chapter  31
Test and measurement
Pages 18

Just as diodes can become open-or short-circuit, so similar faults can arise with BJTs between base and collector and base and emitter. Individual electrodes can become open-circuit and a condition known as punch-through can occur where a short-circuit develops between collector and emitter when the base region is ruptured, usually through excessive current and hence heat dissipation. Other faults that may be encountered are changes in input and output impedances and gain. In general, these are due to overdriving and the consequent dissipation of excessive heat.