varactor

A varactor, short for variable capacitor, is a diode designed to provide a capacitance that varies with applied reverse-bias voltage. In a reverse-biased p-n junction, the depletion region widens as the reverse voltage increases, reducing the junction’s capacitance. The capacitance is nonlinear with voltage and can be tuned over a wide range by changing the reverse bias. Varactors are commonly implemented as silicon p-n junction or PIN diodes and are marketed as varactor diodes or varicaps. Their capacitance may range from fractions of a picofarad to tens of picofarads, depending on device size, doping profile, and design.

Doping profiles such as abrupt or hyperabrupt are used to tailor the C–V characteristics, maximizing the capacitance change per volt for specific applications. Varactors are intended for radio frequency and microwave use, and their performance is influenced by parasitics such as series resistance, package inductance, and lead lengths. They also exhibit temperature sensitivity and a non-ideal, voltage-dependent Q factor, which can affect linearity and loss.

Common applications include tunable RF filters, voltage-controlled oscillators (VCOs), phase shifters, and frequency synthesizers. In VCOs, the varactor forms part of an LC tank whose reactance varies with the control voltage to sweep the output frequency. In tunable filters, arrays of varactors electronically adjust the passband. For high-frequency operation, alternatives such as MEMS varactors or switched-capacitor networks may be used to achieve higher Q and better linearity.