Q.What is thermal runaway? Explain.

Thermal runaway is a self-reinforcing heating process that can destroy a transistor. …

[!TLDR]

Thermal runaway is a regenerative temperature-current cycle that destroys the transistor.

Both $\beta$ and $V_{BE}$ are temperature dependent, so the operating point is inherently unstable with temperature. The collector current is $I_C = \beta I_B + (1 + \beta) I_{CBO}$. As temperature increases, $I_C$ increases; the larger $I_C$ raises the collector-junction temperature; the higher temperature increases the reverse saturation current $I_{CBO}$; this further increases $I_C$ and the junction temperature. This cumulative cycle shifts the operating point toward saturation and, if unchecked, the junctions overheat and burn out beyond their maximum ratings. This self-destruction due to rising temperature and leakage c …