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.