Varactor Diode | Electronic Devices - Electronics and Communication Engineering (ECE) PDF Download

Introduction

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 the varactor diode.

What is a 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 conditions and is a voltage-dependent semiconductor device. Several names know varactor diode as Varicap, Voltcap, Voltage variable capacitance, or Tunning diode.

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 has two parallel lines representing the capacitor’s conductive plates. The gap between the plates shows their dielectric.

Operation of Varactor Diode

• The function of the varactor diode is to store charges, so it is always operated in reverse bias conditions. 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 the 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 the 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 compared to a normal variable capacitor.

Varactor Diode Formulae

• Capacitance of Varactor Diode
  
• Quality factor of the Varactor Diode
  

Notations Used In The Varactor Formula

• C_j is the diode capacitance
• C is the diode capacitance when the device is unbiased
• V is the applied voltage
• V_b 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; therefore, there is no conduction. They are voltage-controlled capacitors and are 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 to enable high-range changes in capacitance. Varactor diodes are categorized based on the properties of the diode junction.

Varactor Diode Applications

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

• They are used in the RF design arena and provide a method of varying the capacitance within a circuit by applying 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 – Tuning filters using varactor diodes is possible. Tracking filters can be required in front-end receiver circuits wherein the filters are enabled to track incoming signals’ frequency.
• Voltage-controlled oscillators (VCOs) – VCOs are used for many applications, and oscillators within a phase-locked loop are the major region. VCOs are present in almost all wireless receivers and radios.