Q. Write a short note on heat sink

For transistors handling small signals ,the power dissipated at the collector is small.Such transistors have little chances of thermal runaway .However in power transistors ,the power dissipated at the collector junction is larger .This may cause the junction temperature to rise to a dangerous level .We can increase the power handling capacity of a transistor if we make a suitable provision for rapid conduction of heat away from the junction .This can be achieved by using a sheet of metal called heat sink.As the power dissipated within a transistor is predominantly the power dissipated at its collector base junction ,sometimes the collector of the power transistor is connected to its metallic base The case of the transistor is then bolted on to a sheet of metal .The sheet serves a heat sink. Connecting heat sink to a transistor increases the area from which heat is to be transferred to the atmosphere .Heat moves from the transistor to the heat sink by conduction ,and the it is removed from sink to the ambient by convection and radiation.