A varactor diode is a diode with a capacitance that varies as the voltage across the terminals is changed. Varactor diodes are often found as voltage-controlled capacitors in frequency modulated radio transmitters and phase-locked loops in radio and television tuners. They're also common components in cellular phones.Continue Reading
Under the right conditions, an ordinary p-n junction can be made to operate as a variable capacitor. It doesn't offer the same performance characteristics as a commercially manufactured varactor, but it performs the same function. Quality diodes are readily available from many electronics supply sources.
In order to function as a variable capacitor, the p-n junction diode must be operated under reverse bias conditions. The anode end should be negative in respect to the cathode end; this create three distinct electron density regions. The p and n terminals of the diode are conductive, but the central region of the diode is a depleted zone where conduction cannot occur.
Under reverse bias conditions, the varactor diode works just like a capacitor. The p and n regions function as the plates, and the depletion zone functions as the space between the pates. By varying the bias, you moderate the size of the depletion zone. This has the same effect as changing the space between the plates of a capacitor to modulate the capacitance.