Question E6B04:
What type of semiconductor device is designed for use as a voltage-controlled capacitor?  

Choices:  
A: Varactor diode  
B: Tunnel diode  
C: Silicon-controlled rectifier  
D: Zener diode    

Correct Answer is  A: Varactor diode  

Correct answer: A — Varactor diode

1) Why A is correct
- A varactor (varicap) diode is a PN-junction diode used in reverse bias so its junction (depletion) capacitance varies with applied reverse voltage. Increasing reverse bias widens the depletion region, reducing capacitance, so it functions as a voltage-controlled capacitor.
- Typical capacitance–voltage behavior:
  $$C_j(V_R) = \frac{C_0}{\left(1 + \frac{V_R}{V_j}\right)^m}$$
  where:
  - $C_0$ = zero-bias junction capacitance
  - $V_R$ = reverse-bias voltage
  - $V_j$ ≈ junction potential (about 0.6–0.8 V for Si)
  - $m$ ≈ 0.5 for abrupt junctions, ≈ 0.33 for linearly graded junctions
- Used for tuning VCOs, PLLs, and RF filters as a voltage-controlled reactance.

2) Why the others are incorrect
- B: Tunnel diode — A negative-resistance device used for high-speed oscillators and amplifiers; not intended as a controllable capacitor.
- C: Silicon-controlled rectifier (SCR) — A thyristor switch/rectifier; it latches on, not a reactive element.
- D: Zener diode — Used for voltage regulation in reverse breakdown; not designed for variable capacitance behavior.

3) Useful formulas and rules of thumb
- Varactor capacitance decreases as reverse voltage increases: higher $V_R$ → lower $C_j$.
- Resonant tuning with a varactor:
  $$f_0 = \frac{1}{2\pi\sqrt{L\,C_j(V_R)}}$$
  So increasing $V_R$ (decreasing $C_j$) raises $f_0$.
- Always operate a varactor reverse-biased; forward bias defeats the voltage-controlled capacitance behavior.