The saturation characteristics of n-p-ν-n power transistors

The transport equations and charge-control concepts are applied in an analysis of a static conductivity-modulation mechanism occurring in the collector region of n-p-ν-n power transistors. This results in an expression for collector-emitter saturation voltage as a function of terminal currents and device parameters. An expression is derived which describes the current gain characteristics of saturated epitaxial and triple-diffused devices. The analysis is also used to illustrate the relationship between the emitter metallization resistance, collector charge storage, and the turn-off crowding mechanism experienced by high-frequency saturating transistor switches. An analysis of a time-dependent collector conductivity modulation process is used to derive an expression which describes the repetition frequency dependence of the collector-emitter saturation voltage of an epitaxial (or triple-diffused) transistor switching a square wave of collector current. It is concluded that frequency-dependent edge-crowding mechanisms occur only at much higher frequencies than those considered in this study.