Varactor Diode

Varactor diodes or Varicap diodes are semiconductor devices that are widely used in the electronics industry. They are also used within the RF design arena. In this article, we will learn about the characteristics and applications of varactor diode.

What is a Varactor Diode?

Varactor diode

Varactor Diode
Varactor diode is a type of diode whose internal capacitance varies with respect to the reverse voltage. It always works in reverse bias condition and is a voltage-dependent semiconductor device. Varactor diode is known by several names as Varicap, Voltcap, Voltage variable capacitance or Tunning diode.

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Symbol of Varactor Diode

From the diagram given below, it is evident that the symbol of the varactor diode is similar to that of the PN-junction diode. The diode has two terminals: anode and cathode. One end of the symbol consists of the diode, and the other end has two parallel lines that represent the conductive plates of the capacitor. The gap between the plates shows their dielectric.

Varactor diode Symbol

Operation of Varactor Diode

The function of the varactor diode is to store charges, so it is always operated in reverse bias condition. When a forward bias voltage is applied, the electric current flows, as a result the depletion region becomes negligible, which is undesirable.

The junction capacitance of a p-n junction diode is inversely proportional to the width of depletion layer. In other words, if the width of the depletion layer is less, then, the capacitance is more, and vice versa. So if we need to increase the capacitance of a varactor diode, the reverse bias voltage should be decreased. It causes the width of depletion layer to decrease, resulting in higher capacitance. Similarly, increasing the reverse bias voltage should decrease the capacitance. This ability to get different values of capacitances just by changing the voltage applied is the biggest advantage of a varactor diode when compared to a normal variable capacitor.

Varactor Diode Formulae

Capacitance of Varactor Diode \(C_{j}=\frac{CK}{(V_{b}-V)^{m}}\)
Quality factor of the Varactor Diode \(Q=\frac{F}{f}\)

Notations Used In The Varactor Formula

  • Cj is the diode capacitance
  • C is the diode capacitance when the device is unbiased
  • V is the applied voltage
  • Vb is the barrier voltage at the junction
  • m is the constant depending upon the material
  • K is the constant equal to 1
  • F is the maximum operating frequency
  • f is the operating frequency

Varactor Diode Characteristics

Varactor diodes are mostly operated under reverse bias conditions and therefore, there is no conduction. They are voltage controlled capacitors and sometimes known as varicap diodes, although the word varactor is widely used.

Variable Capacitance Characteristics

Variable capacitance effect is shown by normal diodes, but varactor diodes are preferred for providing the required capacitance changes. The diodes are uniquely optimized and manufactured such that they enable high range changes in capacitance. Varactor diodes are categorized based on the properties of diode junction.

What are Varactor Diode Applications?

Some applications of varactor diode are given in the points mentioned below.

  • They are used in RF design arena and provide a method of varying the capacitance within a circuit by the application of control voltage. It provides them with special capability due to which varactor diodes are used in the RF industry.
  • These diodes are used in many circuits and find applications in two main sectors.
  • FR filters – It is possible to tune filters by using varactor diodes. Tracking filters can be required in front-end receiver circuits wherein the filters are enabled to track the frequency of incoming received signals.
  • Voltage controlled oscillators (VCOs) – VCOs are used for many applications and oscillator within a phase locked loop is the major region. VCOs are present in almost all wireless receivers and radio.

Frequently Asked Questions on Varactor Diode

What is abrupt varactor diode?

Abrupt varactor diode exhibits the inverse square of C-V function and the doping concentration is constant.

What is a hyperabrupt varactor diode?

Hyperabrupt varactor diode is used as it has greater capacitance for the given change of voltage.

Which layer can be used for the detection of bad frames?

The link layer is used for the detection of bad frames.

What is the major use of varactor diode?

The major use of varactor diode is for tuning of the circuit.

What happens to plate separation when the depletion region in the varactor diode widens?

As the depletion region in the varactor diode widens, the plate separation increases.

Which type of diode is commonly used in electronic tuners in TVs?

Varactor diode is the commonly used diode in electronic tuners in TVs.


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