varactor

A varactor is a semiconductor diode deliberately designed so that its junction capacitance changes as you vary the reverse voltage across it. Unlike ordinary rectifier diodes, where capacitance is a parasitic nuisance, a varactor turns that voltage-dependent behavior into the primary function. The device acts as a voltage-controlled capacitor, with no moving parts and response times in the nanosecond range.

The effect comes from the depletion region at the p-n junction. As reverse bias increases, the depletion width grows, and the capacitance drops. Typical varactors span a capacitance range of 5:1 to 10:1 or more across their working voltage span. Silicon varactors commonly operate from 2 volts to 30 volts reverse bias, delivering capacitance values ranging from a few picofarads up to several hundred picofarads depending on the doping profile and junction geometry. The relationship between voltage and capacitance is not linear; it follows a power law, typically C = K / (V + φ)n, where n lies between 0.33 and 0.5 depending on the doping type.

Radio frequency tuning is the classic application. A varactor in parallel with an inductor forms a voltage-tuned LC tank circuit, allowing frequency adjustment without mechanical capacitor gangs or stepper motors. Phase-locked loops and frequency synthesizers rely on varactor tuning for stable, electronically controlled oscillators. In the microwave and UHF bands, varactors enable rapid frequency switching and beam steering in phased arrays. Parametric amplifiers exploit the nonlinear reactance for signal amplification with minimal noise.

Common failure modes and trade-offs

Temperature drift is a constant headache: capacitance typically changes 0.2% to 0.5% per degree Celsius. Junction breakdown under excessive reverse voltage is destructive and permanent. Series resistance, though small (typically 1 to 10 ohms), limits Q factor and adds loss at high frequencies. Frequency pushing, where oscillator output shifts under load variation, demands careful circuit design. Leakage current increases sharply near breakdown, degrading sensitivity in sensitive detector circuits.

Varactors come in single-junction and multi-junction variants; stacking multiple junctions in series allows higher voltage ratings at the cost of lower capacitance. Abrupt junction varactors (made with step junctions) give steeper voltage dependence; hyperabrupt types push the exponent toward 0.33, useful for linearizing tuning voltage. The name traces to variable reactor, though modern terminology favors variable capacitance diode in some contexts, but varactor remains standard in frequency control and microwave literature.