The advent of varactor diodes has made a huge impact in many areas of electronic design, which is only too evident in todays consumer products. Formerly, where bulky or unreliable mechanical methods were used, the size, reliability and excellent tracking abilities of the varactor has led to smaller, cheaper and more elaborate circuitry, previously impossible to attain.
The varactor diode is a device that is processed so to capitalise on the properties of the depletion layer of a P-N diode. Under reverse bias, the carriers in each region (holes in the P type and electrons in the N type) move away from the junction, leaving an area that is depleted of carriers.

Thus a region that is essentially an insulator has been created, and can be compared to the
classic parallel plate capacitor model. The effective width of this depletion region increases with reverse bias, and so the capacitance decreases. Thus the depletion layer effectively creates a
voltage dependent junction capacitance, that can be varied between the forward conduction region and the reverse breakdown voltage.
Variable capacitance diodes can be used in any tuned circuit application where previously mechanical methods were utilised, and provide a size, cost and performance advantage.

Read and download: http://www.zetex.com/3.0/appnotes/apps/an9.pdf

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