As the power transistors deal with a very high current, they always heat up during operation, ie there is a heat dissipation during the process of its operation.
Since the semiconductor do not support high temperatures, the heat generated to be dissipated to the environment so as to maintain the temperature within the allowable limits, so does not compromise its functioning, or even causing its burning.
For the heat dissipation, semiconductors are generally fixed on a metal plate (aluminum) so that additional heat is transferred for this plate, which serves to dissipate heat, this card is known as heat sink.
The heat sink
Most of the heat is produced within the transistor collector at the junction, in this case the heat sink has the function of increasing surface area of this joint and allow the heat to escape easily from the junction of the collector.
The result is that the temperature of the transistor is low, so, hardly any heat of a transistor is produced at the junction collector / base. Temperature is allowed to exceed or limit, this burning junction and therefore the transistor also.
Most of the energy is dissipated at the junction collector / base, This is because the voltage of the collector / base is much higher voltage than the base / emitter, although the currents through the two joints are nearly the same. The heat sink is a practical means of combating undesirable thermal effects in semiconductors.
The capacity of the heat sink to transfer heat to the environment depends on your material, volume, area, shape, contacting the semiconductor and the air movement around the sink.
The aluminum heat sinks have a better heat transfer / cost than other materials. One should realize that the use of heat sink alone may not be enough to avoid thermal problems, the project should take all the working conditions into account.