Assertion (A) : The bias instability occurs in transistors due to thermal variations. Reason (R) : The reverse saturation current doubles for every 18 °C temperature rise. Due to this the reverse saturation current further heats the junction. As a result there is a thermal run-away.
(A) is true, but (R) is false.
Assertion — TRUE. The quiescent point of a bipolar transistor drifts with temperature, through three separate mechanisms:
\(I_{CO}\ \text{rises},\qquad V_{BE}\ \text{falls by about }2.5\ \text{mV/}^{\circ}\text{C},\qquad \beta\ \text{increases}\)
all of which push IC upward. That is precisely why bias-stabilised circuits — voltage-divider bias with an emitter resistor — are used, and why the stability factor
\(S=\dfrac{\partial I_C}{\partial I_{CO}}\)
is a design parameter.
Reason — contains the right mechanism but the wrong number. The runaway description is sound: a rise in ICO increases IC, which increases the power dissipated in the collector junction, which raises the junction temperature, which raises ICO again — a positive feedback loop that can destroy the device unless the thermal resistance and the bias network hold it in check.
But the standard figure is that the reverse saturation current doubles for every 10 °C rise, not 18 °C:
\(I_{CO}(T)=I_{CO}(T_0)\times2^{(T-T_0)/10}\)
(The number 18 is what you get by quoting the same rule in Fahrenheit — 10 °C is 18 °F — so the figure has been carried across with the wrong unit attached.) Since the reason states a quantitatively wrong rule, it is taken as false.
So the assertion stands but the reason does not.
The practical safeguard against runaway is to keep the junction temperature under control — heat sinking to reduce θJA, an emitter resistor to provide d.c. negative feedback, and the condition that the rate of heat generation must not exceed the rate at which the package can remove it.
Hence, (A) is true, but (R) is false.
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