Current gain and switching speed in a three-terminal nonequilibrium superconducting device

A three-terminal nonequilibrium superconducting device controlled by the tunnel-injection of quasi-particles is fabricated and experimentally investigated. The device used in the present experiments has a sandwich structure consisting of NbN and Pb-alloy films. The dc current gain is measured as a function of the current density of the two junctions and the injected power. A current gain larger than unity is obtained when the current density ratio (the acceptor to the injector current) increases. The turn-on time is measured in an experimental circuit using a Josephson sampling technique. The output waveform rises succeeding a turn-on delay of 300 ps after the input current changes. The rise time of the output waveform consists of two parts. About 70 percent of the full output amplitude occurs within 300 ps and the remaining is slow with a time constant of over 1 ns. Also pointed out in the paper are the problems of obtaining a device with large device gain and with high switching speed. Finally, the possibility of using this device in logic circuits is discussed.