Q. Explain the theory of thermal runaway?

          The maximum average power Pdmax which a transistor can dissipate depends upon the transistor construction and may lie in the range from a few milli watts to 200W.This maximum power is limited by the temperature that the collector to base junction can withstand.For silicon transistors this temperature is in the range 150 degrees to 225 degrees and for germanium it is between 60 to 100 degrees.The junction temperature may rise either because the ambient temperature rises or because of self heating.The maximum power dissipation is usually specified for the transistor enclosure or maximum temperature of 25 degrees.The problem of self heating results from the power dissipated from the collector junction.

         As a consequence of the junction power dissipation, the junction temperature rises  and in turn increases the collector current with a subsequent increase in power dissipation. If this phenomenon named thermal runaway continues then it may result  in permanently damaging the transistor.