Analysis of stress in power bipolar devices during inductive turn-off

The turnoff transients under inductive load of bipolar junction transistors (BJTs) and bipolar-mode field effect transistors (BMFETs) have been calculated, with the aim of investigating the factors which limit the reverse bias safe operating areas of these devices. Results of 2D simulations which include impact ionization in a self-consistent way show that different device structures exhibit different voltage limitations at high currents, and that, for the same maximum voltage, large differences in the power density distribution can account for the difference in ruggedness which is observed experimentally. The inclusion of a buffer layer in the BJT improves its turnoff performance, provided that this layer is sufficiently thick and doped. In BMFETs the sustaining voltage locus is vertical, and there is no sensitive power concentration at the junction epilayer-substrate, even if no buffer layer is included