Impurity Effects on Transistor Behavior at Low Temperature

Unusual low temperature behavior has been observed in NPN silicon transistors when gallium is employed as the base dopant. The current gain exhibits a pronounced "hump" near 76°K in these devices, which enables them to be studied and utilized at this low temperature. Near the hump temperature, the tolerance of the gallium doped devices to fast neutrons is observed to be approximately a factor of five greater than at room temperature. The peak in transistor gain at low temperature has been explained in terms of the change in emitter injection efficiency with temperature as a result of the different rates of carrier freeze-out in the base and emitter regions. In the model proposed, the increased radiation tolerance results from a decrease in the recombination in the emitter space charge region at low temperatures.