capacity diode
A capacitance or varactor diode is a diode that performs the function of a variable capacitor. The pn layer acts as the capacitance, which is operated in the reverse direction and whose space charge zone is controlled.
When a voltage is applied, an electric field builds up in the junction, which increases when the voltage is increased. This also increases the width of the charge-free zone, causing the junction capacitance to decrease. The capacitance change is about 5 pF/V to 10 pF/V in the picofarad range and is proportional to the square root of the applied reverse voltage over a wide voltage range. The distance between the capacitor plates changes as a function of the thickness of the junction and this in turn changes as a function of the magnitude of the applied reverse voltage. The capacitor plates themselves depend on the construction of the capacitance diode.
Capacitance diodes are used as controllable capacitors in oscillating circuits and voltage-controlled oscillators( VCO) and replace the mechanical variable capacitors in radio receivers.