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References
VAN BERLO, W.,
JANSON, M.,
LUNDGREN, L.,
MORNER, A.-C.,
TERLECKI, J., GUSTAVSSON, M., GRANESTRAND, P., and SVENSSON, P.:
‘Polarization-insensitive, monolithically 4 x 4 InGaAsP/InP laser
amplifier gate switch matrix’, IEEE Photonics Technol. Lett., 1995,
7, pp. 1291-1293
JANSON, M., LUNDGREN, L , MORNER, A:C,
RASK, M., STOLTZ, B.,
GUSTAVSSON, M., and THYLEN, L.: ‘Monolithically integrated 2 x 2
InGaAsP/Inp laser amplifier gate switch arrays’, Electron. Lett.,
1992, 28, pp. 776-778
SHERLOCK, G.,
BURTON, J.D.,
KELLY, A.E , and ROBERTSON, M.J.:
FIDDYMENT, P.J.,
SULLY, P.C.,
‘Integrated 2 X 2 optical switch
with gain’, Electron. Lett., 1994, 30, pp. 137-138
SCHIMPE, R , KRISTEN, G., PROEHL, S., STRZODA, R., and RIEGER, J.:
‘Compact 2 X 2 switch with optical amplifier gates’. Tech. Digest
CLE0’94, Anaheim, California, 1994, 8, Paper CTul4, pp. 76-77
LAUBE, G., SCHILLING, M., LACH, E., DAUB, K., BAUMS, D., IDLER, W.,
WEBER, J , NOWITZKI, A., KORNER, U,, and WUNSTEL, K.: ‘Selective
area growth of Q/Q-MQW structures for active/passive 2 1994, 2
space switch matrices’. Proc. 7th Euro. Conf. Int. Opt. (EC10’95),
1995, Paper Th-A3, pp. 527-530
HAMAMOTO, K., and KOMATSU, K.: ‘Insertion-loss-free 2 x 2
InGaAsPhp optical switch fabricated using bandgap energy
controled selective MOVPE’, Electron. Lell., 1995, 31, pp. 17791781
DORGEUILLE, F., MERSALI, B., BRANDON, J., SLEMPKES, S., FILOCHE, M.,
and : ‘A new double buried heterostructure laser diode for InPbased photonics integrated circuits’, Appl. Phys. Lett., (submitted)
MERSALI, B.,
BRUCKNER, H J ,
FEUILLADE, M.,
SAINSON, S.,
OUGAZZADEN, A., and CARENCO, A : ‘Theoretical and experimental
studies of a spot-size transformer with integrated waveguide for
polarisation insensitive optical amplifiers’, J. Lightwave Technol.,
1995, LT-13, pp. 1865-1872
undoped 5nm GaSbiundoped 5nm In,,Ca, ,As double cap struG
ture, which is needed to prevent oxidation of the AlSb donor
layer. The Hall mobility of the starting material at 300K was
9,900cmW-s and the sheet carrier concentration was 2.2 x
10’2cm-2.
The devices were fabricated using AuGeNiiPUAu (50/15/35/50
nm) source and drain ohmic contacts which were formed by rapid
thermal alloying. The Schottky-gate contact is Cr/Au formed
using tri-level resist e-beam lithography. Device isolation was
achieved by wet chemical etching, and to reduce leakage current a
gate air bridge was formed at the mesa edge. The devices were
treated with a brief acetic acid-based etch at the end of the process, which significantlyreduced gate leakage current due to surface
conduction.
The double cap layer approach is similar to that which was
reported in other work [2], and was used owing to the InGaAs
surface being less susceptible to reaction during processing than
GaSb. H
all measurements showed a negligible change in mobility
and an increase in the sheet charge density of -10% as a result of
the acetic acid etch.
The ohmic contact alloy time and temperature were optimised
to limit the lateral diffusion and minimise the contact resistance.
The Pt layer also reduced the extent of the reaction by preventing
the top layer of Au from diffusing during the alloying. After
processing was completed, the contact resistance was measured to
be -9.lQmm using transmission line measurements. Using this
contact metallisation, the use of the double cap layer was found to
give consistently lower contact resistances than a single GaSb cap
layer.
AISb/lnAs HEMTs with high
transconductance and negligible kink effect
J.B. Boos, W. Kruppa, D. Park, B. Molnar and
B.R. Bennett
Indexing term: Ohmic contacts, Field effect transistors, High
electron mobility transistors
AlSbLnAs HEMTs with a 200nm gate length have been
fabricated and exhibit a low-field source-drain resistance of 0.6
Qmm, a transconductance as high as 1.3S/mm, and an effective
electron velocity of 3.5 X 107cm/s.The HEMTs also have a
negligible kink effect.
AlSbflnAs-based HEMTs have the potential for hgh-speed applications owing to the large conduction band discontinuity (hE, =
1.35ev) and attractive transport properties of the InAs channel.
Although improvements have been made in recent years, the
growth and processing technology in this material system is relatively immature. A common problem in AlSb/lnAs HEMTs is a
kink effect which affects the charge control, particularly in short
gate length devices, and restricts the usable operating voltage
range [l 31. The reported exceptions of HEMTs exhibiting no
kink effect have been for gate lengths of 1 pm or longer [l, 41. The
reduction of the HEMT source-drain access resistance is a second
area in which improvement is needed. The AlSb/InAs material system is particularly suitable for achieving low access resistance
owing to the high mobility, high sheet charge density, and low
contact resistance values which are obtainable. In this Letter, we
report on 200nm AlSb/InAs HEMTs with a negligible kink effect
and low access resistance. These HEMTs also exhibit transconductances higher than 1Simm.
The AlSbLnAs HEMT material was grown by molecular beam
epitaxy at 510°C on an undoped (100) GaAs substrate. A 2 . 3 ~
undoped AlSb buffer layer was used to accoinmodate the 7% lattice mismatch. Then, in order of growth, the structure consisted of
a l0nm undoped InAs channel; a 12.5nm AlSb donor layer, where
the donors are supplied by a Te planar doping sheet which was
inserted 2.5nm above the InAs quantum well; and finally an
~
688
0-0
0.5
1
drain
.o
1-5
voltage, V
2 .o
Fig. I AlSb/InAs HEMT drain characteristics
L , = 0 . 2 W~G =
~ low,Los
1 . 9 ~V,,,
,
0.1V
The HEMT drain characteristics are shown in Fig. 1. The
HEMT exhibits a low-field source-drain resistance of 0.6Qmrn,
and a maximum on-state breakdown voltage of 2V at a drain current density of 1.4A/mm. The transconductance values against
gate voltage are shown in Fig. 2. The g, is as h g h as 1.3S/mm
and is above lS/mm throughout most of the gate voltage range.
Based on the measured source resistance value of 0.15Qmm, the
corresponding intrinsic transconductance g,‘ is 1.6Simm. Using
the relationship v = g,’(d+Ad)/&,where d is the gate-channel separation and Ad is 112 the InAs quantum well thickness, the effective
electron velocity under the gate is calculated to be 3.5 x l07cds.
Based on measured S-parameters and equivalent circuit modelling,
the HEMTs exhibit a maximum g,/gd3 value of 8 at 1GHz. The f T
is -70GHz and is limited by the gate bonding pad capacitance and
the gate leakage current associated with holes generated by impact
ionisation. The onset of this hole leakage current begins at a drain
voltage of -0.5V [3]. At V,, = 4 . 8 V the gate current increases
substantially from 20 at V, = 0.5V to 470pA at V,, = 1V.
The HEMTs also exhibit negligble kink effect in the drain characteristics. The kink is commonly attributed to weak impact ionisation and associated hole accumulation in the AlSb layers.
Measurements we have made on a variety of HEMTs of different
material designs show that HEMTs with increasing degree of kink
have correspondingly less drain current saturation and larger g,H
ELECTRONICS LETTERS
28th March 1996
Vol. 32
No. 7
References
1 BRAR, B., and KROEMER, H.: ‘Influence of impact ionization on the
drain conductance in InAs-A1Sb quantum well heterostructure
field-effect transistors’, ZEEE Electron Device Lett., 1995, 16, (12),
pp. 548-550
2 BOLOGNESI, C.R., CAINE, E.J., and KROEMER, H.: ‘Improved charge
control and frequency performance in InAs/AISb-based
heterostructure field-effect transistors’, ZEEE Electron Device Lett.,
1994, 15, (l), pp. 16-18
3 BOOS, J.B., KRUPPA, w., SHANABROOK, B.v., PARK, D., DAVIS, J.L., and
DIETRICH, H.B.: ‘Impact ionization in high-output-conductance
region of 0 . 5 AlSb/InAs
~
HEMTs’, Electron. Lett., 1993, 29,
(21), pp. 1888-1889
4 LI, X., LONGENBACH, K.F., WANG, Y., and WANG, W.I.: ‘Highbreakdown-voltage
AISbAs/InAs
n-channel
field-effect
transistors’, IEEE Electron Device Lett., 1992, 13, (4), pp. 192-194
I
I
I
-1 .o
-0.6
I
-0.2
gate vo[tage,V
Fig. 2 HEMT transconductance against Vcs
I
I
I
I
I
‘
I
30
High frequency and low noise C-doped
GalnP/GaAs heterojunction bipolar
transistor grown by MOCVD using TBA and
TBP
Q
E
Y.F. Yang, C.C. Hsu and E.S. Yang
E-20
tJ
3
Indexing terms: Bipolar transistors, Chemical vapour deposition,
Heterojwzction bipolar transistors
r
e
u1 0
A C-doped GaInP/GaAs heterojunction bipolar transistor (HBT)
grown by MOCVD using TBP and TBA is (demonstrated. A
current gain of 60, a cutoff frequency of 59GHz, and a maximum
oscillation frequency of 68GHz were obtained for a 5 X 1 5 p 2
self-aligned HBT. A minimum noise figure of 1.62.6 was
measured in the frequency range of 2-18GHz. The results show
that TBA and TBP are suitable MOCVD sources for growing
high quality HBT materials.
0
0.2
0.6
drain voltage, V
1925131
Fig. 3 AlSb/InAs HEMT drain characteristics
L, = 0.25pn1, W, = 4 0 p , LDS= 1.9pm,
= 0.1V
v,,,
Introduction: Carbon-doped GaInP/GaAs heterojunction bipolar
compression near V,, = 0. These features are consistent with the
parasitic bipolar transistor explanation reported in [2]. HEMT
drain characteristics with no kink effect were also observed, as
shown in Fig. 3, on the same wafer as the device reported above.
Studies are in progress to understand the variation of the kink
effect observed across the wafer. It should be noted that the
HEMTs reported here did not have an epitaxial p-type back gate
in the material design to remove the holes generated by impact
ionisation.
The broad g, characteristic at V,, = 1.8V for a non-optimised
structure is believed to be caused by the good carrier confinement
resulting from the large A&, and indicates the potential for highly
linear operation. The low on-resistance and high current capability
inherent in this material system are particularly attractive for lowbias-voltage applications.
Acknowledgments: This work was supported by the Office of
Naval Research. The authors would like to thank S.C. Binari and
H.B. Dietrich for helpful discussions, and C. Falkowski and M.
Goldenberg for measurement and material growth assistance,
respectively.
0 IEE 1996
Electronics Letters Online No: 19960408
I March I996
J.B. Boos, D. Park, B. Molnar and B.R. Bennett (Naval Research
Laboratory, Washington, DC 20375-5320, USA)
W. Kruppa (SFA, Inc., Landover, MD 20785, USA)
ELECTRONICS LETTERS 28th March 1996
Vol. 32
transistors (HBTs) have been shown to be potential candidates for
high speed and microwave integrated circuits [14]. Although
phosphine and arsine are the common group V precursors for
growing GaAs and InP based materials by MOWE, MOMBE
and CBE, both are extremely toxic gases with high pressure, which
is unsuitable in a production environment. Recently, the less toxic
and liquid tertiarybutylarsine @BA) and tertiarylmtylphosphine
(TBP) have received increased attention as a replacement for phosphine and arsine for the growth of high quality III- V materials [5,
61. Some promising results have been reported on the GaInP/
GaAs and InPiInGaAs heterostructure bipolar transistors (HBTs)
grown by MOMBE [7] and CBE [7, 81 using TBE’ and TBA. In
our early work, we demonstrated GaInP/GaAs heterostructureemitter bipolar transistors (HEBTs) [9-111 and pseudomorphic
Ga, Jn,, 8P/Ga,,,In, ,,As/InP HEMTs [121 grown by MOCVD
using TBA and TBP. The results showed that TBP and TBA were
promising precursors for growing those devices. Although there
are several reports on R F performance [24] and a few on the low
noise property of GaInPiGaAs HBT [13, 141, all devices were
grown using phosphine and arsine. In this Letter, we present a
high frequency and low noise C-doped GaInP/GaAs HBT grown
by MOCVD using TBA and TBP.
Device growth and fabrication: Epitaxial growth was performed in
a commercial (Aixtron) M O W E reactor. Triethylgallium (TEG),
trimethylindium (TMI), TBA and TBP were used as the precursors. Disilane and carbontetrachloride were used a:; n-type and p type dopant sources, respectively. The growth temperature was
600°C at a reactor pressure of 200mBar. The group V to group I11
ratio in the vapour phase was in the range of 10 to 20 during
growth. The substrate was a commercially available ‘epi-ready’
semi-insulating GaAs wafer nominally oriented in the [1001 direction. The epitaxial structure consists of a 5000w n-GaAs subcol-
No. 7
689
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