The maximum average power Pcmax Which a transistor can dissipate depends upon the constructión oft the transistor and may lie in the range of a few milliwatts and 200W. This maximum power is limited by the temperature that the collector-base junction can withstand.
For silicon transistor, this temperature is in the range of 150 225°C and for germanium, it is between 60°C and 100C. The maximum power dissipation is usually specified for the transistor enclosure (or casing) or ambient temperature of 25°C. The junction temperature may rise either because of rising in ambient temperature or because of self-healing. The problem of self-heating arises due to the dissipation of power at the collector junction. The leakage current IcBo is extremely temperature dependent and increases with the rise in temperature of the collector-base junction. An increase in ICBO causes an increase in collector current Ic considerably. With the increase in collector current Ic, collector power dissipation increases which raise the junction temperature that leads to further increase in collector current Ic.
The process is cumulative and may lead to the eventual destruction of the transistor. This phenomenon is known as a thermal runaway of the transistor. In practice, thermal runway prevented in a well-designed circuit by using stabilization circuitry.