Noise and Bandwidth of 0.5-THz Twin Vertically Stacked SIS Junctions

Twin SIS (superconductor-insulator-superconductor) junctions, with a simple structure, yet large bandwidth and low noise temperature, have been widely used in millimeter- and submillimeter-wave heterodyne mixers. With increasing frequency, however, the length of the tuning inductance connecting the two individual SIS junctions becomes short while the junction area remains fixed. With a relatively short tuning inductance, the effect of the junction's spreading inductance becomes non-negligible, and device fabrication becomes more difficult. By adopting vertically stacked SIS junctions (VSJs), which have an equivalent geometric capacitance inversely proportional to the junction number, it becomes feasible to increase the junction tuning inductance. In this paper, we report on the design, fabrication, and characterization of twin Nb/Al-AlOx/Nb/Al-AlOx/Nb VSJs for the 500-GHz frequency band.