Avalanche injection and second breakdown in transistors

A rapid type of second breakdown observed in silicon n⁺-p-n-n⁺transistors is shown to be due to avalanche injection at the collector n-n⁺junction. Localized thermal effects, which are usually associated With second breakdown, are shown to play a minor role in the initiation of the transition to the low voltage state. A useful tool in the analysis of avalanche injection is the n⁺-n-n⁺diode, which exhibits negative resistance at a critical voltage and current. A close correspondence between the behavior of the diode and the transistor (open base) is established both theoretically and experimentally. Qualitative agreement with the proposed model is obtained for both directions of base current flow. It is shown that transistors having thin, lightly doped collector regions are particularly susceptible to avalanche injection, which suggests that some compromise may be necessary in the design of high-frequency power transistors.