Varicap

A varicap diode, varactor diode or tuning diode is a type of diode used in electronic circuits. It is principally used as a voltage-controlled capacitor, and its diode function is secondary. It is operated reverse-biased[?] so no current flows through it, but since the depletion region[?] varies in proportion to the applied bias voltage, the capacitance of the diode can be made to vary. All diodes exhibit this phenomenon to some degree, but specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite.

Varactors are commonly used in voltage controlled oscillators[?] as part of Phase locked loops and Frequency Synthesisers.