RF CHARACTERIZATION OF JOSEPHSON FLUX-FLOW TRANSISTORS - DESIGN, MODELING, AND ON-WAFER MEASUREMENT

Josephson flux-flow transistors based on 0-32 degrees [001]-tilt YBa2C u3O7 bicrystal junctions have been fabricated with Au loops as the con trol current lines. In order to measure the S-parameters of the transi stors, no matching elements are implemented and 50 Ohm coplanar lines are used as the input and output ends of the transistors. The rf prope rties of the transistors have been modeled on an HP microwave CAD desi gn system, with the junction parameters taken from our dc measurements . Simulations indicate that the transistors can have a maximum stable gain of 20 dB at 7 GHz and 6 dB around 35 GHz under matched conditions . An 'on-wafer' transmission-reflection-load (TRL) method is used for calibrating the S-parameters of the transistor. A low temperature micr owave probe station has been built: a pair of microwave probes inside a cryostat are moveable in the xyz-directions. Together with a Wiltron 360B vector network analyzer, this setup provides a convenient way fo r doing on-wafer characterization of the transistors in the frequency range from 40 MHz to 60 GHz, and in the temperature range from 20 to 8 0 K.