<P> Transistors have "maximum ratings", including power ratings (essentially limited by self - heating), maximum collector and base currents (both continuous / DC ratings and peak), and breakdown voltage ratings, beyond which the device may fail or at least perform badly . </P> <P> In addition to normal breakdown ratings of the device, power BJTs are subject to a failure mode called secondary breakdown, in which excessive current and normal imperfections in the silicon die cause portions of the silicon inside the device to become disproportionately hotter than the others . The electrical resistivity of doped silicon, like other semiconductors, has a negative temperature coefficient, meaning that it conducts more current at higher temperatures . Thus, the hottest part of the die conducts the most current, causing its conductivity to increase, which then causes it to become progressively hotter again, until the device fails internally . The thermal runaway process associated with secondary breakdown, once triggered, occurs almost instantly and may catastrophically damage the transistor package . </P> <P> If the emitter - base junction is reverse biased into avalanche or Zener mode and charge flows for a short period of time, the current gain of the BJT will be permanently degraded . </P>

In pnp transistor n region is the terminal