A quantitative study of emitter ballasting

A theoretical and experimental study is carried out to quantitatively analyze the effect of emitter ballasting on thermal instabilities in high power density transistors. The analysis includes factors such as thermal resistance, emitter and base resistances, collector dissipation, etc., affecting thermal runaway. In particular, numerical computations are presented to describe current-voltage characteristics as they relate to thermal instability with emitter ballast resistance and the collector bias voltage as parameters. The agreement between theory and experiment is shown to be excellent. The study yields a minimum value of ballast resistance above which there is unconditional thermal stability.