Superconducting niobium variable thickness bridges: Characteristics and properties as microwave mixer

Niobium variable thickness bridges designed for millimeter wave mixing were fabricated and tested. The junction characteristics were studied by measurements of I-V and dV/dI vs. V as functions of temperature, microwave power and microwave mixing. Near T sub C these bridges behaved as nearly ideal Josephson junctions but at lower temperatures their characteristics are determined by flux flow. Hysteresis of the I-V curves, depression of the energy gap, a feature interpreted as the onset of a self-sustaining hot spot, and bolometric mixing are described by a simple heating model. The mixing experiments both at 10 GHz and 85 GHz were described as a measurement of the device properties at high frequencies. Junctions made on semiconductor substrates to provide better cooling were used in 85 GHz mixing experiments.