Noise Performance of a 200–280 GHz Mixer with Nb-AlN-NbN Superconducting Tunnel Junctions

Superconductor-insulator-superconductor (SIS) mixers remain the choice of heterodyne mixers for single-dish telescopes and interferometers at millimeter and submillimeter wavelengths. Compared with conventional Nb/AlOx/Nb superconducting tunnel junctions, Nb/AlN/NbN ones have larger gap voltage and may reach critical-current density beyond 10kA/cm(2), which are both of particularly interest in developing broadband SIS mixers. Here we report on the design and measurement of an SIS mixer based on Nb/AlN/NbN parallel connected twin junctions (PCTJ) incorporating NbTiN/SiO2/Al microstrip circuit. The junctions have a gap voltage of 3.18mV and a critical-current density of 15kA/cm(2). The measured receiver noise temperatures reach 5h nu/k(B) among 200-260GHz band, and the mixer's fractional bandwidth is about 40% centered at 230GHz.