On the relationship between Schottky barrier capacitance and mixer performance at cryogenic temperatures

The flat-hand voltage is the Schottky junction voltage required to shrink the depletion width to zero, At cryogenic temperatures, mixer diodes are generally biased and/or pumped beyond the Rat-hand condition to minimize conversion loss and noise figure, This occurs despite the presumed sharp increase in junction capacitance near flat-band, which should instead limit mixer performance. Past moderate forward bias, the diode C-V relationship is difficult to measure, A simple analytic expression for C(V) is usually used to model and predict mixer performance, This letter provides experimental data on C(V) at 77 K based on a microwave measurement and modeling technique, Data is also provided on the conversion loss of a singly balanced mixer optimized for 77 K operation, The connection between junction capacitance, flat-hand potential, and conversion loss is examined. It is shown that the analytic expression greatly overestimates the junction capacitance that occurs as flat-band is approached.