When the diode is reverse biased, there is a region at P-N junction having dielectric properties. As you know from the last chapter, this is called the depletion region, as it has a shortage of majority charge carriers. The width of zone depends on many things, including reverse voltage. As long as reverse bias is less than avalanche voltage, varying bias can change the width of depletion region. This results in a change in capacitance of junction. The capacitance, which is quite small, varies inversely with the square root of reverse bias.
Some of the diodes are manufactured especially for the use as variable capacitors. These are varactor diodes. Sometimes you will hear them being called as varicaps. They are made from silicon or gallium arsenide.
A common use for a varactor diode is in the circuit known as voltage-controlled oscillator (VCO). A voltage tuned circuit, by using a coil and a varactor, is shown in the figure given below. This is a parallel tuned circuit. The fixed capacitor, the value of which is large as compared with that of varactor, serves to keep coil from short circuiting the control voltage across varactor. Notice that symbol for varactor has two lines on cathode side. This is signature of it, so that you know that it is a varactor, and not an ordinary diode.
Figure-- Connection of a varactor diode in a tuned circuit.