Power and energy limitations of bipolar transistors imposed by thermal-mode and current-mode second-breakdown mechanisms

Two distinct second-breakdown modes observed in bipolar power transistors are described. Experimental results obtained on a number of power-transistor types are presented and interpreted. The conditions under which each of these second-breakdown modes occurs, as evidenced by a very sudden collapse in voltage, are shown to be the result of the application of relatively high voltage and current to the device. Thermal-mode breakdown, the classical mechanism, is shown to be related to the total energy absorbed by the device; the primary parameters are voltage, current, and duration of application of these. Current-mode breakdown, which occurs essentially at the instant that critical current and voltage levels are achieved, is shown to be primarily related to the instantaneous power the device is forced to handle. The validity of the term current mode is established by the contention that, although the voltage collapse is caused by the presence of an excessive field, the mechanism is actually induced by the forcing of very high currents through the device under bias conditions that produce a local critically high current density.