Secondary breakdown characteristics in the reverse base current region

At constant forward or reverse base currents the pulse length of a collector current pulse, which forces the transistor into secondary breakdown, is measured. For all transistors measured we plotted the logarithm of the collector pulse length,logDelta t, versus collector pulse height, with base current as a parameter. According to our observations one can discern between three types of transistors as far as their secondary breakdown behaviour is concerned. 1. Good epitaxial ones show a linear dependence oflog Delta tto Ic with a negative slope. The curves for reverse base current are displaced parallel to the ones for forward base current, the energy of the pulses not being much different for both cases. 2. Bad epitaxial ones show an S shaped curvelog Delta tversus Ic for reverse base currents. The dip occurs mostly between 100 and 300mA collector current. These transistors fail in practical circuits where a reverse base current is allowed to flow. 3. For homogeneous material thelog Deltatversus Ic curves for all base currents practically coincide. For devices with a small collector series resistance the emitter to collector voltage just prior to the occurrence of secondary breakdown is practically a constant, regardless of collector current. Measurements of secondary breakdown characteristics as a function of case temperature have been performed. A discussion of the results will be presented.