Numerical model of the thyristor turn off

Numerical modelling of semiconductor device operation has become a useful tool for obtaining a better understanding of physical behavior and for optimizing device performance. In the field of thyristors only a few aspects of transient processes have so far been described. In this paper, results will be presented of the turnoff behavior of power thyristors, based on a numerical one-dimensional time-dependent model of a p+pnpn+structure. The phenomenological semiconductor equations are solved including realistic mobility dependencies and arbitrary base width, doping profile, carrier lifetime distribution, and current commutation rate. Emphasis is laid on the reverse current phase. It is shown: to what extent the reverse current depletes the base - the building up of the blocking voltage and the corresponding reverse current peak - the reverse recovery phase - the dependencies on lifetime distribution, initial current and current commutation rate, which can partly be expressed by analytical formulae. Conclusions concerning the most favorable lifetime profiles are drawn. The calculations are in good agreement with voltage versus time measurements performed on standard devices.