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Above 20har a saturation was observed which is due to the onset
of avalanche breakdown of the transistor. We obtained a sensitivity of 200mV/har which is lower than the theoretically predicted
value. This is due to the pressure signals having a frequency well
below 700Hz. In special equipment, pure thermal pulses of 5kW
were applied which corresponds to the temperature stress in a
motor running at 75% of its power. A negative pressure signal
corresponding to 0.63bar was detected which can be attributed to
a distortion of the membrane in the test fixture. It is noteworthy
that the thermoelectric effect which is quite large in GaAs has no
considerable influence due to the arrangement of the sensors and
the fact that the thermal gradient and hence the thermoelectric
field are almost perpendicular to the piezoelectric field
5
MOCK. R., and MEIXNER. H.: ‘A miniaturized high-temperature
pressure sensor for the combustion chamber of a spark ignition
engine’, Sensors and Actuators A, 1991, 25, (27). pp. 103-106
3 FRICKE. K., and HARTNAGEL. H.L.: ’Pressure measurement by GaAs
piezoelectric sensors’, Electron. Lett., 1990, 26, (1 I), pp. 693-694
4 PRICKE. K.: ‘Piezoelectric properties of CaAs for application in
stress transducers’, J. Appl. Phys. 1991, 70, (2). pp. 916918
2
First hydride free GalnP/GaAs carbon doped
HET grown by CBE using DMAAs and TBP
B. Lamare, J.L. Benchimol, R. Driad and P. Launay
4
Indexing f w m x Heterojunction hipolur tran.~i.stors,Chemical beam
epifusial grow,th
3
A current gain of 120, for a base sheet resistance of400 CUD, is
reported in a carbon doped base heterojunction bipolar transistor
grown by chemical beam epitaxy without hydride sources. This
result is to the authors’ knowledge, the best obtained with hydride
free CBE for this device.
.’
0
m
-0
I
92
c
a
3
.3.
pressure ,bar
Fig. 4 Dependence of output voltage on applied pressure at 3 m A droin
current
Conclusion: We have demonstrated that a GaAs MESFET can be
a used as a piezoelectric sensor with an integrated charge amplitier. The sensor gives reproducible signals which compare perfectly
with commercially available reference sensors, which are far more
bulky and more sensitive to disturbing signals. The temperature
sensitivity is low enough so that the sensor can he used in environments with rapidly changing temperatures. An improvement in the
membrane could eliminate the remaining signal. The sensor can be
applied not only in combustion engines but everywhere a dynamic
pressure measurement is performed and the signal has to be amplitied near the sensor due to wiring problems. We can also imagine
that small signal processing circuitry and a data transmitter will he
integrated on the same chip so that the sensor can he used in hostile environments or wherever wiring problems occur, e.g. in a
rotating system.
Introduction: Various techniques, such as metal organic vapour
phase epitaxy (MOVPE), metal organic molecular beam epitaxy
(MOMBE) or chemical beam epitaxy (CBE) have been used to
grow GalnPIGaAs heterojunction bipolar transistors (HBT) with
hydrides (ASH,, PH,) as the source of group V elements. Owing to
the extreme toxicity of these sources, safer alternatives for 111-V
epitaxy are highly desirable. The first hydride free epitaxial process for this application was investigated by Beam et al. [I] in
MOMBE, with elemental Ga and In as group 111 sources and tertiarybutylarsine (TBAs) and tertiaryhutylphosphine (TBP) as
group V sources. Recently, Ng et al. [2] have shown reasonable
static and dynamic characteristics for devices grown by CBE using
group I11 metal organic sources, TBAs and TBP as group V
sources.
In this Letter, we report a hydride free CBE epitaxial process,
using TBP and trisdimethylaminoarsenic (DMAAs). This arsenic
precursor has the advantage over TBAs of being less thermally
stable, so that it can be used without precracking, and of allowing
the growth of GaAs layers with low background residual doping
using triethylgallium (TEG) [3]. This work shows that TBP and
DMAAs are compatible with the growth of GaInPiGaAs HBT
structures, and that a carbon doped GaAs base can be grown
from DMAAs and TMG.
Table 1: GalnP/GaAs HBT structure and name of organometallic
precursors used for growth
Acknowledgments: First of all we wish to thank P.KieSlich for his
invaluable help in the production of all mechanical parts. Special
thanks is due to the Deutsche Forschungsgemeinschaft for financing the project under SFB241.
0 IEE 1994
IO June I994
Electronics Letters Online No: I9940910
G. Schweeger, C. bang, K. Fricke and H. L. Hartnagel (fnstitut.fur
Hochfrequenztechnik, TH Darmstadt, Merckstr. 2s. 642x3 Darmstudt.
Germany)
R. Dolt and G. Hohenberg (Fachgebier Verbrennungskruftmaschinen.
TH Darmstadt. Perersenstr. 30, 64287 Durmstudt, Germany)
References
I
HOHENBERGG., and DOLT,R.: ‘Ein Konzept m r adaptiven
SteuerungIRegelung von Verbrennungsmotoren unter Verwendung
eines Online-Brennverlaufrechners’. Proc. Meeting Integrierte
Mechanisch-Elektronische Systeme, 2-3 March 1993, (VDI-Verlag,
Darmstadt), pp. 58-69
1356
Growth condirions: The HBT structure shown in Table I was
grown on a 2inch semi-insulating (100) GaAs substrate. TEG,
TMG and trimethylindium (TMI) were used as group I11 precursors, DMAAs and TBP as group V precursors, solid Si as the ntype dopant and TMG as the p-type dopant. All metal organic
sources were introduced into the growth chamber without carrier
gas and controlled by a mass flow controller, except for the
DMAAs which was introduced by a leak valve due to its low
vapour pressure (1.35 torr at 20°C).
The originality of this work is the carbon doping of GaAs for
the HBT base using uncracked DMAAs and TMG, as opposed to
the work by Ishikura et ai. [4], who find precracking necessary for
carbon incorporation. The maximum doping level obtained at a
growth temperature of 490°C and a VI111 ratio of -1, was 4 x
at .cm as measured by SIMS. The Hall mobilities as a function of doping level were equivalent to those obtained with ASH,
~31.
ELECTRONICS LE77ERS
4th August 7994 Vol. 30 No. 76
The high carbon incorporation observed with T M G is in contrast with previous studies of Abernathy et al. [SI, who reported
- 3 TMG. This difference
carbon incorporation below 1 0 1 6 ~with
might be attributed to the absence of hydrogen carrier gas for
transporting DMAAs in the present work.
The GaInP emitter was grown with TBP, which was injected
into a low pressure home made cracking cell held a t a temperature
of 800°C. This temperature corresponds to a complete decomposition of TBP into P, and a minimal impurity incorporation. The
growth rate was l w h at a temperature of 510°C. The relative
lattice mismatch between GaInP and GaAs was 5 x IO“.
-
of 110 Aicm’, with a maximum value of 120. This result is equivalent to those obtained with structures grown from hydrides in
our laboratory [6]and better than the recent result reported with
all metal organic precursors [2] (Fig. 2). We attribute these better
results to the use of DMAAs unstead of TBAs in the growth of
carbon doped GaAs base. The emitter-base threshold voltage is
1.3V, which demonstrates no diffusion of p-type dopant towards
the emitter layer. A typical Gummel plot of the same device is
shown in Fig. 3. The collector and base current ideality factors (n,
and na) are 1.05 and 1.94, respectively.
Device technology: Large contact devices were realised using a tri-
ple mesa technology. Standard optical lithography and selective
wet etching techniques were applied for device fabrication. The
emitter-collector contact was AuGeNilAgiAu. AuMn was used to
contact the base layer, without removing the GaInP layer, which
must be thin enough (< MOA) to allow manganese diffusion into
the base layer during the ohmic contact annealing (300’C).
1
Oo5t
0-8
1-0
12
1-4
16
Fig. 3 Gummelplot ofdevice
0-0002
Conclusion: This work has shown that DMAAs is a convenient
arsenic precursor for the growth of a carbon doped GaAs base in
the 1019cm~3
range with TMG and n-GaAs collector using TEG.
DMAAs and TBP have proved to be safer alternatives to hydrides
for CBE growth of carbon doped GaInPiGaAs HBTs with state
of the art static performances.
0~0001
O 00
1.
2
O 3
4 I
5
C6
%e. v
Fig. 1 DC current-voltage characteristics as function of base current
Acknowledgments: The authors would like to thank M. Juhel for
SIMS measurements, G. Le Roux for X-ray measurements, C.
Besombes, L. Bricard for support in technology, J.P. Medus for
device measurements and F. Alexandre for helpful discussions.
0 IEE 1994
Electronics Letters Online No: I9940844
18 May 1994
B. Lamare, J.L. Benchimol, R. Driad and P. Launay (France Telecom,
CNETIPAB, Laboratoire de Bagneux, 196 A V Henri Ravera, BP 107,
92225 Bagnarx Cedex. France)
C?
References
102
base sheet resislancqnlo
Fig. 2 DC current gain against base sheet resistance
Comparison of HBT structures grown by CBE using different group V
sources
0 CNET (AsH3+PH3) [6]
Thomson (AsH3+TBPI
Thornson
(AsH3+TBP) 171
[7]
U. of Mic&gan/Thoms&’(TBA+PH3)
U
Micbigaflhomson (TBA+PH3) 121
0U
U. of Micbiganmhomson (TBA+TBP) [2
CNET (TBP+DMAAs) (this work)
+
*
Device re.sults: DC testing was performed on completed devices
~ ~ sheet resistance RO,
with an emitter area of 90 x 1 6 0 Base
with low dispersions (4%), were deduced from the transmission
line model measurements (TLM). R O values of 400i22/0 were
obtained for a base doping level of 4 x 1019cm-3.Fig. I shows the
common emitter I-V characteristics. The offset voltage of lOOmV,
which is a typical value for a GaInPiGaAs HBT, confirms the
interface quality. The D C current gain was 90 a t a current density
ELECTRONlCS LE7TERS
4th August 1994
Vol. 30
1
BEAM, E . A . HENDEKSON,T.S., SEABAUGH, A.C.,
3
LAMARE, B., RENCHIMOL, J.L., IUHEL, M., AKAMATSU, B., LEGAY, P.,
and YANG, J.Y.: ‘The
use of tertiarybutylphosphine and tertiarybutylarsine for the
metalorganic molecular beam epitaxy of the InGaAshP and
InGaPiGaAs material systems’, J. Crystal Growth, 1992, 116, pp.
436-446
2 N G . G . 1 , P A V L I D I X D , SAMELIS, A., PEHLKE, D . , GARCIA. J.C., and
HIRTZ. J.P.: ’Demonstration of GaInPiGaAs HBT grown with
reduced toxicity all metalorganic molecular beam precursors’. Proc.
6th InP and related materials Conf. IEEE, Santa Barbara, March
1994, pp. 3 9 9 4 2
and ALEXANDRE, F.: ‘High carbon doping of GaAs using DMAAs
and TMG in CBE, to be published in J. Crystal Grow’th
4 ISHIKURA. K., TAKEUCHI, A., KURIHARA. M., and MACHIDA, H.:
‘Growth condition dependence of carbon reduction in GaAs
chemical beam epitaxy using trisdimethylaminoarsine and
trimethylgallium’, J. Jpn. Appl. Phys.. 1994, pp. L49U96
5
ABERNATHY, C.K., WISK, P.W., PEARTON, S.J., REN. F., BOHLING. D . A . ,
and MUHR, G.T.: ‘Alternative group V sources for growth of GaAs
and AlGaAs by MOMBE (CBE)’, J. Crystul Growth, 1992, 124,
pp. 64-69
6
BENCHIMOL. J.L., ALEXANDRE, F , DUBON-CHEVALLIEK, C., HELIOT, F.,
BOURGIGA, R., DANGLA, I., and SERMAGE. 8.: ‘Very high gain in
carbon-doped base heterojunction biopolar transistor grown by
CBE, Electron. Lett., 1992, 28, pp. 1344-1345
No. 16
1357
7
REGREW, PH.,
DELAGE, s.L.,
BLANCK, H.,
and
'Chemical beam epitaxy of GaInP using
tertiaryhutylphosphine', J. Crystal Growth, 1993, 127, pp. 255-257
GARCIA, KH.,
HIRTZ,I.P.:
Low-temperature characteristics of 0.35ym
AISb/lnAs HEMTs
W. Kruppa and J.B. Boos
Inde-Xing terms: High electron mobility transistors, Impact
ionisation, Cryogenics
The effects of low temperature on the characteristics of 0 . 3 5 ~
gaie-length AISbllnAs HEMTS are reported. Measurements down
to 15K reveal an increase in transconductance and low-field
source-drain conductance. The commonly-observed impact
ionisation and its associated gate current were found to decrease
significally ai low temperature apparently due to a increase in
bandgap.
Recent interest in the use of InAs for HEMT chanels is based on
several desirable properties of this material [l]. Compared to the
more commonly used GaAs and InGaAs, electrons in InAs have
lower effective mass, higher mobility, and larger r-Lvalley separation, which contributes to a higher peak velocity. Moreover, the
AISh/InAs heterojunction has a very large conduction band discontinuity (1.35eV), which yields high values of 2-DEG sheet carrier concentration. Unfortunately, owing to the narrow bandgap
(0.35eV at 3DOK), InAs has impact ionisation rates several orders
of magnitude larger that the competing materials. In this Letter
the results of measurements are reported which examine the characteristics of 0 . 3 5 AlSh/InAs
~
HEMTs at low temperature and
provide additional insight into the impact ionisation mechanism
and its effects.
The device layer structure is grown on a (100) undoped GaAs
substrate. A thick 2 . 3 AlSh
~
buffer layer is used to accommodate the 7% lattice mismatch. Then, in order of growth, the structure is formed by a 15nm undoped InAs channel. a 50nm AlSh
donor layer. where the donors are supplied by an As soak technique, and finally a 7nm undoped GaSb cap layer, which is
needed to prevent oxidation of the AlSh donor layer. Additional
growth and fabrication details are given elsewhere [2]. In these initial fabrication runs the donor layer is significatly thicker than
optimum, and consequently the transconductance is lower and the
threshold voltage higher than values anticipated in future runs.
The measured Hall mobility of these layers is 21000 (67000) cmY
V.s at 300K (77K) and the sheet carrier concentration is 2.1 x IO',
(1.9 x 1012)cm-z.The device has a gate length of 0 . 3 5 ~and a
source-drain spacing of 3 . 5 ~ The
.
total channel width is 5 0 ~ .
Based on measured S-parameters, the fT and fmx for these devices
are both equal to SOGHz.
saturation mechanism, and there is a large kink phenomenon, particularly where the drain voltage pulls the device out of pinch-off.
These features have also been reported by other researchers. The
drain characteristics at 15K are shown in Fig. 2. Compared to
Fig. 1, there is a doubling in the low-field source-drain conductance and a significant increase in the peak transconductance, both
of which can he attributed to the increased mobility. The saturation, which occurs at 15K for small gate voltages, can be partially
explained by a reduction in impact ionisation at low temperature.
It should be recalled that in InAs the bandgap expands from 0.35
to 0.42eV when the temperature is lowered from 300K to 77K
and consequently the impact ionisation in this material actually
diminishes at low temperature [3]. Another possible reason for the
improved saturation is better carrier confinement at the lower
temperature. It should he noted that although the reduced impact
ionisation yields a saturation effect, the large kink phenomenon
remains or is even enhanced. It appears that this kink may be
caused by weak impact ionisation with associated hole trapping
which increases the channel charge.
Fig. 2 HEMT drain characteristics at ISK
The effect of temperature on the gate current is given in Fig. 3.
The large reduction in the current as the temperature is lowered
from 300K reflects a reduction in both the gate leakage current
and channel impact ionisation at lower temperatures. Additional
measurements on these and other devices indicate that for the
larger drain voltages in Fig. 3, a large component of the gate current consists of hole current due to impact ionisation [2].
3001
20
4
E.
E5
10
U
.-C
P
0
U
100
200
ternperatu1e.K
m300
Fig. 3 Gate current as function of temperature for various values of
drain voltage
0
0
05
dram voltage. V
1.o
1485111
15
Fig. 1 HEMT drain charactenstus ut 300K
The drain characteristics at 300K are shown in Fig. 1. Two
unusual features are that at small gate voltages, there IS almost no
1358
A novel way to examine the impact ionisation is to measure the
R F output impedance as a function of frequency and bias [2]. The
values of S,, for four temperatures over the frequency range from
300kHz to lGHz are shown on a section of the Smith chart in
Fig. 4. It should he noted that the two lowest-temperature curves
are segmented into two frequency ranges, separated by a cusp
ELECTRONICS LETTERS
4th August 1994
Vol. 30
No. 16
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