Study of current mode second breakdown in inductively loaded power transistor switches

Abstract

A major problem in operating epitaxial bipolar transistors in reverse biased condition 18 the occurrence of current mode second breakdown. The base current reversal may be brought about by the conditions: i) operation above open-base breakdown voltage BVCEO and ii) during turn-off, where charge is being extracted from the base-emitter region. The field distribution within the collector arising from the mobile space charge causes voltage collapse accompanied by an internal current constriction and this, in turn, can very rapidly produce damage to the device. In the case of power transistor switch driven with an inductive load at the end of the storage time the magnitude of the collector-enlitter voltage increases rapidly until at some critical voltage and simultaneously at some critical current density, avalanche injection occurs at the n~n+ interface. These two parameters are very important ln determining the susceptibility of a device to failure by current mode second breakdown. Therefore, there is a need to know the dependence of the critical voltage and current density upon the collector doping density and epitaxial layer thickness in order to determine the resistance of power transistor to current mode second breakdown and also, to design protective circuit. The study permits the evaluation of critical current density and voltage of bipolar power transistor switches with inductive loads. Results show the dependence of this two important parameters upon collector doping density and epitaxial layer thickness.