Two-dimensional computer simulation for switching a bipolar transistor out of saturation

A two-dimensional numerical analysis for the turnoff of a bipolar transistor from high injection level (VBE= 900 mV) is carried out. VBCis being kept constant at 1 V. Distributions of potential, electron, and hole density are interpreted and lead to a subdivision of the total transient time into four time regions, each governed by a single phenomenon. These phenomena are 1) fast discharge of the sidewall transistor, 2) the "lateral wave" which has the dominating influence in the total switching time, 3) the vertical discharge, and 4) the emitter discharge. The transient behavior is essentially ruled by two-dimensional lateral effects. Hence one-dimensional models are not adequate for switching a transistor out of saturation.