Second breakdown prediction by two-dimensonal numerical analysis of BJT turnoff

An isothermal two-dimensional numerical calculation of the potential and current distribution in an n⁺-p-n-n⁺bipolar power transistor driving an inductive load during its turnoff transient has been carried out. The transistor is initially considered to be in a heavily saturated ON-state and is then turned off by extracting a nearly constant base current. The simulation shows that during the turnoff transient, current constriction to the center of the emitter together with the increasing collector-emitter voltage produce a high electric field near the collector n-n⁺junction which can initiate avalanche injection. It has been found that the collector-current density is not uniform vertically (from collector to emitter) due to the current spreadout in the collector n-region. Previous one-dimensional analytical analyses of second breakdown did not consider this important effect. Thus, for an accurate prediction of reverse second breakdown voltage, the two-dimensional current flow should be considered.