Some new aspects of thermal instability of the current distribution in power transistors

An important characteristic of second breakdown in p-n junctions is the current constriction to a small region. This may be caused by a thermal feedback mechanism, as discussed by Scarlett and Shockley, and by Bergmann and Gerstner. A brief review of this theory is given, illustrated by experimental results of a simple model arrangement consisting of three thermally coupled transistors. The essential parameters influencing the thermal stability of the current distribution are device geometry, power density, and temperature dependence of current. It is widely known that second breakdown occurs at high voltages at a much lower power level than at low voltages. To allow a more detailed discussion of this effect in view of thermal stability, we determined experimentally the temperature coefficient of transistor current for various Si planar transistors as a function of current, voltage, and junction temperature. The experimental procedure is described and the results are discussed. The experimental values of the temperature coefficient range from 0.08 to 0.01 1/°C. The values for high currents are much lower than predicted by the theory of Ebers and Moll. It thus can easily be understood why, in the case of high current, and low voltage, the thermal stability of the current distribution is much better than in the case of low current and high voltage.