Varactor

(electronics) A semiconductor device characterized by a voltage-sensitive capacitance that resides in the space-charge region at the surface of a semiconductor bounded by an insulating layer. Also known as varactor diode; variable-capacitance diode; varicap; voltage-variable capacitor.

A solid-state device which has a capacitance that varies with the voltage applied across it. The name varactor is a contraction of the words variable and reactor. Typically the device consists of a reverse-biased pn junction that has been doped to maximize the change in capacitive reactance for a given change in the applied bias voltage. The device has two primary applications: frequency-tuning of radio-frequency circuits including frequency-modulation (FM) transmitters and solid-state receivers, and nonlinear frequency conversion in parametric oscillators and amplifiers. See also Capacitance; Frequency modulator; Parametric amplifier; Reactance.

A pn junction in reverse bias has two adjacent microscopic space-charge or depletion regions which function like the plates of a capacitor. These depletion regions get larger as the applied reverse-bias voltage is increased; however, the increase in the width of the depletion region is not linear with bias voltage, but instead is sublinear, the exact nature of the relationship depending upon the doping profile in the pn junction. For example, in a pn junction with constant doping density, the depletion region width varies as the square root of the applied reverse-bias voltage. Because the capacitance of the device is proportional to the width of the depletion region, the nonlinear relationship between bias voltage and depletion width results in a nonlinear voltage-capacitance relationship as well. The pn junction doping profile is adjusted by the device designer to obtain the desired capacitive nonlinearity.

The frequency response of the varactor is governed by the relationship between the series linear resistance of the diode and its nonlinear capacitance. The highest frequency for which the device will function properly is that at which the capacitive reactance (the reciprocal of the product of nonlinear capacitance and frequency) is equal to the series resistance of the device. Thus, designing a varactor for maximum frequency response involves choosing a doping density high enough for a small series resistance but low enough so that the capacitance of the device is small.

Varactors, as well as other solid-state devices, possess the advantage that they are compact and robust, permitting their use in hostile environments, as well as improving the reliability of the circuits in which they are employed. See also Junction diode; Microwave solid-state devices; Semiconductor; Semiconductor diode.