A Superconductor-Insulator-Superconductor (SIS) mixer using two junctions connected in parallel through a stripline inductance has been studied. The essential point of the two-junctions device is that the capacitance of the junctions was tuned out by the inductance to obtain a broadband operation without mechanical tuning elements. It has been shown by theoretical analysis that the performance of this type of device is excellent and nearly quantum-limited performance of the mixer can be obtained. It has been demonstrated that the double sideband (DSB) noise temperature of a receiver employing this type of device was less than 40 K over the bandwidth of 90-120 GHz and that the lowest receiver noise temperature of 18 K, which is only 3.2 times as large as the quantum limited photon noise was obtained around 118 GHz. Junctions used in the two-junctions device have significantly larger area, i.e. larger capacitance, and smaller normal resistance than conventional ones. In order to obtain a good impedance match between the source and the junctions, an impedance transformer made of a superconductiong stripline was integrated with the junctions. This type of two-junctions device can easily be scaled to submillimeter frequency without using submicron-sized SIS junctions.

Millimeter- and submillimeter-wave low-noise superconducting receivers, such as superconductor-insulator-superconductor (SIS) mixers, hot-electron bolometer (HEB) mixers, and superconducting direct detectors, are addressed in this paper. Some general topics on the development of SIS mixers, including SIS junction and integrated tuning circuitry, mixing circuitry, and mixer-performance simulation, are extensively discussed. A tuneless waveguide SIS mixer developed at Nobeyama Radio Observatory (NRO) and its performance are presented. The fundamental mechanisms of diffusion- and phonon-cooled HEB mixers and recent advances in HEB mixers are briefly reviewed. Finally, incoherent detectors with superconducting tunnel junctions are discussed. Results for a direct detecting experiment at 500 GHz are given.

In this paper we focus on the numerical simulation of the mixing behaviors of distributed superconducting junction arrays at 1.2 THz. A novel type of superconducting tunnel junctions, i.e., NbN/AlN/Nb, which have a relatively high gap voltage (4.3 mV) and can reach a critical current density as high as several tens of kA/cm2, are proposed for this characterization along with conventional Nb/AlOx/Nb junctions. The former is incorporated with a NbN/SiO2/Al tuning circuit, and the latter with a Nb/SiO2/Al and a NbTiN/SiO2/Al tuning circuits. The noise performance, local-oscillator power requirement, IF bandwidth, and optimum embedding impedance are thoroughly characterized for the two types of distributed superconducting junction arrays.