thermal runaway
Thermal runaway is a vicious feedback loop that can destroy a BJT. A transistor passing current gets warm; as it warms, it conducts MORE current for the same drive; more current makes it warmer still; which makes it conduct even more — and round and round it spirals until the device cooks itself. It is the opposite of a thermostat: instead of self-correcting, the loop is self-reinforcing, like a microphone howling into its own speaker, building until something breaks.
The physical engine is the base-emitter voltage's temperature dependence. For a fixed base-emitter voltage, a silicon transistor's collector current roughly doubles for about every 10 degrees Celsius of temperature rise — equivalently, the base-emitter voltage needed for a given current FALLS by about 2 millivolts per degree. So if a transistor is biased by a fixed voltage and it heats up, it draws more current, dissipates more power (P = Vce times Ic), heats up further, and the cycle accelerates. It is most dangerous where a transistor carries large current and dissipates real power: power output stages, linear regulators' pass transistors, and any device run near its limits. Parallel transistors are especially at risk — the slightly hotter one hogs more of the shared current, gets hotter, and hogs even more.
The cure is negative feedback that fights the temperature drift. A small emitter resistor (emitter degeneration) is the classic fix: if the current tries to rise, the extra drop across the emitter resistor reduces the base-emitter voltage and throttles the transistor back — exactly the self-policing of voltage-divider bias, now doing thermal duty. Other safeguards: heat sinks to keep the temperature down, thermally coupling the bias circuit to the power transistor so the bias falls as the transistor warms, current limiting, and in power chips an explicit thermal-shutdown circuit. The honest lesson behind all of this is the recurring theme of BJT design: never trust the bare device to behave, because temperature and beta both conspire against you — build in feedback that makes the circuit stable regardless.
A power transistor in a linear regulator with no emitter resistor and an inadequate heat sink starts at 100 mA, warms up, climbs to 150 mA, warms more, climbs again — within seconds it is too hot to touch and soon fails short. Add a 0.22 ohm emitter resistor and a proper heat sink and the current sits stable, because every rise is pushed back.
A self-reinforcing heat-and-current loop — broken by emitter resistors and good cooling.
Thermal runaway is a BJT affliction rooted in its negative-temperature-coefficient base-emitter voltage. Power MOSFETs are largely immune at high currents because their on-resistance rises with temperature, naturally sharing current — one reason they dominate modern power switching.