Session S6: Schottky Diodes and Mixers - Department of Physics

21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010
Two diode-MMIC wafers were fabricated at JPL using the
very successful membrane process developed for Hershel
HIFI [3]. The wafers had epi-layer doping of 5x10 17 /cm 3
and 2x10 17 with identical circuit layouts. Variations of the
design were scaled to as high as 3.5 THz, but tests reported
here are of only the 1.5 THz design. Simulations of the
performance of these designs indicate a relatively flat RF
fractional bandwidth of 25%. Upconverted power is limited
by the input power which is very constrained in a circuit with
just two anodes, since the anodes must be very small for
proper impedance matching. The required capacitance Cj(0)
is 1 fF at 1.5 THz yielding an anode diameter of 0.5 m
(scaling downward with increasing frequency). However, it
is practical to add anodes in series, as has been done with
multiplier circuits [4] In principle, using two anodes in series
increases the maximum input power by a factor of four,
assuming that the failure mechanism is related to power
density within the device. This scaling comes about because
for the same impedance level, each device can have twice the
area if two devices are in series. In the case of failure due to
heating, the scaling is much less obvious, since it depends on
thermal pathways within the complete circuit, not just local
device properties, but there is still some significant
advantage. Four-anode devices were fabricated but the
anode size was only 1.5 times larger so the maximum power
in this case should only increase a factor of three.
The best results were obtained with the 5x1017 doping
devices, but this wafer yield is quite low, ~35% for 2-anode
devices, and the yield seems to be near zero for 4-anode
devices. The wafer with 2x10 17 doping works acceptably at
1.5 THz, but it is clear that the doping is too low. This wafer
has a high yield of 2-anode devices and 30% yield of 4-anode
devices.
As is apparent from Fig 2, these are very small devices.
While the GaAs membrane can not be touched, the beam
leads can be handled safely, and installation in the block is
done by positioning the device with a micro-manipulator.
Once positioned, conductive epoxy is applied to the IF and
bias port leads. Because of the small contact area this can
result in a high contact resistance, and a solder contact would
be preferred, but is much more difficult to produce.
The block was machined in brass as an E-plane split-block
using conventional tools with a CNC micro-milling machine
[5] and the block was then gold plated. Both waveguides are
80 x 160 um, stepping up to square cross section before
transitioning to diagonal feedhorns on both input and output
ports. The diagonal aperture sizes of each were 1.5 mm.
Details of the block are shown in Figs 3 and 5. The diode
was installed with the beamleads clamped between the halves
of the block. The IF port used a K type connector to allow
tests up to 40 GHz, but in fact no dimension or circuit
capacitance inherent to the design limits the bandwidth below
~150 GHz. For practical reasons of testing and machining,
the input waveguide was bent to align with the axis of the
output guide. This adds LO loss but this is not a concern
with laser sources.
III. UPCONVERSION RESULTS
Upconversion tests were done using an optically pumped
laser at 1561 GHz, with an available power level of >10 mW
and a variable attenuator before the mixer input. The IF was
10 GHz from a synthesizer. For the tests with the 5x10 17
doping devices, only 2-anode devices were available. Four
mixers were assembled with these devices. These were
driven with a laser power of ~1.5 mW (as measured by a
coupling aperture comparable to that of the feed horn).
Diodes were forward biased in operation, with the
conversion efficiency rising with bias up to 1 mA (which was
the maximum used). With an IF power of 1.5 mW, the
output power from the best device in the two sidebands is 45
µW with only 5 µW of LO feedthrough (~25 dB LO-RF
isolation). The feedthrough power was measured with the IF
signal off, but there is the possibility that the feedthrough
may increase with IF applied. In previous tests with a similar
mixer a very narrow band FPI etalon was used to separate the
LO from the sideband signals. Use of this filter showed that
there was no change in LO feedthrough with the IF applied.
The THz input and output power were measured with a
waveguide calorimeter (Erickson Instruments PM4) [6] built
in WR10 waveguide with no transition between the horn and
the waveguide in the sensor. The size of the horns and
WR10 waveguide are fairly similar so coupling was expected
to be good. For two other devices taken from very nearby
on the wafer, output power was within 10%, while the fourth
device from another region produced only 18 W.
Fig 4 shows the power output vs IF drive power at the
maximum laser power used of ~1.5 mW. Output power is
compressed by ~1dB at 1 mW IF level, but is still increasing
up to the maximum applied. There is the potential for higher
output power, particularly with increased laser power, but
diode failure due to heating is likely within a factor of 1.5
higher input. RF dissipation within the diodes is difficult to
estimate from diode bias since most of the THz input power
is lost in parasitic resistances. Simply summing all the input
power is the only way to estimate dissipation, which is ~4
mW at the maximum output.
Fig. 4 Output power vs 10 GHz IF drive with laser power of 1.5 mW. The
solid line is a spline fit, with no physical model.
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