A varactor diode, also known as a voltage-variable capacitor diode or varicap, is a reversebiased PN junction whose operation depends on the variation of junction capacitance with reverse bias. Special dopant profiles are grown in the depletion layer to enhance this capacitance variation and minimize series resistance losses.
The varactor is made from a semiconductor material whose dopant concentration is graded throughout the device, with the heaviest concentration in the regions adjacent to the junction. The junction region is small to take advantage of the variation of junction capacitance with reverse voltage. Varactor diodes have very low internal resistance so that the PN junction, when reverse biased, acts as a pure capacitor. Because the junction is abrupt, junction capacitance varies inversely as the square root of the reverse voltage.
Most varactor diodes are made from silicon, but gallium-arsenide varactors offer higherfrequency response. Low-power varactors serve as voltage-variable capacitors in electronic tuners, and do phase shifting and switching in the VHF and microwave circuits. They also function as very low frequency multipliers in solid-state transmitters and do limiting and pulse shaping.
Standard varactors can provide 12 W at 1 GHz, 7 W at 2 GHz, 1 W at 5 GHz, and 50 mW at 20 GHz. Efficiencies of 70 to 80 percent have been obtained at 1 and 2 GHz. The dimensions of a varactor’s package depend on its operating frequency and power dissipation.
