Thermal Runaway and the use of Heat sink in BJT
The maximum average power in which a transistor can dissipate depends upon the construction of transistor and lie in the range of few milliwatts and 200W. The maximum power is limited by the temperature that the collector Base junction can withstand. The maximum power dissipation is usually specified for the transistor enclosure is 25 degree celsius. The junction temperature may increase either because of rise in ambient temperature or because of self heating. The problem of self heating arises due to dissipation of power at the collector junction.
The leakage current Icbo is extremely temperature dependent and increases with the rise in temperature of collector-base junction. With the increase in collector current Ic, collector power dissipation increases which raises the junction temperature that leads to further increase in collector current Ic. The process is cumulative and may lead to the eventual destruction of transistor. This phenomenon is known as THERMAL RUNAWAY of transistor. In practice the Thermal Runaway can be prevented by a well designed circuit called as STABILIZATION Circuitry.
When the power dissipation at the collector-base junction of a transistor is small, as in case of a small signal transistor, the surface area of the transistor case is normally large enough to allow all of the heat to escape. But for the large power dissipation that can occur in high power transistor the transistor surface area is not enough and junction temperature may rise to a dangerous level. However, power handling capacity of a transistor can be increased by making suitable provision for rapid conduction of heat away from the transistor junction. This is achieved by selecting a sheet of metal called the HEAT SINK which increases the area of contact With the atmosphere.
Heat sink is a direct practical means of combating the undesirable Thermal effects. It may be noted that the ability of any heat sink to transfer heat to the surroundings depends upon its Material, volume, area, shape, contact between case and sink and movement of air around the sink.
Take a look on Power Transistor Tutorial