Acronym for variable reactor. A p-n junction diode or Schottky diode operated with reverse bias so that it behaves as a voltage-dependent capacitor. The depletion layer at the junction acts as the dielectric and the n- and p-regions form the plates (see diagram). A diode intended for use as a varactor generally has a particular impurity profile designed to give an unusually large variation of junction capacitance and to minimize the series resistance.

In a diode with an abrupt junction the voltage dependence is given by

where C is the capacitance and V the reverse voltage. See also hyperabrupt varactor.

Energy bands in a varactor