THERMAL INSTABILITY IN POWER TRANSISTOR STRUCTURES

A time-dependent formulation is obtained for the two-dimensional heat-flow problem applied to a model for an interdigited power transistor structure. The treatment predicts that lateral thermal instability can arise at high power densities from the interaction of a local temperature rise within the transistor volume with the temperature dependence of the emitter current. A solution is offered which permits calculation of the effect of transistor design on the tendency toward thermal instability. It is seen that the presence of series emitter and base resistance can significantly improve device stability, especially at high currents where the dynamic emitter resistance becomes small and these resistances tend to limit the emitter current. This partly explains the more stable transistor operation at a given power level observed at high current-low voltage compared to high voltage-low current. Lateral instability can cause an order of magnitude reduction in the power-handling ability of a transistor compared to the case where only steady-state heating and maximum junction operating temperature are considered. A many-fold improvement in power capability is calculated for a transistor with emitter heat-sinking rather than the more conventional collector heat-sinking. © 1969.