Q. What is meant by thermal runaway in a transistor ?

When current flows through the collector circuit, it produces heat at the collector junction. This increases the temperature of the collector junction. More minority carriers are generated in base collector region (since more bonds are broken).The leakage current ICBO increases. This increase is very high since the leakage current is extremely temperature dependent. It almost doubles for every 6^o rise in temperature in germanium and for every 10^o rise in silicon.

                       I_C= B I_B +I_CEO.
                       I_CEO = (1+B) I_CBO.

So any increase in ICBO (current from collector to base when emitter open) is magnified (1+B) times that is 200 to 300 times. This increases the collector current This increase in the collector current produces an increase in the power dissipated at the collector junction.

This in turn further increase the collector current. This process is cumulative or cyclic which would eventually destroy the transistor permanently. This is called thermal runaway.

In practice, thermal runaway is prevented by the use of stabilization circuitry. For transistors, handling small signals the power dissipated at the collector is small. Such transistors have little chances of thermal runaway. However, in power transistors the power dissipated at the collector junction is larger. So the chances of thermal runaway is also higher.