# Varactor Diode

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## What is a Varactor Diode?

A varactor diode is a p-n-junction diode, in which internal capacitance can be altered as a result of the application of reverse bias voltage. The varactor diode works in the reverse bias condition. It is also known as Varicap diode or voltCap diode indicating varying capacitance. The varactor diode is a voltage-controlled device, i.e., the output of the varactor diode is depending on the input voltage supplied, because its function is determined by the applied voltage. The varactor or varicap diodes are used to store charges.

Diodes are the electronic components used to conduct the flow of electrons in one direction. Diodes are two-terminal devices, they are mainly used to protect electronic circuits by controlling voltage and also by transforming AC to DC.  Although there are many varieties of diodes, they are all having the same basic structure of diode, i.e., sandwiching a p-type material with n-type, further depending upon the requirement and their characteristic nature they have been classified into rectifier diodes, Zener diodes, Light-emitting diodes(LED’S), Schottky diodes, varactor diodes, etc.

### Symbol of Varactor Diode

Since it is a type of p-n-junction diode, its symbolic representation is similar to that of the p-n-junction diodes. It is a two-terminal device having an anode and a cathode as shown in the figure below. One terminal consists of a diode symbol followed by parallel lines indicating the capacitor and the gap between the two parallel plates is their dielectric.

### Operation of Varactor Diode:

The varactor diode consists of p-type and n-type semiconductor components. The majority of charge carriers in n-type and p-type semiconductor materials are electrons and holes respectively. The diodes are designed by sandwiching a p-type material with an n-type material that creates a p-n-junction and the depletion layer is formed at the junction. The width of the depletion region depends on the type of biasing technique used. If it is in forward bias condition, the depletion region will be narrow and the conduction current takes place. In reverse biasing, the depletion region will be wide and will block the current.

The varactor diode is used only in a reverse-biased condition and the current will not flow as the depletion region will block its flow. The main function of a varactor diode is to store charge, if we use forward biasing the density of charges that can be stored will be zero as they are allowed to flow which is undesirable, thus it is always operated in the reverse bias condition.

### Varactor Diode Formulae

The junction capacitance of the varactor diode is inversely proportional to the width of the depletion layer, i.e., if the width of the depletion region is negligible or narrow, the capacitance of the junction will be high. Thus to increase the storage capacity of the varactor diode, the depletion layer must be narrow.

The mathematical equation that gives the junction capacitance of the varactor diode is,

C = εA/W  …..(1)

Where,

C- Junction capacitance

ε - Permittivity of the material

A- Area of p-n-junction

W - Width of the depletion region

Further, the relationship between the junction capacitance and the reverse bias voltage is given by the equation:

C = C’K/(V - V’)$^{m}$

Where,

C - Diode capacitance

C’’ - Diode capacitance before biasing

K - Constant and is equal to 1

V - The barrier voltage

V’ - Reverse bias voltage

m - Constant depends upon which type of semiconducting material is used.

The Quality factor of the Varactor diode is given by,

Q = F/f

Where,

F - The Cut-off frequency or maximum Operating frequency

f - The Operating frequency

### Varactor Diode Characteristics

The varactor diodes are manufactured with the intention of resulting in a definite C-V curve that can be obtained by changing the intensity of doping during the process of manufacture. Depending upon the type of doping whether it is linearly doped or non-linearly doped the varactor diodes are classified into the following two types.

1. Abrupt Varactor diodes

2. Hyper-Abrupt Varactor diodes

The characteristic curve of the varactor diode is given as shown below,

By studying the graph we can explain that the varactor diode is operated in reverse bias condition and as the width of depletion region increases by increasing reverse voltage ‘V’ resulting in a decrease of junction capacitance ‘C’. The varactor diodes are typically used to provide high range variation in capacitance.

The advantages of varactor diodes are as follows.

1. They are manufactured in such a way that the size of varactor diodes are compact and lightweight.

2. They are affordable and economical.

3. They produce relatively less noise in comparison with other diodes.

### Did You Know?

• Why is it Termed as a Varactor Diode?

Ans: The word Varactor indicates varying reactance. As these diodes are mainly used because of their peculiar property of varying capacitance by regulating the reverse-biased voltage, they are termed as Varactor diodes.

• What are the Applications of Varactor Diodes?

Ans: The varactor diodes are widely used in electrical circuits, and the following are some remarkable applications of varactor diodes.

1. The varactor diodes are used in tuning circuits, they are used as a replacement for classic capacitor tuning in old FM radio.

2. They are used in tank circuits or LC circuits.

3. They are used in signal modulation and demodulation processes.

4. They are used as frequency multipliers.

5. They are used in small remote control circuits.

The applications of varactor diodes are plenty, they are almost used in every host of electronic circuits in general electrical circuit design as well as RF designs.