Q. What is thermal runaway.

The problem of self heating arises due to dissipation of power at the collector junction. The leakage current ICBO is extremely temperature dependent and increase with the rise of  temperature of collector base junction. An increase in ICBO causes an increase in collector current  IC considerabely. With the increase in collector current ,collector power dissipation increase which raises the junction temperature which leads to further increase in collector current. The process is cumulative  and may lead to the eventual destruction of the transistor . This phenomenon is called thermal runaway.