Junction Capacitance:

Under circumstances of reverse bias, a p-n junction can perform as a capacitor. A special kind of diode known as a varactor (a contraction of the term variable reactor) is produced with this property especially in mind. The junction capacitance of a varactor can be diverse by changing the reverse-bias voltage since this voltage influenced the width of the depletion area. The greater is the reverse voltage, the wider the depletion area gets, and the smaller the capacitance becomes. A good varactor can display a capacitance which fluctuates quickly, obeying voltage variations up to high frequencies.