The Madistor-A Magnetically Controlled Semiconductor Plasma Device

Abstract

The madistor is a new active device which makes use of the effects of a magnetic field on an injection plasma in a semiconductor. The formation of an injection plasma has been observed in p-type InSb at temperatures below 100°K as donor traps become saturated by electrons injected through a forward biased n+p junction. In an appropriately designed n+pp+ diode, the saturation of traps and the subsequent increase in electron lifetime bring about an abrupt decrease of base resistance, and a negative resistance region is observed in the current-voltage characteristic. Because of the high mobility of electrons in InSb (5×105 cm2/vsec) the plasma can be appreciably deflected and deformed by transverse magnetic fields of the order of 10 gauss. The possibility of controlling the position of a plasma inside a solid by means of a magnetic field can be utilized in a number of different types of madistors in which the input circuit is isolated from the output. The operation at 77°K of four types of InSb madistors has been studied. The first makes use of a specially designed n+ pp+ diode mounted in the air gap of a small ferromagnetic-core electromagnet. A small change in the electromagnet winding current produces a magnetic field at the diode and causes a larger change in diode current. Typically an increase in mmf of 200-ma turns produces an additional magnetic field intensity of 5 gauss which decreases the diode current by 10 ma.