Low-Noise Waveguide-Type NbN/AlN/NbN SIS Mixers Approaching Terahertz Frequencies

In this paper, we present the development of low-noise waveguide mixers with NbN/AlN/NbN tunnel junctions at a frequency approaching 1 THz. The mixer was designed to be compatible with MgO substrate. Mixers of the presented design demonstrated much improved receiver sensitivity. This improvement is attributed to the reduction in signal dissipation in waveguide and leakage into an intermediate frequency port by adopting a full-height waveguide and an effective RF choke filter, respectively. Two types of tuning circuits, namely parallel-connected twin-junction (PCTJ) and half-wavelength self-resonance distributed junction (DJ), are designed and evaluated. Both theoretical and experimental results show that the PCTJ design is superior to the DJ design in terms of gain and sensitivity due to smaller loss in the tuning circuit.