varaktor

A varaktor (also varactor or varicap) is a semiconductor diode designed to exploit the voltage-dependent capacitance of a reverse-biased p–n junction. When biased in reverse, the width of the depletion region changes with the applied voltage, producing a controllable junction capacitance used for electronic tuning and frequency control in radio-frequency circuits.

Varactors are manufactured as silicon p–n junctions with doping profiles tailored to give a desired capacitance-versus-voltage characteristic. Common types include abrupt and hyperabrupt devices; the latter provide a stronger change in capacitance per volt and are useful where wider tuning ranges are required. The capacitance typically ranges from fractions of a picofarad to several tens of picofarads, depending on device geometry and bias. The relationship between capacitance and reverse voltage is often approximated by C ∝ (V + V0)−m, where m depends on the doping profile.

Practical considerations include series resistance, quality factor (Q), maximum reverse voltage, and temperature dependence. Varactors must remain reverse-biased in operation; forward bias destroys the tuning behavior. Nonlinear capacitance can introduce harmonic distortion and intermodulation in high-level RF applications, so biasing networks, linearization techniques, or multiple diodes are used to mitigate these effects.

Typical applications are voltage-controlled oscillators, phase-locked loops, FM radio tuners, tunable filters, frequency multipliers, and antenna matching networks. Varactors are compact, inexpensive components widely used wherever electronic tuning or voltage-dependent reactance is required.