Relative Roles of Charge Accumulation and Interface States in Surface Degradation (NPN Planar Transistors)

The relative roles of the radiation-induced interface states and oxide charges on the surface degradation of NPN planar transistors have been determined by using gate controlled measurement techniques. Three main conclusions resulted. 1) In general, the new interface states are the more important factor in the surface degradation of NPN transistors with oxide properties similar to those existing in the present study as long as a strong inversion of the base surface is not developed which causes a "channel" between the emitter and base contact. Charge buildup would then become the primary factor if extensive channeling did develop; this primarily would occur at high doses. 2) Extensive "channeling", affecting the high current gain, did not fully develop in our NPN transistors up to very high doses; however, the possible existence of channeling in other different oxide preparations cannot be ruled out. 3) The nonuniform nature of the charge buildup and the charge of the interface states can contribute significantly to low-current gain degradation.