Performance of NbN superconductive tunnel junctions as SIS mixers at 205 GHz

This paper reports on the performance of NbN superconductive tunnel junctions as SIS mixers at 205 GHz. The authors have fabricated small area (≤1 μ2) high current density NbN/MgO/NbN tunnel junctions with I- V characteristics suitable for high frequency mixers. Mesa- geometry junctions with an area of about 1 μ2 and critical current density of 5-10kA/cm2 are integrated with superconducting microstrip lines designed to resonate out the junction capacitance. Accurate values of junction capacitance and magnetic-penetration depth are required for the proper design of the microstrip. The authors studied the mixer gain and noise performance near 205 GHz as a function of the inductance provided by the microstrip line