Frequency controlWhen a diode is reverse-biased, there is a region at the P-N junction with dielectricproperties. As you know from the last chapter, this is called the depletion region, be-cause it has a shortage of majority charge carriers. The width of this zone depends onseveral things, including the reverse voltage.As long as the reverse bias is less than the avalanche voltage, varying the bias canchange the width of the depletion region. This results in a change in the capacitance ofthe junction. The capacitance, which is always quite small (on the order of picofarads),varies inversely with the square root of the reverse bias.Some diodes are manufactured especially for use as variable capacitors. These arevaractor diodes. Sometimes you’ll hear them called varicaps. They are made from sil-icon or gallium arsenide.A common use for a varactor diode is in a circuit called a voltage-controlled oscil-lator (VCO). A voltage-tuned circuit, using a coil and a varactor, is shown in Fig. 20-9.This is a parallel-tuned circuit. The fixed capacitor, whose value is large compared withthat of the varactor, serves to keep the coil from short-circuiting the control voltageacross the varactor. Notice that the symbol for the varactor has two lines on the cath-ode side. This is its “signature,” so that you know that it’s a varactor, and not just an or-dinary diode.376 Some uses of diodes20-8At A, two diodes can work as a limiter. At B, the peaks are cut off by the action of thediodes.20-9Connection of a varactordiode in a tuned circuit.