Q.What is thermal runaway?

Thermal runaway is the self-destruction of a transistor caused by a regenerative, cumulative rise of temperature and leakage current. It happens because the transistor's parameters $\beta$ and $V_{BE}$, and its leakage current $I_{CBO}$, are all temperature dependent, which makes the operating point inherently unstable with heat.

The collector current is $I_C = \beta I_B + (1 + \beta)I_{CBO}$. The cycle runs as follows: a rise in $I_C$ raises the collector-junction temperature; the higher temperature increases the reverse saturation current $I_{CBO}$; the larger $I_{CBO}$ increases $I_C$ still further; the still-larger $I_C$ raises the temperature again. Each turn of this loop reinforces the last, so the operating point is driven steadily toward saturation. …