A $ \text{P-N} $ junction exhibits two internal capacitances: Transition (Depletion) Capacitance ($ C_T $) and Diffu · Electronics MDCAT Past Paper MCQ

Physics Electronics PMDC Conceptual Practice

PMDC Verified Question 407 of 494

A $ \text{P-N} $ junction exhibits two internal capacitances: Transition (Depletion) Capacitance ($ C_T $) and Diffusion Capacitance ($ C_D $). Which capacitance dominates under forward-bias and reverse-bias conditions, respectively?

$ C_T $ dominates under forward bias, and $ C_D $ dominates under reverse bias

$ C_T $ dominates under both forward and reverse bias

$ C_D $ dominates under both forward and reverse bias

$ C_D $ dominates under forward bias, and $ C_T $ dominates under reverse bias

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Official Correct Choice:

Option D: $ C_D $ dominates under forward bias, and $ C_T $ dominates under reverse bias

Concept:

Transition capacitance arises from the fixed space-charge layer, whereas diffusion capacitance arises from minority carrier charge storage injected during forward conduction.

Formula:

$$C_T = \frac{\varepsilon A}{W} \quad (\text{Reverse Bias}), \quad C_D = \frac{\tau I_f}{\eta V_T} \quad (\text{Forward Bias})$$

Solution:

In forward bias, large numbers of minority carriers are injected and stored near the junction boundaries, making diffusion capacitance $ C_D $ (proportional to $ I_f $) dominant.

In reverse bias, current is negligible, so $ C_D \approx 0 $. Concurrently, the widened depletion layer acts as a dielectric, making transition capacitance $ C_T $ dominant.

Why other options are incorrect:

Option A: The dominant regimes are inverted.
Options B & C: Each capacitance mechanism depends on different physical processes that dominate under opposite biasing conditions.

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