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An attempt was also made to perform the bidirectional transmission without the subcarrier modulation. In this case,
however, the performance was severely impaired by the previously mentioned coherent Rayleigh noise, and an error floor
developed around 10~6BER.
narrowband optical filter, detector and modulator. The tunability of the amplifier/detector/modulator in combination with
the tunability of the laser source allows for a communication
system with wideflexibility.For example, at the remote end,
the subscriber can choose which transmitter to listen or transmit to by tuning to the desired wavelength. Similarly, at the
laser end of the system, one can choose the remote location to
communicate with by selecting the appropriate wavelength of
the laser. The use of subcarrier modulation gives an additional
degree of freedom which is completely controlled from the
laser end, and transparent at the remote location. Some of
these features may be of interest for high-density local-areanetwork systems.
N. A. OLSSON
L. L. BUHL
17th November 1986
AT&T Bell Laboratories
Holmdel, NJ 07733, USA
Fig. 4 Received bit stream at laser end
Top trace with laser on; bottom trace with laser blocked
Discussion: The resonant amplifier used in the above experiments has several longitudinal modes or resonant frequencies
on which it can operate for a given drive current and temperature. Therefore, in a large system with many channels, the
amplifier should be combined with a coarse optical filter that
selects only one of the longitudinal modes. The width of this
filter, however, can be as wide as the mode spacing of the
amplifier (~ 12 A) and can be easily constructed from standard
optical components. The amplifier can also, of course, be
operated above threshold as a regular laser. In this mode of
operation, however, the selectivity is lost. This letter has
demonstrated the versatility of optical amplifiers in local-areanetwork systems. A single amplifier can be used as amplifier,
SELF-ALIGNED AIGaAs/GaAs HBT WITH
InGaAs EMITTER CAP LAYER
Indexing terms: Semiconductor devices and materials, Bipolar
transistors
A new self-aligned AIGaAs/GaAs HBT with an InGaAs
emitter cap layer is presented. This HBT has nonalloyed
ohmic contacts to the emitter and base that are formed
simultaneously and in a self-aligned manner. The low emitter
contact resistance of 1-4 x 10" 7 £3 cm 2 and the high transconductance per unit area of 3-3mS//«n 2 demonstrate the
effectiveness of this structure.
References
1
D'AURIA, L., CHEVALIER, G., and MAILLOT, P.: 'Libido using Eros:
half-duplex optical link', Electron. Lett., 1979,15, pp. 820-821
2 WOOD, T. H., et al.: 'Bidirectional fibre-optical transmission using a
multiple-quantum-well (MQW) modulator/detector', ibid., 1986,
22, pp. 528-529
3
4
5
DUTHIE, P. J., WALE, M. J., BENNION, i., and HANKEY, J.: 'Bidirectional
fibre-optic link using reflective modulation', ibid., 1986, 22, pp.
517-518
ALPING, A., and ENG, S. T.: 'Detection at Gbit/s rates with a TJS
GaAlAs laser', Opt. Commun., 1983, 44, pp. 381-383
ALFERNESS, R. C , KOROTKY, S. K., BUHL, L. L., a n d DIVINO, M. D . :
'High speed, low-loss, low-drive-power travelling-wave optical
modulator', Electron. Lett., 1984, 20, pp. 354-355
6
OLSSON, N. A., and GARBINSKI, P.: 'High-sensitivity direct-detection
receiver with a 1-5 nm optical pre-amplifier', ibid., 1986, 22, pp.
1114-1116
epitaxy on an n+-GaAs substrate. The emitter cap layer consisted of three layers, an /i + -GaAs layer (1000 A), an
n+-InxGa!_xAs (x = 0 - 0-5) graded layer (500 A) and an
n+-In0.5Ga0.5As uniform layer (500 A). The graded layer was
inserted to smooth out the conduction-band discontinuity.
These layers were doped with Si to 2 x 10 19 cm" 3 . The base
l
dd
layer was a graded
(y = 0 - 0 1 22
) layer
doped
with Be to 2 x 1019cm
n
SiO2sidewall
777Zz777Ziri
cap
emitter
*-' n Q.5 G a 0 5 A s
n + -In x Ga,. x As
GaAs
? ///////l
base
collector
Introduction: In realising the high switching speed of heterojunction bipolar transistors (HBTs), minimising the parasitic
elements is very important. To this end, self-alignment and ion
implantation techniques have been successfully used to reduce
base resistance and base-collector capacitance.1"3 To attain
further improvement, however, the reduction of emitter resistance is also necessary.4
The adoption of an InGaAs cap layer on GaAs has been
found to be effective in obtaining very low ohmic contact to
n-GaAs without alloying.5"7 Therefore, the use of an InGaAs
emitter cap layer in an HBT is expected to reduce the emitter
resistance drastically. A nonalloyed ohmic contact is already
being applied to p-GaAs (or AlGaAs) base layer in HBTs. 26
The adoption of nonalloyed ohmic contacts to both emitter
and base layers makes it possible to form emitter and base
electrodes in a one-step process and in a self-aligned manner.
By combining the InGaAs emitter cap layer with a previously
developed self-aligned structure (sidewall-separated base electrode structure2), we can realise a reduction in the emitter
resistance while keeping other parasitic elements low.
This letter describes the fabrication and DC performance of
a new self-aligned AIGaAs/GaAs HBT with an InGaAs
emitter cap layer.
The device structure shown in Fig. 1 was fabricated based
on the SSBE technique. After emitter mesa-etching using a
Si3N4 mask and SiO2 sidewall formation, the emitter and base
electrodes were formed in the following steps. First, the
surface of the emitter was revealed by removing the Si3N4
emitter mesa mask, then Ti/Pt/Au was evaporated on to the
whole surface, and finally the metal on the SiO2 sidewall was
removed by angled Ar ion milling. The base mesa was made
by Ar ion milling of the Ti/Pt/Au layer and sequential reactive
ion beam etching with BC13 of the AlGaAs (base) and GaAs
(collector) layers. The collector electrode was formed on the
back surface of the substrate by depositing AuGe/Ni/Ti/Au.
Sintering was carried out in an ambient of N 2 at 350°C for
30 s.
Fabrication: The transistor structure investigated is shown in
Fig. 1. The epitaxial layers were grown by molecular beam
Device performance: The fabricated HBT has emitter dimensions of 6/im x 6/*m. Its DC characteristics are shown in
64
n + - GaAs substrate
Fig. 1 Schematic structure of self-aligned HBT with InGaAs emitter
cap layer
ELECTRONICS LETTERS 16th January 1987 Vol. 23 No. 2
Fig. 2. Fig. 2a is VCE/IC characteristics with a series of base
current steps. The current gain /? of 20 was obtained at a
collector current density Jc of 2 x 104 A/cm2.
The transconductance of the device was measured for evaluation of its parasitic resistance. VCEJlc characteristics with a
series of emitter-base voltage steps is shown in Fig. 2b. In a
bipolar transistor the intrinsic transconductance g^ is
expressed by
9mo
kT
where q is electron charge, k is Boltzmann's constant and T is
temperature. In an actual device with parasitic resistances, the
transconductance gm is expressed by
9m =
(R
where REE is emitter resistance and RB is base resistance. Since
the collector current density is relatively high in an HBT in
comparison with that in an Si bipolar transistor, the influence
of emitter resistance is more pronounced in the practical operation range. In our device gm per unit area was 3-3mS//mi2 at
Jc = 4 x 104 A/cm2. In spite of the rather large emitter size of
6 /mi x 6 pan. this value is still very large and comparable with
the highest value of 3-75mS//mi2 reported in a 1-6/mi x 5/mi
emitter HBT.8 From 3-3mS//mi2 the effective parasitic resistance in the device, REE + RB/P, is calculated to be 6-7 Q.
conductance of the fabricated HBT is limited by the base
resistance.
The base resistance can easily be reduced by scaling down
the transistor size and by increasing the doping in the base
layer. Therefore, further improvement in the device performance should be attained in smaller-size HBTs with lower
base contact resistance.
Conclusion: A new self-aligned AlGaAs/GaAs HBT with an
InGaAs emitter cap layer has been successfully fabricated. In
this structure the nonalloyed emitter contact enabled forming
the emitter and base electrodes simultaneously and in a selfaligned manner. The transconductance per unit area as high
as 3-3mS//im2 and the emitter contact resistance as low as
1-4 x 10" 7 Qcm 2 obtained demonstrate the effectiveness of
the proposed InGaAs cap layer HBT structure.
Acknowledgments: The authors wish to thank Y. Yamauchi
and S. Adachi for valuable discussions and K. Hirata for his
helpful suggestions.
K.
O.
T.
H.
T.
NAGATA
NAKAJIMA
NITTONO
ITO
ISHIBASHI
12th November 1986
NTT Electrical Communications Laboratories
3-1, Morinosato Wakamiya
Atsugi-shi, Kanagawa 243-01, Japan
References
1
CHANG, M. F., ASBECK, P. M., MILLER, D. L., a n d WANG, K. C . : ' G a A s /
(GaAl)As heterojunction bipolar transistors using a self-aligned
substitutional emitter process', IEEE Electron Device Lett., 1986,
EDL-7, pp. 8-10
2
NAGATA, K., NAKAJIMA, O., YAMAUCHI, Y., a n d ISHIBASHI, T.: 'A n e w
self-aligned structure AlGaAs/GaAs HBT for high speed digital
circuits'. Inst. Phys. Conf. Ser., 1986, no. 79, pp. 589-594
3
Fig. 2 Typical VCE/IC characteristics (a) with series of base current
steps and (b) with series of emitter-base voltage steps
4
To evaluate each resistance (REE, RB), sheet and contact
resistances for emitter and base layers were measured in the
TLM patterns fabricated on the same wafer. The emitter
contact resistance as low as 1-4 x 10~7Qcm2 was obtained,
as shown in Fig. 3. This value is much lower than the value
for a conventional AuGe/Ni contact to n+-GaAs
(~1 x 10~6ficm2). The specific contact resistance leads to
REE = 04Q, in a 6/im x 6/mi emitter device. On the other
hand, the contact resistance and sheet resistance for the base
layer were 1-3 x 10~5Qcm2 and 408 Q/D, respectively. Using
these values, RB/f} is calculated to be 5-6fi.The sum of the
parasitic resistances fits very well to the experimental data of
REE + RB/P = 6-7 Q in gm measurement. From these results it
is noted that (i) the emitter resistance is drastically reduced by
using the InGaAs emitter cap layer and (ii) the trans10
1
R j = 28 A /
5
ASBECK, P. M., MILLER, D. L., ANDERSON, R. J., a n d EISEN, F. H.: ' G a A s /
(Ga, Al)As heterojunction bipolar transistors with buried oxygenimplanted isolation layers', IEEE Electron Device Lett., 1984,
EDL-5, pp. 310-312
YAMAUCM, Y., and ISHIBASHI, T.: 'Dependence of switching performance on emitter resistance and current gain in GaAs/AlGaAs
HBT ECL gates', Trans. IECE Jpn., 1986, E69, pp. 286-287
WOODALL, J. M., FREEOUF, J. L., PETTIT, G. D., JACKSON, T., a n d
KIRCHNER, p.: 'Ohmic contacts to n-GaAs using graded band gap
layers of Gaj^In^As grown by molecular beam epitaxy', J. Vac.
Sci. & Technoi, 1981,19, pp. 626-627
6
RAO, M. A., CAINE, E. j . , LONG, s. i., and KROEMER, H. : 'AlGaAs/GaAs
heterostructure bipolar transistor with non-alloyed graded-gap
ohmic contacts to the base and emitter'. Paper IIIA-5 presented at
the 44th device research conference, 1986
7
NITTONO, T., ITO, H., NAKAJIMA, o., and ISHIBASHI, T. : 'Extremely low
resistance non-alloyed ohmic contacts to w-GaAs using compositionally graded In^Ga^^As layers', Jpn. J. Appl. Phys., 1986, 25,
pp. L865-L867
8
ASBECK, P. M., MILLER, D. L., ANDERSON, R. J., DEM1NG, R. N., CHEN, R.
T., LIECHTI, c. A., and EISEN, F. H. : 'Application of heterojunction
bipolar transistors to high speed, small-scale digital integrated circuits'. Tech. dig. of 1984 IEEE GaAs IC symp., 1984, pp. 133-136
1
D
• R^ = i A x 1 0 7 i l c m 2
CALCULATION OF THE RADIATION LOSSES
OF GROOVE-GUIDE E-PLANE BENDS
6 .
U
Indexing term: Waveguides
1
•
40
i
-
•
10
|570/3|
Fig. 3 Sheet and contact resistances for emitter layer measured by
TLM method
ELECTRONICS LETTERS 16th January 1987 Vol. 23 No. 2
An approximate closed-form expression for the evaluation of
groove-guide radiation losses is presented and confirmed by
measurements.
Introduction: In a recent publication some basic studies have
been made concerning the radiation phenomenon of JE-plane
groove-guide bends.1 As has been found empirically these
bends exhibit radiation losses whenever a characteristic transverse cutoff frequency is exceeded. To the author's knowledge
65
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