When the transistor is operational, almost the whole heat is produce at the collector-base junction.
This power dissipation causes the junction temperature to rise.
This successively increases the collector current since more electron-hole pairs are generate.
Because of the increase in temperature.
This produces an increased power dissipation within the transistor and consequently an extra rise in temperature.
Unless adequate cooling is provided or the transistor has built-in temperature compensation
circuits to forestall excessive collector current rise, the junction temperature will still increase
until the most permissible temperature is exceed.
If this example occurs, the transistor are going to be permanently damage.
The unstable condition where, thanks to rise in temperature, the collector current rises and continues to extend is know as thermal runaway.
Thermal runaway should be avoided. If it occurs, permanent damage is cause and therefore the transistor must be replace.