Varactor diode is a type of PN junction diode that behaves as a variable capacitor under varying reverse-biased voltage applied to its terminals. It shows this capacitive effect under reverse biased condition only. It has two terminals i.e. anode and cathode. It is also known by other names such as tunning diode or voltage variable capacitance diode or varicap diode.
When the varactor diode is reversed biased, the PN junction shows a characteristic of an internal capacitor due to space-charges existing in the depletion region. This internal capacitance can be varied by the varying the applied reverse-biased voltage. Hence this diode is a voltage-dependent semiconductor device. The applied voltage controls the capacitance value.
The working mode of a varactor diode is basically the reverse-biased condition.
Varactor diode symbol
The symbol of a varactor diode is shown in figure 1. This symbol is different from a normal diode. The two parallel lines depict the capacitive behaviour.
Varactor diode working
This diode under no biased condition has a depletion layer. Under the forward biased condition, the depletion region almost disappears. Hence allow this diode to conduct as a normal diode.
Under the reverse biased condition, this diode does not conduct. When we increase the reverse voltage across the terminals, the depletion region width increases. As we know that the capacitance is inversely proportional to the separation of plates (depletion region width in this case). Hence the capacitance value decreases on increasing the reverse voltage.
On another hand, if we decrease the reverse voltage across the terminals, the depletion region width decreases. Hence the capacitance value increases on decreasing the reverse voltage.
Therefore, it can be said that the capacitance value can be varied by varying the reverse-biased voltage. The varactor diode act as a variable capacitor. Hence this property of varactor diode is utilized in several applications.
The capacitance of a varactor diode can be calculated as
$C=\epsilon \frac{A}{W}$
where C refers to the capacitance
A refers to the area of the junction
W refers to the width of the depletion region
and, ε refers to the permittivity of semiconductor material used in the diode.
The above equation shows that the capacitance of the varactor diode is directly proportional to the junction area and inversely proportional to the width of the depletion region.
Characteristics
In the characteristic graph, plotted between the capacitance of the varactor diode and reverse voltage, it is observed that the capacitance value decreases with the increase in the applied reverse-biased voltage.
Varactor diode advantages
The advantages of the Varactor diodes are
1. The reliability of the varactor diode is more than that of other diodes.
2. It can vary the capacitance of the devices.
3. The cost of a varactor diode is cheap.
4. This diode is light-weighted and its size is also small.
5. The production of noise or disturbances due to the use of this diode is less.
6. Less loss of power occurs in a varactor diode.
7. The use of a varactor diode is economically efficient.
Varactor diode disadvantages
The disadvantages of the Varactor diode are
1. The diode works only on reverse-biased condition.
2. In forward-biased condition, the diode cannot exhibit the capacitance property.
Varactor diode applications
The applications of a varactor diode are as follows.
1. It can be used as a bandpass filter as it contains capacitor plates.
2. In local oscillators, the varactor diodes are used for signal generation.
3. It can control frequency, hence used in controllers and control circuits.
4. A varactor diode is used as a phase shifter in radiofrequency applications.
5. For the tuning of electrical and electronic circuits, varactor diodes are used. The tuning of the circuits depends on the varactor diode capacitance.
6. Due to the tuning property of the varactor diodes, these are used in television like electronic appliances.
7. The varactors diodes are used in frequency multipliers and amplifiers.
8. Automatic frequency control circuits use varactor diodes.
9. The use of varactor diodes is found in LC resonant circuit in frequency multipliers.
10. It is used in the varactor diode modulator.
Author
Sunmoni Gohain
NIT Silchar
