Definition: thermal runaway

A repeating cycle in which excessive heat causes more heat until the operation ceases or an explosion occurs. For example, lithium-ion batteries have been known to exhibit thermal runaway and explode.

In germanium-based bipolar transistors, high temperatures cause current to leak. In turn, this causes the transistor to heat even more and eventually become unstable or self-destruct. Obviously, after thermal runaway appears in new systems, the product must be re-engineered to prevent it.