Thermal runaway in a BJT

We know that in a transistor , power is dissipated in the collector and hence it is made physically larger than the emitter and base region. As the power is dissipated , there is a chance for the collector base junction temperature to be raised. As the temperature at collector base junction increases, the reverse leakage current ICBO increases. This is because ICBO arises due to the flow of minority carriers which are thermally generated across reverse biased collector-base junction (reverse biased pn junction) . As the temperature increases, thermal generation increases, ICBO increases..

I_C  = αI_E + ICBO

 So, as ICBO increases,  I_C  increases. Power dissipated =I² * R. 

So, as collector current increases, power dissipated increases which in turn increases the collector base junction temperature. So the process is cumulative leading eventually to the destruction of the transistor.

Thermal runaway can be prevented by using a heat sink.