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.