Experimental demonstration and theory of a corrective to second breakdown in Si power transistors

This paper reports experimental measurements on second breakdown behavior of transistors which were altered from conventional structure so as to allow the addition of externally mounted, variable resistors to a finely divided emitter. It is found that forward-biased second breakdown can be improved so that with suitable resistances, the safepower vs. voltage curve becomes a straight line, allowing the transistor power dissipation to be thermally limited at all voltages rather than second break limited at higher voltages. Data are presented showing the variation of obtainable power and the degree of nonuniform current distribution as a function of the resistance in series with the divided emitter. The significance of the extent of emitter subdivision, i.e., how small the discrete areas are, is also shown by data taken using the emitter sites individually and in clusters. The theory supporting these experiments is presented. What is essentially required is a load line analysis applied simultaneously to the various IEvs. VBEcurves of the individual emitter regions. This is complicated by the shifting of these curves both with temperature and the application of collector voltage.