The Cryosistor-A Field-Effect Controlled Impact Ionization Switch

The cryosistor is a new three-terminal device in which the low-temperature (4.2°K) impact ionization of impurities in germanium is controlled by means of the depletion field effect of a reverse-biased p-n junction. Compensated germanium has an inherent bistable voltage-current characteristic at liquid helium temperature. With such a material, bistable switching by means of pulses applied to the gate-junction located between two ohmic contacts is shown to be possible. The critical gate voltages may be calculated from elementary consideration and a wide range of values of the switching voltages, as well as power dissipation, is shown to be possible with suitable choice of material and physical dimensions. The switching speed is basically limited by the ionization and recombination times of the bulk material and may be as fast as a few nsec, depending upon the amplitude of the switching pulse. Possible applications of the device at 4.2°K include computer components such as binary counters, logic functions, and memory arrays, in addition to pulse amplifiers and trigger circuits of the thyratron type. The localized nature of the ionization should permit the construction of a large number of independent cryosistors on a single germanium wafer.