RF characterization of Josephson flux-flow transistors: design, modeling, and on-wafer measurement

Josephson flux-flow transistors based on 0-32/spl deg/ [001]-tilt YBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal junctions have been fabricated with Au loops as the control current lines. In order to measure the S-parameters of the transistors, no matching elements are implemented and 50 Omega coplanar lines are used as the input and output ends of the transistors. The rf properties of the transistors have been modeled on an HP microwave CAD design 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 microwave 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 for 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 80 K.<
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