Thermal runaway in a transistor biased in the active region is due to...
When an input signal is applied, the output signal should not move the transistor either to saturation or to cut-off. However, this unwanted shift still might occur, due to the following reasons
1. Parameters of transistors depend on junction temperature. As junction temperature increases, leakage current due to minority charge carriers (ICBO) increases. As ICBO increases, ICEO also increases, causing an increase in collector current IC. This produces heat the collector junction. This process repeats, and, finally, the Q-point may shift into the saturation region. Sometimes, the excess heat produced at the junction may even burn the transistor. This is known as thermal runaway.
2. When a transistor is replaced by another of the same type, the Q-point may shift, due to changes in parameters of the transistor, such as current gain (β) which varies slightly for each unique transistor and also temperature dependent.
3. Thermal runaway in a transistor biased in the active region is due to change in reverse collector saturation current due to rise in temperature.