Comparison of one and two-dimensional models of transistor thermal instability

The IC-VCElocus predicted by a one-dimensional model of thermal instability is compared with the locus predicted by a numerical model which accounts for non-uniform heat generation over the transistor area and also two-dimensional heat flow within the heat sink. Both models include the effects of distributed emitter and base ballast resistance, as well as the magnitude and temperature dependence of current gain. An important result obtained from this comparison is that the forward safe operating areas (SOA) predicted by the two models are very nearly the same. This suggests that the one dimensional model can be used to predict a particular SOA, even though the assumptions used in its derivation are not valid at the onset of thermal instability. Confirmation of this approach has been obtained by demonstrating good agreement between measured SOA and that predicted by the one-dimensional model.