PN junctions and diodes
Which expression gives the depletion (junction) capacitance C of a reverse-biased PN junction in terms of junction area A, depletion width W and semiconductor permittivity ε?
- AC = εA / W
- BC = εW / A
- CC = ε / (A × W)
Show answer and explanation
Answer: A. C = εA / W
The depletion layer behaves like an insulating layer with charge stored on both sides, so it can be expressed like a parallel-plate capacitor as C = εA/W. Increasing the reverse bias widens W and reduces the capacitance.
- A. Correct. It has the same form as a parallel-plate capacitor.
- B. A wider W means a smaller capacitance, so W belongs in the denominator.
- C. A larger A means a larger capacitance, so A belongs in the numerator.