Submicron-long HTS hot-electron mixers

We have measured the device length and temperature dependence of the interm ediate frequency (IF) bandwidth and noise of hot-electron bolometer (HEB) m ixers made from a high-T-c superconductor. Mixer devices with lengths (L) b etween 50 nm and 1 mu m were fabricated from 25-35 nm thick YBCO films on M gO and sapphire substrates. Bandwidth measurements were made using signal a nd local oscillator (LO) frequencies in the range 1-20 GHz. At low operatio n temperatures the IF bandwidths were about 100 MHz and several hundred MHz for devices on MgO and sapphire, respectively. At higher operation tempera tures, where self-heating disappeared and flux-flow effects define the shap e of the l-V characteristic, the measured IF bandwidth increased significan tly. The temperature and IF dependence of absolute conversion efficiencies determined from noise measurements are in good agreement with the bandwidth data. At 2.7 GHz LO frequency the single-side-band mixer noise temperature of a 50 nm long HEB on MgO was about 8000 K.