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

Small area (⩽1-μm²), high-current-density NbN-MgO-NbN tunnel junctions with I-V characteristics suitable for high-frequency mixers have been fabricated. Mesa-geometry junctions with an area of about 1 μm² and critical current density of 5-10 kA/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. A study was made of the mixer gain and noise performance near 205 GHz as a function of the inductance provided by the microstrip line. This has confirmed, at a high millimeter-wave frequency, values of junction capacitance of 85 fF/μm² and recently measured values of a magnetic penetration depth of 380 nm. Mixer noise temperatures as low as 134 K at 1.5 K have been obtained for properly tuned junctions. A significant improvement in mixer performance on cooling from 4.2 K to 1.5 K was observed. Edge-geometry junctions with an area of 0.3 μm³ and critical current density of 18-25 kA/cm² have also been fabricated. These junctions give a mixer noise temperature of 145 K at 4.2 K without the use of integrated tuning elements. These are the best results ever achieved for NbN-based SIS mixers