Concept:Under reverse bias, the depletion layer acts as an insulating dielectric of width \( W \) between the conductive P and N regions, forming a parallel-plate capacitor whose capacitance decreases as the depletion layer widens.
Formula:$$C_T = \frac{\epsilon_s A}{W} \propto \frac{1}{\sqrt{V_0 + V_R}}$$
Solution:- Increasing the reverse voltage \( V_R \) widens the depletion layer (\( W \propto \sqrt{V_0 + V_R} \)).
- Because capacitance is inversely proportional to plate separation (\( C = \epsilon A / W \)), \( C_T \) decreases as \( V_R \) increases.
- This voltage-dependent capacitance is the operating principle of varactor (varicap) diodes.
Why other options are incorrect:- Option B: Transition capacitance decreases with reverse voltage; it does not increase.
- Option C: \( C_T \) decreases toward a small minimum value rather than approaching infinity.
- Option D: Capacitance depends on depletion layer width, which changes with reverse voltage.
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