Avalanche multiplication in forward- and reverse-active mode bipolar junction transistors

Abstract

Impact ionization in the reverse-biased base-collector space-charge layer of the bipolar junction transistor (BJT) can cause an avalanche collector current as well as a reverse (or negative) base current. We develop analytical models to predict and compare avalanche phenomena in the BJT biased under forward-active and under reverse-active operations. The models consider a position-dependent electric field in the space-charge layer, the effect of the non-uniform doping profile, and the excess free carriers associated with the current passing through the space-charge layer. For the specified device make-up and parameters used, our results suggest that avalanche is more prominent in the reverse-active BJT than the forward-active BJT. The physics underlying this occurrence is given, and experimental data obtained from an advanced BJT is included in support of the models.