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. In compenstated germanium, impact ionization breakdown occurs at a critical electric field value with a resulting bistable voltage-current characteristic and the formation of a filamentary ionized current path. In a structure resembling the unipolar transistor a reverse biased gate junction can reduce the area of a conducting channel, between the source and drain ohmic contacts, to below the minimum value of filament area necessary to sustain breakdown. This provides the possibility of bistable switching in and out of the ionized state by means of pulses applied to a biased gate. The critical gate voltages can be calculated from elementary considerations, and with suitable choice of physical dimensions and material, a wide range of values of switching voltages and of power dissipation is possible, The switching speed is limited by the ionization and recombination times of the bulk material and may be as fast as a few nanoseconds, depending upon the amplitude of the switching pulse.