Second breakdown in diodes and simplified transistor structures

The use of high frequency silicon power transistors with interdigitated comb emitter structure was successful for proving the essential features of the theory of lateral thermal instability and second breakdown. For a more detailed study of the appearance of hot spots and their relationship to the onset of second breakdown, the use of a simpler structure looked advisable. This paper describes an uncomplicated geometry which offers many possibilities for modifications. Calculations, experimental data and photographs are presented which show the means by which second breakdown and possible burn-out occurs after the formation of a hot spot. In particular, measurements demonstrate the localized thermal runaway in the collector-base diode and the relationship to the presence of the emitter. The role of the emitter and base layer resistances as well as the resistance of the contacting metal layers (series and contact resistance) in reducing the thermal instability is shown. The shorted emitter structure proved to be a successful method to postpone or even avoid the appearance of second breakdown. The tool for the determination of current and voltage distributions within the small silicon structures was a potential microprobing apparatus with a resolution of 2 microns.