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2018-02-27 19:36  
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What is a varactor?A varactor (or a tuning diode) is a diode designed to be used as a voltage controlled capacitor. Its schematic symbol is shown in Figure SB1. A diode starts as a piece of intrinsic mono-crystalline material. Intrinsic means it contains no atoms of other material. Mono-crystalline means its one single crystall with all the electrons trapped in covalent bonds between atoms. Basically, it’s piece of rock and it’s an insulator.
To form the PN junction that makes a diode, half the piece is doped with a trace amount of “donor atoms” that have one more electron than the instrinsic stuff to make N-material.
varactor symbol
The other half is doped with a trace amount of “acceptor atoms” that have one less electron than the intrinsic stugg; they form “holes” in the crystal that can trap electrons. Those holes make P-material. Since the doped material contains charde carriers, it conducts a bit. It’s a semiconductor.
To see how a varactor works, look at the reverse-biased PN junction in Figure SB2.
varactor PN junction

You had “loose electrons” moving around the N side as far as the junction. Likewise, you had “loose holes” moving around the P side as fac as the junction. But the battery pulls electrons out of the N side and pushes electrons into the P side untile the charge stored in the diode balances the battery voltage. That forms a region on both sides of the junction – the depletion region – that contains no loose charge carries. The depletion region cannot conduct, so it’s an insulator. On either side of that insulator you have conductive regions. Two conductors separated by an insulator form a capacitor, so a reverse-biased PN junction is a capacitor. The equation for capacitance is:
varactor capacitance equation
- C is the capacitance in Farads
- A is the area of each plate measured in square meters
- Er is the relative permittivity of the insulator
- Eo is the permittivity of free space
- d is the separation between the plates in meters.
But d is the length of the depletion region which depends on V – the reverse-bias voltage. As V increases, d increases and C decreases. There’s your voltage controlled capacitor. Any diode can be used this way but most diodes are designed to minimize junction capacitance.Varactorsare specifically designed to be used as capacitors.

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