Q. What is thermal runaway? State one method to prevent the same?

The effect of changes in leakage current (ICO)  and current gain(b) on the dc bias point is given by the equation

I_C = bI_B + (1+b) I_CO.

The collector current increases with the increase in temperature since b increases with increase in temperature.

           I_CO doubles in value for every 10⁰ C increase in temperature.

This leads to increased power dissipation with further increase in temperature. Being a cumulative process it can lead to thermal runaway resulting in burn-out of the transistor. The self-destruction of an unstabilized transistor is called thermal runaway. However, if by some technique, IB is made to fall with increase in temperature automatically, then decrease in the term bIB can be made to neutralize the increase in the term (1+b)I_CO, thereby keeping IC almost constant. This will achieve thermal stability resulting in bias stability.

Stabilization technique makes use of a resistive biasing circuit that permits such a variation of base current (biasing current) I_B as to maintain the collector current I_C almost constant in spite of variations in reverse saturation current I_CO and b.

D_T = T_j - T_A = θP_D

where, PD = power in watts dissipated at the collector junction

T_A = ambient temperature

T_j = junction temperature

θ = constant of proportionality called thermal resistance