[Physics Class Notes] on Varactor Diode Pdf for Exam

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What is a Varactor Diode?

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

The Diodes are the electronic components used to conduct the flow of electrons in one direction. Diodes are two-terminal devices that are mainly used to protect electronic circuits by controlling voltage and also by transforming AC to DC, but there are many varieties of diodes and they are all having the same basic structure of diode, i.e., sandwiching a p-type material with n-type, also 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.

Operation of Varactor Diode

A Varactor diode consists of p-type and n-type semiconductor components respectively. The majority of charge carriers in n-type and p-type semiconductor materials are the electrons and holes respectively. These 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 the diode is in forward bias condition, the depletion region will be narrow and the conduction current takes place and in reverse biasing, the depletion region will be wide and will block the current.

These varactor diodes are used only in a reverse-biased condition and the current will not flow as the depletion region will block its flow. The primary 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

Advantages of Varactor Diodes

The advantages of varactor diodes are as follows:

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

• They are affordable and economical.

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