A Varactor Diode is a semiconductor device whose capacitance can be varied. You might have heard about variable resistor. Varactor diode is similar and is a variable capacitor. Varactor diodes are constructed using semiconductor fabrication technology similar to manufacturing other ordinary diodes but special attention is given to the junction capacitance as the reverse bias is increased.

Varactor Diode are also called Varicap, Variable Capacitor Diodes or epicaps. They were developed during 1950s and were used afterwards in FM signal generation(see FM generation using varactor diodes). Nowadays they are used in RF circuit design and microwave circuits components like frequency multipliers, oscillators. A class C amplifier for example has an LC tuned filter wherein the varicap can be used as the capacitor part. Electronics Principle book by Malvino explains such circuit design examples.

The operation of varactor diode is as follows. When you connect a varactor diode in a circuit and you apply reverse voltage then its junction capacitance increases. This is due to the fact that when a diode is reversed biased the depletion region increases and therefore the separation between the charges at the junction and this gives the diode its capacitance. 

In more details, as you perhaps know the capacitance of a capacitor is proportional to the area. So in case of varactor diode, the area of the depletion junction will determine the capacitance of the diode. Also you should know that the capacitance of a diode is inversely proportional to the width of the capacitor so in case of varactor diode the width of the depletion region also determines the capacitance of the diode.

Thus during the manufacturing of the varactor diode, the area and width of the depletion region or the junction is made so as to match the range of capacitance for which the diode is designed for.

The figure below shows how a varactor diode is constructed. This varactor diode is constructed using GaAs(Gallanium Arsenide).

The reason for choosing the GaAs as the semiconductor material is because it has higher operating frequency and can withstand large range of operating temperature.


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