varactor diode

A varicap diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals

Varactors are operated reverse-biased so no current flows, but since the thickness of the depletion zone varies with the applied bias voltage, the capacitance of the diode

 

can be made to vary. Generally, the depletion region thickness is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region thickness. Thus, the capacitance is inversely proportional to the square root of applied voltage.

varicap diodes are a particularly useful form of semiconductor diode. Finding uses in many applications where electronically controlled tuning of resonant circuits is required, for items such as oscillators and filters