Question

The junction capacitance of a PN junction depends on

• A. applied voltage only
• B. barrier potential
• C. both doping concentration and applied voltage
• D. doping concentration only

Hint:

Capacitance depends on depletion width.

Answer 1

The Correct Option is C

Step 1: Model the junction as a parallel-plate capacitor.
The depletion region has fixed charges on either side with no free carriers. This acts like a parallel-plate capacitor: $C_j = \varepsilon A / W$, where $W$ is the depletion width and $A$ is the junction area.

Step 2: Show voltage affects $W$.
The depletion width is $W \propto \sqrt{V_{bi} + V_R}$, where $V_{bi}$ is built-in potential and $V_R$ is reverse bias. Higher reverse voltage widens $W$, reducing $C_j$. So applied voltage directly controls the capacitance.

Step 3: Show doping also affects $W$.
The depletion width also depends on doping: $W \propto 1/\sqrt{N}$, where $N$ is doping concentration. Heavier doping narrows $W$, increasing $C_j$. Both voltage AND doping determine $C_j$. \[ \boxed{\text{Both doping concentration and applied voltage}} \]