Varicaps

Varicaps, short for varactor-capacitors, are diodes whose junction capacitance varies with applied reverse bias. They are commonly referred to as varactor diodes or varactors and are used as voltage-controlled capacitors in RF and microwave circuits, notably in tuners, filters, phase-locked loops, and voltage-controlled oscillators.

The capacitance arises from the depletion layer of a p–n junction. Increasing reverse bias widens the depletion region, reducing capacitance. The C-V characteristic is nonlinear, often described by a hyperabrupt or abrupt doping profile to tailor tuning range and linearity.

Most varicaps are silicon or gallium arsenide diodes designed for reverse-bias operation. They are available with different junction profiles: abrupt, hyperabrupt, or step-cut; die sizes and packages vary; used as discrete components or in integrated circuits.

Used in RF tuning networks, such as adjustable LC tanks in receivers, VCOs in PLLs, and channel scanners; in digital phase-locked loops for frequency synthesis; in automatic tuning circuits of radios and transmitters.

Key parameters include varicap capacitance range, quality factor, reverse breakdown voltage, series resistance, and temperature coefficient. Higher tuning ranges often trade off linearity and Q. Temperature compensation or tuning networks may be used to stabilize performance; bias supply noise can modulate capacitance and introduce phase noise.

The term varactor originated in the 1960s; 'varicap' is a common synonym, historically used by manufacturers. Varicaps are distinct from MEMS or ferroelectric tuners, though all can serve as voltage-controlled capacitors in modern RF systems.