photoluminescence
characterization of nonradiative recombination in carbon-doped
Appl. Phys. Lett., 1992,60, pp. 1597-1599
Rapid photo-deposition of silicon dioxide
films using 172 nm VUV light
E: quasielectric field in base, p: DC commonemitter current ain
measured at emitter current densitv of J. = 1.5 x lo)A/cmz. RA.:fase
sheet resistance, V, +,, Bv,, ,BP cbmmonemtter offset voltage
and breakdown voftages of m t t e r % e and basecollector junctions
(measured at leakage cumnt level of 5 x IWA/cm’), respectively
The offset voltage (V, ,@.,) of the transistors increases monotonically as the base grading is increased because the effective barrier height of the emitter-base junction increases as the grading is
increased. The breakdown voltages of the emitter-base and basecollector junctions increase slightly as the base grading is increased
primariljdue to the increased-effective bandgap of the p+-AI,Ga,.
.As base layer at the junctions.
In conclusion, we have investigated the characteristics of carbondoped GaInP/GaAs HBTs having three different compositional base gradings and compared them with those of a GaInPi
GaAs HBT without grading. The transistor characteristics of the
graded base GaInP/GaAs HBTs for x values up to 0.2 were as
good as those of an HBT without grading. The result indicates the
potential of graded base GaInF’/GaAs HBTs for achieving
enhanced high-speed performance due to the reduced base transit
27 January 1994
Q IEE 1994
Elecironics Letters Online No: 19940386
J.4. Song, C. Caneau, W.-P. Hong and K . B. Chough (Bellcore, 331
Newman Springs Road, Red Bank, NJ 07701. USA)
HONG, w.-P.,
‘Characterisation of GaInP/GaAs double heterojunction bipolar
transistors with different collector designs’, Electron. Lett, 1993,
29, pp. 1881-1883
2 REN,F.,
WSK. P.w., FULLOWAN. T.R., TSENG, B., and CHEN,Y.K.: ‘Small area
InGaP emittericarbon doped GaAs base HBTs grown by
MOMPBE, Electron. Lett.. 1992,28, pp. 2250-2252
LIU, W., FAN,%-K., HENDERSON.T., and DAVITO,D.: ‘Microwave
performance of a self-aligned GaInPGaAs heterojunction bipolar
transistor’, IEEE Elecfron Device Left., 1993, 14, pp. 176478
and EISEN. F.H.:
‘(GaA1)MGaAs heterojunction bipolar transistors with graded
composition in the base’, Elecrron. Lett., 1983, 19, pp. 367-368
ZHANG.Q.M., TAN,G.L., XU, J.M.,and DAY,D.I.: ‘Current gain and
transit-time effects in HBT’s with graded emitter and base regions’,
IEEE Electron Device Leri.. 1990, 11, pp. 508-510
YAMAHATA.~., and
MATSUOK& Y.: ‘Suppressed base-widening in AIGaAdGaAs
ballistic collection transistors’. IEEE 48th DRC Tech. Dig., 1990,
Paper VIIB-3
P.Bergonzo and I.W. Boyd
lndpxlnn rerm Chemrrul Yoour &Dosinon. Slhcon comooundr
DieIeciGc thin film,Insulot& thin >h,
Silicon dioxide
A new method is presented for the rapid direct photodeposition
of silicon dioxide using silane and oxygen mixtures and 172nm
radiation merated from a xenon excimer
r T h e ~.
obtained -&h 500kmin at 300’C. some 200% faster than
previously achieved using either low temperature CVD or
traditional optical sources.
High temperature chemical vapour deposition (CVD) is the conventional method used for producing silicon dioxide thin films for
their many applications in microelectronics and currently many
low temperature processing techniques are being studied. Of these,
photo-deposition is very promising since the processed surfaces
and growing films are not subjected to damaging ionic bombardment such as can be present in plasma-assisted processing systems
Several groups have previously reported the use of ultraviolet
lasers [3, 41 and lamps [ H I to induce the required photo-CVD
heterogeneous reactions that encourage thin f h growth. However, the layers grown with an acceptable quality at low temperatures have been obtained using silane and nitrous oxide precursor
mixtures, from photo-enhanced processes which exhibit intolerably
slow growth rates (20A/min) [6-9]. Also, generally when silane
and oxygen are used, the high reactivity between these precursors
allows far higher growth rates to he attained (up to hundreds
of kmin), but the f h s grown at temperatures below 500°C
exhibit poor stoichiometry, density, porosity, hydrogen content,
and uniformity [1&12]. In recent work, we have reported the use
of novel excimer lamps that provide a new route to growing good
quality layers at an improved growth rate using mixtures of silane
and nitrous oxide [9]. In this Letter,we extend this work by using
different gas mixtures, namely silane and molecular oxygen, to
produce growth rates of good quality silicon dioxide which are
some 200% faster than previously achieved using either low temperature CVD or traditional optical sources 113-151.
The recent development of novel excimer lamps has received
considerable attention during the past few years [la], producing
wavelengths extending to the vacuum ultraviolet (VLIV) at higher
intensities than can otherwise be currently routinely achieved.
Because in photo-CVD there is no need for purely monochromatic
coherent light, complicated and expensive optical resonators are
not required. Such lamps, therefore, which can be operated continuously and cheaply and can produce intensities of 100mW/cm~,
bode well for photo-CVD applications. The physics behind the
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