As power transistor handle large currents, they always heat up during operation. Since transistor is a temperature dependent device, the heat generated must be dissipated to the surrounding in order to keep the temperature within permissible limits. Generally, the transistor is fixed on a metal sheet (usually aluminum) so that additional heat is transferred to the Aluminum sheet. The metal sheet that serves to dissipate the additional heat from the power transistor is known as heat sink.
|Fig-1: Heat Sink (Aluminum Sheet)|