ON THE REVERSE BIAS SAFE OPERATING AREA OF POWER BIPOLAR-TRANSISTORS DURING INDUCTIVE TURN-OFF

The turnoff transient of power bipolar transistors, under inductive load,has been analyzed by a 2D numerical simulator in the region where impact ionization is present, with the purpose of investigating the factors which limit their reverse bias safe operating area (RBSOA). Results show that the geometry of the device elementary cell, as well as its doping concentration profile have a strong effect on the maximum voltage the device can withstand at high currents and on the stress the device undergoes during turnoff. The steady-state (or sustaining) voltage locus, calculated while the device is avalanching, allows to understand a basic reason for the decrease of maximum voltage at high currents, which is observed in the RBSOA of some devices, and allows to predict the shape of turnoff voltage waveforms, that are experimentally observed in recently developed devices. The use of power density maps allows to identify the basic differences among the different device structures, in terms of power concentration resulting from current crowding and impact ionization effects.