Q.Explain the terms thermal runaway, leakage current and heat sink.

These three terms describe temperature effects in a transistor: leakage current is the small reverse minority-carrier current, thermal runaway is the cumulative self-heating that can destroy the device, and a heat sink prevents it by removing heat. …

[!TLDR]

Leakage current is the reverse minority-carrier current; if uncontrolled it drives thermal runaway (self-destruction by cumulative heating); a heat sink removes the heat to prevent it.

1. Leakage current: In a BJT a small current flows due to minority carriers across the reverse-biased collector-base junction even with no useful input. The two kinds are collector-to-base leakage $I_{CBO}$ (emitter open) and collector-to-emitter leakage $I_{CEO}$ (base open). Leakage current increases strongly with temperature — $I_{CBO}$ roughly doubles per 10 degree Celsius rise.

2. Thermal runaway: Since $I_C = \beta I_B + (1+\beta)I_{CBO}$, a rise in $I_C$ raises the collector-junction temperature, which increases the reverse saturation current $I_{CBO}$, which further increases $I_C$ and the temperature. This cumulative process shifts the operating point toward saturation and, if unchecked, the junction temperature keeps rising until the junctions burn out. This self-destruction of a transistor due to increasing temperature and hence leakage current is called thermal runaway. …