Modeling of electrical and thermal instabilities in a bipolar transistor under reactive circuit conditions

The current density and temperature distribution in a bipolar power transistor operating in the switching mode under transient conditions has been computed as a function of circuit environment. A modeling was done of the turn-off of the transistor in a circuit containing resistive and inductive elements. Of particular interest was the study of the local current and temperature distribution achieved in the transistor during turn-off in a circuit with a large inductance: in the process of shut-off this inductance maintains the transistor collector current at a high value as the collector junction undergoes avalanche multiplication due to the high voltage induced across this junction by the inductive load. The method of computation was to solve numerically the electrical carrier flow as well as Poisson and the heat flow equations in a two-dimensional model n(+)-p-n-n(+) of a transistor structure as a function of time.