Jw. Kooi et al., A 850-GHZ WAVE-GUIDE RECEIVER EMPLOYING A NIOBIUM SIS JUNCTION FABRICATED ON A 1-MU-M SI3N4 MEMBRANE, IEEE transactions on microwave theory and techniques, 46(2), 1998, pp. 151-161
We report on a 850-GHz superconducting-insulator-superconducting (SIS)
heterodyne receiver employing an RF-tuned niobium tunnel junction wit
h a current density of 14 kA/cm(2), fabricated on a 1-mu m Si3N4 suppo
rting membrane, Since the mixer is designed to be operated well above
the superconducting gap frequency of niobium (2 Delta/h approximate to
690 GHz), special care has been taken to minimize niobium transmissio
n-line losses, Both Fourier transform spectrometer (FTS) measurements
of the direct detection performance and calculations of the IF output
noise with the mixer operating in heterodyne mode, indicate an absorpt
ion loss in the niobium film of about 6.8 dB at 822 GHz. These results
are in reasonably good agreement with the loss predicted by the Matti
s-Bardeen theory in the extreme anomalous limit, From 800 to 840 GHz,
we report uncorrected receiver noise temperatures of 518 or 514 K when
we use Callen & Welton's law to calculate the input load temperatures
, Over the same frequency range, the mixer has a 4-dB conversion loss
and 265 K +/-10 K noise temperature, At 890 GHz, the sensitivity of th
e receiver has degraded to 900 K, which is primarily the result of inc
reased niobium film loss in the RF matching network, When the mixer wa
s cooled from 4.2 to 1.9 K, the receiver noise temperature improved ab
out 20% 409-K double sideband (DSB), Approximately half of the receive
r noise temperature improvement can be attributed to a lower mixer con
version loss, while the remainder is due to a reduction in the niobium
film absorption loss. At 982 GHz, we measured a receiver noise temper
ature of 1916 K.