Short note: Varactor diode

A varactor diode (varicap, voltage-variable capacitor) is a PN junction diode designed to be used as a voltage-controlled capacitor. It is operated in reverse bias.

Principle

In reverse bias, the depletion region acts as a dielectric between the p and n regions, which act as the capacitor plates. When the reverse voltage VR increases, the depletion region widens, so the capacitance decreases. When VR decreases, the capacitance increases.

C = C0 / (1 + VR/V0)ⁿ

where C0 is the capacitance at zero bias, V0 is the barrier potential and n = 1/2 for an abrupt junction (1/3 for a graded junction).

 C
 |\
 | \
 |  `.
 |    `--.___
 +---------------- VR (reverse)

 Symbol:  --|>|--| (diode with a capacitor symbol)

Characteristics

• Capacitance range from a few pF to about 100 pF, tuning ratio about 2:1 to 10:1.
• Needs a reverse dc bias; the signal must be small enough not to turn the diode on.
• High Q (low losses).

Applications

• Electronic tuning of radio and TV receivers (replacing tuning capacitors).
• Voltage controlled oscillators (VCO) and phase-locked loops.
• Frequency modulation (FM) generators and frequency multipliers.
• Automatic frequency control (AFC).