The reverse saturation current in semiconductor devices changes with temperature. The reverse saturation current approximately doubles for every 100 c rise in temperature. As the leakage current of transistor increases, collector current (Ic) increases The increase in power dissipation at collector base junction. This in turn increases the collector base junction causing the collector current to further increase. This process becomes cumulative. & it is possible that the ratings of the transistor are exceeded. If it happens, the device gets burnt out. This process is known as „Thermal Runaway‟.
Thermal runaway can be avoided by 1) Using stabilization circuitry 2) Heat sink