varactors

A varactor, or varactor diode, is a semiconductor device that acts as a voltage-controlled capacitor. It consists of a reverse-biased p-n junction or a MOS capacitor; the capacitance changes with the applied reverse bias. Increasing reverse bias widens the depletion region, reducing capacitance. In junction varactors the C–V curve can be tailored by the doping profile (abrupt, step, or hyperabrupt) to balance tuning range and linearity.

MOS varactors use a metal-oxide-semiconductor structure and offer integration advantages and different temperature behavior. They are typically reverse-biased and must avoid forward conduction, which would destroy the varactor function.

Applications include tuning RF and microwave circuits such as voltage-controlled oscillators, phase shifters, tunable filters, and impedance matching networks. They provide large capacitance changes with relatively small DC control voltages, with typical C values ranging from tens of femtofarads to a few picofarads depending on process and area. Performance is affected by nonlinearities, finite Q, parasitic inductance and resistance, and temperature sensitivity, requiring careful biasing and RF design.

Alternatives and related devices include specialized PN-junction varactors and MOS varactors; high-frequency operation uses III-V materials in some cases. MEMS-based tunable capacitors offer different trade-offs in power handling and linearity.