Varactor Diode

In this chapter, we are going to learn about the Varactor Diode, its working principle, the working of the Varactor Diode, and its applications. So let’s start with the definition of varactor diode.

Varactor Diode Definitions

Definition 1: A Varactor Diode is a specialized p-n junction diode with a suitable impurity concentration profile that changes its level of capacitance depending on the value of the reverse bias voltage applied to the diode.

Definition 2: A Varactor Diode is a specialized voltage-dependent variable capacitor p-n junction diode.

Symbol of Varactor Diode

The schematic symbol of a varactor diode and its equivalent circuits are shown in the figure below:

Note: The other name of Varactor Diode are varicap diode, variable capacitance diode, voltage variable capacitor(VCC), variable reactance diode, or tunning diode.

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Working Principle of Varactor Diode

• A varactor diode is a reverse-biased junction diode whose mode of operation depends on its capacitance.

• When a p-n junction diode is reverse-biased, the depletion region acts as a capacitor dielectric.

• In addition to this, under the reverse biased condition on either side of the depletion region, the resistance of the p and n region gets reduced and acts like the plates of parallel plate capacitors, as shown in the figure below.

• Thus, within a p-n junction diode, all the components of the parallel plate capacitor are available. This junction capacitance is called transition capacitance or space charge capacitance ( C_T or C_pn).

• The capacitance of the varactor is given by the relation;

C = \frac{\epsilon A}{d}

where

• \epsilon is the permittivity of the depletion region of the semiconductor diode.
• A is the area of the junction and
• d is the width of the depletion region.

• The capacitance value can be changed by changing any one of the above three factors or by changing all three factors. Thus junction diode will act as a variable plate capacitor.

Note: In a varactor diode, the capacitance parameter can be controlled by the method of doping in the depletion region or the size and geometry of the diode construction.

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Working of Varactor Diode

• The variation of capacitance with the applied reverse bias voltage is shown in the above figure. From the graph, it is seen that the junction capacitance is maximum when the reverse voltage is equal to zero and reduces in a non-linear manner as the value of reverse voltage is increased.

• When the reverse bias voltage increases, the depletion region widens. This increases the dielectric thickness which in turn reduces the capacitance.

• On the other hand, when the reverse bias voltage decreases the depletion region gets narrowed. this decreases the dielectric thickness which, in turn, increases the capacitance.

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Applications of Varactor Diode

A varactor diode finds its application in many circuits. Some major applications are listed below:

1. The varactor is used in the FM tuner for automatic frequency control.
2. It is used in electronic tuners for radio, TV, and other commercial receivers.
3. It is used in bandpass filters and parametric amplifiers.
4. It is used in communication equipment as a voltage-controlled capacitor.

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Frequently Asked Questions on Varactor Diode