Thermal Runaway
The reverse saturation current in semiconductor devices changes with temperature. The reverse saturation current approximately doubles for every 100 c rise in temperature. As the leakage current of transistor increases, collector current (Ic) increases The increase in power dissipation at collector base junction. This in turn increases the collector base junction causing the collector current to further increase. This process becomes cumulative. & it is possible that the ratings of the transistor are exceeded. If it happens, the device gets burnt out. This process is known as „Thermal Runaway‟.
Thermal runaway can be avoided by 1) Using stabilization circuitry 2) Heat sink
Concept of thermal runaway:
1.We know that
IC = β IB + (1+ β) ICO
Where ICO is the leakage current and ICO is strongly dependent on temperature. 2. Leakage current approximately doubles for every 100 C rise in temperature. 3 As the leakage current of transistor increases, collector current (Ic) increases (1+ β) times. 4. This increases the power dissipation at collector base junction. 5. This in turn increases the temperature of the collector base junction which will increase the number of minority carriers. 6. So ICO further increases causing the collector current to increase further. 7. This effect is cumulative and in a fraction of a second Ic becomes large causing transistor to burn up.
This self-destruction of an unstabilized transistor is known as Thermal Runaway.
Thermal runaway can be avoided by :
1) Using stabilization circuitry
2) Heat sink