Abstract
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/AlO<sub>x</sub>/Nb superconducting tunnel junctions, Nb/AlN/NbN ones have larger gap voltage and may reach critical-current density beyond 10kA/cm<sup>2</sup>, 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/SiO<sub>2</sub>/Al microstrip circuit. The junctions have a gap voltage of 3.18mV and a critical-current density of 15kA/cm<sup>2</sup>. The measured receiver noise temperature reach 5hν/k<sub>B</sub> among 200-260GHz band, and the mixer’s fractional bandwidth is about 40% centered at 230GHz.
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