Varactor Diode

Varactor Diode

A varactor , also known as tuning diode, a variable capacitance diode, a varicap diode or variable reactance diode, is a diode that exploit the principle of operation of semiconductor diode, to act or behave like a variable capacitor i.e. has variable capacitance, which is a function of the external voltage impressed on its terminals. As we know that a capacitor is basically a device constructed using two parallel plates (conductor) separated by an insulator or a dielectric (insulator) (See: Basic Electronic (Passive Component)).

If we reconsider the electrical characteristics semiconductor diodes as in figure 3.15.

Figure 3.15: Semiconductor diode

The P-side is a good conductor owing to the presence of excess holes; also the N-side is a good conductor due to the presence of excess free electrons. The depletion layer is ridden of charge carriers thus an insulator. This can be likened to the construction of a typical capacitor. 

 

 Figure 3.16: Semiconductor Diode depicted as a capacitor

Also, we will recall that when diode is forward biased, the width of the depletion region becomes narrower with respect to the external voltage supply, the more the voltage the narrower is the depletion region, until the voltage overcomes the built-in barrier and allow current flow.

We can also recall also that; the Capacitance of a capacitor is Proportional to the area A of the plate and inversely proportional to the distance d between the plate in an equation given by;

            ε₀ – permittivity in free space

            ε_r  - dielectric constant (relative permittivity)

            A – area of the plate

            d – distance of plates apart

(See:  Basic Electronics (Passive Components)).

Now, employing the semiconductor diode as a capacitor, the area A of the plates (P and N-sides) can be increased, by reducing the depletion layer (distance d, between the plates) through applying an external voltage of a certain level, thus increasing its capacitance, while we can reduce its capacitance by reducing the applied voltage which increases their distance apart (depletion region) and reduces the cross sectional each sides (P- and N-) of the diode. This means that the capacitance is varied with respect to the applied voltage, thus provides the variable capacitor property and called a Varactor.

See Also:

Light Emitting Diode (LED) 

Photo-diode