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Acknowledgments: This letter has been sponsored in part by
CSELT Torino and Italian MPI.
G. D'ELIA
27th February 1984
IRECE-CNR
Via Claudio, 21
80125 Naples, Italy
G. LEONE
R. PIERRI
Dipartimento di Ing. Elettronica
Universita di Napoli
Via Claudio, 21
80125 Naples, Italy
References
1
2
3
4
BUCCI, o. M., FRANCESCHETTi, c , and PIERRI, R.: 'Reflector antennas
field. An exact aperture-like approach', IEEE Trans., 1981, AP-29,
pp. 580-586
BUCCI, o. M., D'ELIA, C , FRANCESCHETTI, c , and PIERRI, R. : 'Efficient
computation of the far field of parabolic reflectors by pseudosampling algorithm', ibid., 1983, AP-31, pp. 931-937
D'ELIA, G., and PIERRI, R.: 'Iterative method and sampling technique in the analysis of shaped reflectors'. Proceedings of 11th
European Microwave Conference, Amsterdam, Netherlands, Sept.
1981
D'ELIA, G., PIERRI, R., and SCAFURO, N.: 'Analysis of a strip in
Fresnel zone using the pseudo sampling technique'. Proceedings of
7th Colloquium on microwave communication, Budapest,
Hungary, Sept. 1982, pp. 291-294
5
BENCINI, F., D'ELIA, G., and PIERRI, R. : 'Numerical evaluation of
Fresnel zone fields by sampling-like technique'. Digest of AP-S
Symposium, Albuquerque, USA, May 1982, pp. 515-518
60 GHz MONOLITHIC GaAs FRONT-END
CIRCUIT FOR RECEIVER APPLICATIONS
Indexing terms: Semiconductor devices and materials, Integrated circuits, Mixers
A millimetre-wave planar mixer diode compatible with GaAs
MESFET based integrated circuit fabrication has been developed. Selective ion implantation was used to optimise the
diode and FET doping profiles. A novel feature reported
here is the use of a deep implanted buried n+ layer to minimise diode series resistance, yielding diode cutoff frequencies
in excess of 500 GHz. Monolithic balanced mixer diodes
integrated with an MESFET I F amplifier fabricated by this
technique have given 5dB conversion loss at 60 GHz.
Introduction: GaAs mixer diodes are capable of operating at
very high frequencies, well into the millimetre-wave region.
Diode design is based on minimising diode series resistance,
junction capacitance and parasitic capacitance. These are
usually achieved by the use of n+ substrates and a shallow
low-doped layer on which very small Schottky contacts are
formed.1 Planar diodes using semi-insulating substrate tend to
have poor high-frequency performance unless a highly doped
n+ layer is provided under the diodes to minimise the series
resistance. Great care has to be taken to minimise the parasitic capacitance on planar structures and to avoid the
Schottky electrode coming into contact with the edge of the
n+ layer at the mesa step.2 Alternatively proton isolation may
be used to avoid such a step.3 The two-layer optimum diode
structure is not, however, compatible with the well established
GaAs MESFET-based integrated circuit process, where a
high-resistivity layer instead of an n+ layer is required under
the FET channel. Earlier attempts to demonstrate diode/FET
integration had to resort to selective MBE or epitaxial
growth, which are not reproducible processes.4 In this letter
we report the use of a selective ion implantation technique to
enable the separate optimisation of the diode and the FET
layers for monolithic integration. A novel feature is the use of
a deep implanted buried n+ layer to achieve diodes with low
series resistance.
334
Circuit design: Many factors have been taken into consideration in the design of a millimetre-wave monolithic circuit
such as flexibility in circuit layout, maintenance of high Q
values, and line width control to prevent overmoding and
mechanical rigidity. Microstrip was chosen as the most suitable transmission medium compared with suspended stripline
or coplanar waveguide. A balanced mixer circuit design was
adopted since no DC return is required, considerably simplifying chip layout (Fig. 1). The diodes are situated at two adjacent ports of a 3 dB 90° branch hybrid coupler, and each diode
matching quarter-wave stub
FET bias
IF FET amplifier
resistor
mixer diode'
stripline
3dB hybrid
suspended striplines
forming E - field probes
Fig.l 60 GHz GaAs front-end integrated circuit consisting of balanced
mixer diodes, matching quarter-wave stubs, hybrid coupler, IF
MESFET with bias resistor and two waveguide to stripline transistions
Chip measures 8 x 2 m m
is matched by a quarter-wave stub to provide an RF shortcircuit. A conventional coupler design was used with an
impedance ratio of 62 Q/43 Q and a centre frequency at
60 GHz. The impedance of the quarter-wave stubs is also
43ft and this is the lowest impedance that can be used on a
125 /^m-thick GaAs substrate before the transmission-line
linewidth becomes a significant fraction of the wavelength. To
connect RF and LO power into the coupler, 62 Q transmission lines are used. At the extreme end of these transmission
lines there is no ground plane and they serve as probes suspended on GaAs which couple power from the E field of
V-band waveguides. The passive components have been scaled
to X-band on thicker GaAs substrates to validate the designs.
An untuned MESFET IF amplifier was used to provide low
gain with broad bandwidth in this initial experiment. Matching structures would have consumed a large area, making the
chip too broad. This in turn would have given unwanted
coupling between the two waveguides via the slot where the
GaAs IC is positioned (Fig. 2). To minimise the diode parasitic
Fig. 2 60 GHz waveguide test fixture showing the mounting arrangement of the GaAs IC between two waveguides and the IF output port
resistances and capacitances, each mixer diode was subdivided
into ten small fingers.5 Each finger measures 1 /mi wide by
2 ^m long. Several test structures were also incorporated on
the chip to provide data on diode series resistance, junction
and edge capacitances and SB and ohmic contact characteristics.
Technology: The integrated circuits were fabricated using
direct ion implantation into 2 in-diameter LEC semiinsulating substrates. A selective ion implantation technique
compatible with capless annealing was used to separately optimise the FET channel and the low-resistance diodes.6 For
FETs, (Si29)+ was implanted at 100 kV and 5 x 102/cm2 dose.
ELECTRONICS LETTERS 12th April 1984
Vol. 20 No. 8
For diodes, (Si 2 8 ) + + was implanted at 200 kV, equivalent to
400 keV energy, and a 1 x 10 14 /cm 2 dose to obtain a deep
buried n+ layer for low diode series resistance. Peak doping
concentration of this buried layer is approximately 1 x 10 18 /
cm 3 at 0-5 ^m depth, with the doping falling towards the
surface to 10 17 /cm 3 (Fig. 3), where CrAu Schottky barrier
10
(ii) SB diode
'S,d7
MESFET has 2-5 V pinch-off voltage and 100 mS/mm mutual
transconductance and has an untuned gain of 3 dB and 3 dB
noise figure at 2 GHz IF in a 50 Q circuit.
RF measurements were carried out using a swept impatt
source in the CW mode manually scanned from 55-5 to
59 GHz at 0 1 mW and a fixed-frequency Gunn oscillator as
LO at 60 GHz and 20 mW. The typical conversion loss of the
monolithic circuit was 50 dB + 1 dB for IF frequencies
between 1 0 and 4-5 GHz as shown in Fig. 4. Measurements
on chips without the IF FET gave typically 6-5 dB ± 0-5 dB
conversion loss. In both cases the total passive circuit loss is
approximately 10 dB. It is expected that with the incorporation of a tuned IF amplifier, circuits with greater than unity
gain could be obtained.
Conclusion: A millimetre-wave planar mixer diode compatible
with GaAs MESFET-base integrated circuit fabrication has
been developed. It has been shown that deep Si implantation
into LEC semi-insulating substrate can provide an adequate
buried n + contact layer to minimise diode series resistance,
yielding diode cutoff frequencies in excess of 500 GHz. Monolithic balanced mixer diodes integrated with an MESFET I F
amplifier have been demonstrated with this technique, giving
5 dB conversion loss at 60 GHz.
\
\
10
05
depth,j
10
Fig. 3 Doping profiles of MESFET and mixer diode showing deep
buried n+ contact layer for the diode
CV profiling method was used for (i) and Polaron electrochemical
profiler was used for (ii)
diode contacts were made. Ohmic contacts, consisting of
AuGe-Ni, were also made on this layer and good contact
resistance are obtained due to the fact that ohmic metallisation diffuses readily through the thin low-doped surface
layer.
The processing steps consist of etching alignment marks,
selective FET channel and resistor implant, selective diode
and FET n + contact implant, capless anneal with As overpressure at 85O°C for 30 min, and ohmic contact deposition
(0-8 jim) for the passive components. Finally, a layer of polyimide is used to passivate the FETs and resistors but not the
diodes. Wafers are then thinned to 125 /zm thick before dicing.
G. K. BARKER
27th February 1984
M. H. BADAWI
J. MUN
Standard Telecommunication Laboratories Ltd.
London Road, Harlow, Essex, CM 17 9NA, England
References
1
2
BOCCON-GIBOD, D., and HARROP, p.: 'High performance
GaAs
Schottky barrier diodes using a cantilevered metal contact'. Ibid.,
pp. 696-700
3
CHAO, C , CONTOLATIS, A., JAMISON, S., a n d BUTTER, C : '94 G H z
monolithic GaAs balanced mixers'. IEEE Monolithic Circuits
Symposium, Boston, 31 May-1 June 1983, pp. 50-53
4
CHU, A., COURTNEY, w. E., and SUDBURY, R. w.: 'A 31 GHz mono-
lithic GaAs mixer/pre-amplifier circuit for receiver applications',
IEEE Trans., 1981, ED-28, pp. 149-154
5
KELLNER, w., ENDERS, N., and KNIEPKAMP, H. : 'Design conditions of
planar SB diodes', Solid-State Electron., 1980, 23, pp. 9-15
6
Results: DC measurements on the millimetre-wave mixer
diodes showed a barrier height of 0-73 V and an ideality
factor of 117. Typical reverse breakdown voltage was of the
order of 8 V. Diode series resistance for ten parallel diode
fingers with a total area of 20 ftm2 was 5-5 fi, and for 60 fim2
total area was 2 Q. The measured results are within 30% of
the calculated values. The diode junction and fringe capacitances (extrapolated from larger-area diodes) are 50 fF for
20 fim2 and 95 fF for 60 ^m 2 area diodes. This gives a diode
cutoff frequency of 580 to 800 GHz, respectively, well above
the intended operation frequency of 60 GHz. The IF
SCHNEIDER, M. v.: 'Low noise millimeter-wave Schottky mixers'.
8th European Microwave Conference, Paris, 4-8 Sept. 1978, pp.
682-688
BADAWI, M. H., DUNBOBBIN, D. R., and MUN, j . : 'Selective implanta-
tion of GaAs for MESFET applications', Electron. Lett., 1983, 19,
pp. 598-600
COMMENT
SYSTEM REDUCTION BY CAUER
CONTINUED-FRACTION EXPANSION ABOUT
s = a AND s = oo ALTERNATELY
without IF amplifier
J
c 5
o
with IF amplifier
-
2
0
1
2
3
IF frequency ,GHz
Fig. 4 Conversion loss of integrated circuit against IF frequency measured with and without IF MESFET amplifier
ELECTRONICS LETTERS 12th April 1984 Vol. 20 No. 8
Sivanandam and SreeramA have suggested a refinement to the
method of Parthasarathy and Jayasimha 8 for reducing the
order of linear systems by continued-fraction expansion about
s = a and s = cc alternately. They use the idea c of changing
the zeros of the reduced transfer function obtained by this
method by adjusting the constant term in the numerator such
that the steady-state gain is equal to that of the full system.
However, it should be pointed out that this operation ensures
that the final reduced model will not, in general, retain any
terms about s = a and only (k — 1) terms about s = co (for a
fcth-order model) from the full system.
This can be proved by considering a general fcth-order
reduced model, which is assumed to have been derived by the
method of Reference B; i.e.
ak-.l(s-a)k
(A)
335
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