The varicap was developed by the Pacific Semiconductor subsidiary of the Ramo Wooldridge Corporation who received a patent for the device in June 1961.

This can be accomplished by placing a DC blocking capacitor with a capacitance about 100 times greater than the maximum capacitance of the varicap diode in series with it and by applying DC from a high impedance source to the node between the varicap cathode and the blocking capacitor as shown in the upper left circuit in the accompanying diagram.

In time, varicap diodes were developed which exhibited large capacitance ranges, 100–500 pF, with relatively small changes in reverse bias: 0–5 V or 0–12 V. These newer devices allow electronically tuned AM broadcast receivers to be realized as well as a multitude of other functions requiring large capacitance changes at lower frequencies, generally below 10 MHz.

Some designs of electronic security tag readers used in retail outlets require these high capacitance varicaps in their voltage-controlled oscillators.

The three leaded devices depicted at the top of the page are generally two common cathode connected varicaps in a single package.

This is done to keep costs down – two dual packages could have been used, one for the tank and one for the oscillator, four diodes in all, and this is what was depicted in the application data for the LA1851N AM radio chip.

Two lower-capacitance dual varactors used in the FM section (which operates at a frequency about one hundred times greater) are highlighted by red arrows.

If a sine wave current of sufficient amplitude is applied driven through a varicap, the resultant voltage gets "peaked" into a more triangular shape, and odd harmonics are generated.

[clarification needed] Reverse biasing the emitter-base junctions of transistors also is quite effective as long as the AC amplitude remains small.

Before the development of the varicap, motor driven variable capacitors or saturable-core reactors were used as electrically controllable reactances in the VCOs and filters of equipment like World War II German spectrum analyzers.