Germanium P-N junction - Tunnel junction combination devices

A P-N junction tunnel diode (designed P
T
N), in the low voltage region preceding the negative resistance, has current carried across the P-N junction primarily by electron tunneling, with very little minority carrier injection. In the higher current region following the negative resistance, minority carrier injection occurs. Despite some electron tunneling in this region and the very high impurity concentrations on both sides of the P-N boundary, injection efficiencies can be appreciable. Germanium P
T
NP transistors, having a tunnel junction emitter and a normal junction collector, were made by a combination of diffusion and alloying processes. They exhibit a current gain of essentially zero at low emitter currents, jumping abruptly to a finite value at the tunnel junction peak current, and remaining quite constant at higher currents. This effect was applied to control the characteristics of switches, for which P
T
NPN structures were used. The P
T
NPN triode can be made to have the switching current equal to the tunnel junction peak current, and the hold current equal to the tunnel junction valley current. A base current equal to the peak current can be used to turn the device on. These structures can be made to have quite stable properties from-70°C to + 100°C, since germanium tunnel diodes show little change in characteristics over this temperature range.