Varactor Diode

A varactor diode is a pn junction that acts as a variable capacitor under changing reverse bias.

A varactor diode may be considered as a capacitor with n - region and p - region forming oppositely charged plates and with depletion layer acting as a dielectric.

 Varactor Diode Symbol

Varactor Diode

Therefore, a varactor diode is constructed to have high capacitance under reverse bias range.

Conventionally, a varactor diode is always reverse biased.

Total Capacitance, C_T = εA/W_d where:

A represents the cross sectional area.
W_d represents the width of the depletion layer.

When reverse voltage across a varactor diode is increased, the width of the depletion layer increases. Therefore, the total junction capacitance reduces.

On the other hand, if the reverse voltage across a varactor diode is lowered, the width of the depletion layer lowers. Therefore, the total junction capacitance increases.

For this very effect a varactor diode is sometimes called a voltage controlled capacitor which maakes it useful in voltage controlled tuning.