RRB Technician Grade 1 Model Paper 5

Concept:Thermal runaway is a self-reinforcing heating cycle that can damage or destroy a transistor.Explanation:Excess heat produced at the collector-base junction causes thermal runaway.This heat increases the leakage current $I_{C0}$.The collector current is given by:$$I_C = \beta I_B + (1+\beta) I_{C0}$$As temperature $T$ rises, $I_{C0}$ increases.This increases $I_C$, which raises the power dissipated $P_T$, which further increases $T$.So the cycle becomes: $T \uparrow \Rightarrow I_{C0} \uparrow \Rightarrow I_C \uparrow \Rightarrow P_T \uparrow \Rightarrow T \uparrow$This is called thermal runaway.It does not occur simply because the collector is reverse biased or the emitter is forward biased, since these are normal bias conditions.It also does not occur when the transistor is not biased, because no current flows then.Answer:D. None of these