Thermal Runaway

The maximum power dissipation in a transistor depends on transistor construction and may lie in the range from few mW to some Watts.

The power dissipated in a transistor is predominantly the power dissipated at its Collector Base junction. Thus the maximum power dissipated at Collector base junction is limited by the temperature that can Collector base junction bear.

For Sillicon transistor this temperature is in the range 150° to 225° C, and for germanium it is between 60° to 100° C.

The increase in the collector current increases the power dissipated at the collector junction. This, in turn further increases the temperature of the junction and hence increase in the collector current. The process is cummulative and it is referred to as self heating. The excess heat produced at the collector base junction may even burn and destroy the transistor. This situation is called “Thermal Runaway” of the transistor.

As T ↑ ⇒ Ico ↑ ⇒ Ic ↑ ⇒ PT ↑ ⇒ T ↑

(Cummulative Process)

Thermal Resistance
The steady state temperature rise at the collector junction is proportional to the power dissipated at the junction.
∂T = Tj – TA = θ PD
Tj = Junction temperature in °C
TA = Ambient temperature in °C
PD = Power dissipated at Collector base junction
θ = Constant of proportionality referred as Thermal Resistance
θ = ( Tj – TA ) / PD
The unit of thermal resistance is °C / W for a low power. The maximum collector power allowed for safe operation is specified at 25 °C.

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