RRB Technician Grade 1 Model Paper 3

Concept:The depletion layer width changes with bias voltage and doping concentration.Explanation:The depletion width is given by:$$W = \sqrt{\frac{2\epsilon (V_{bi} + V_r)}{q} \cdot \frac{N_a + N_d}{N_a N_d}}$$Here, $V_{bi}$ is the built-in potential and $V_r$ is the applied reverse bias voltage.So, $W \propto \sqrt{V_{bi} + V_r}$.When reverse bias increases, $V_r$ increases, so the depletion width increases.When doping is increased, $N_a$ and $N_d$ increase, which makes $\frac{N_a + N_d}{N_a N_d}$ smaller, so the depletion width decreases.Thus, heavy doping decreases the width, while light doping increases it.The width is therefore not independent of applied voltage.Answer:B. Increases under reverse bias