Varactor Diode

1.    Varactor Diode

1.1.          Definition:

A Varactor Diode is:

A p-n junction diode which acts as a variable capacitance under changing reverse bias.

A p-n junction diode that changes its capacitance and the series resistance as the bias applied to the diode is varies.

1.2.          Symbol:

The symbol of Varactor diode is:

The Varactor diode symbol consists of the capacitor symbol at one end of the diode that represents the variable capacitor characteristics of the diode.

1.3.          Explanation:

Varactor diodes are also termed as varicap diodes, in fact, these days they are usually termed as Varactor diodes. Even though the variable capacitance effect can be exhibited by the normal diodes (P-N junction diodes), but, Varactor diodes are preferred for giving the desired capacitance changes as they are special types of diodes. These diodes are specially manufactured and optimized such that they enable a very high range of changes in capacitance. Varactor diodes are again classified into various types based on the Varactor diode junction properties. And, these are termed as abrupt Varactor diodes, gallium-arsenide Varactor diodes, and hyper abrupt Varactor diodes. The Varactor diode also called a varicap or tuning or voltage variable capacitor diode, is a junction diode with a small impurity dose at its junction, which has useful property that its junction or transition capacitance is easily varied electronically.

A Varactor diode is a specially manufactured P-N junction with variable concentration of impurities in its P-N materials. In a conventional diode doping impurities are usually distributed equally throughout the material. Varactor have a very light dose of impurities near the junction. Moving away from the junction the impurity level increases.

1.4.          Circuit Diagram:

It is impossible to show all the circuits in which Varactor / varicap diodes may be used. However, it is worth providing one example to show how these diodes may be used in a typical circuit. Effectively a capacitor is replaced with the Varactor diode, but it is necessary to also ensure that the tune voltage, i.e. the voltage used to set the capacitance of the diode can be inserted into the circuit, and that no voltages such as bias voltages from the circuit itself can affect the Varactor diode.

Within this circuit D1 is the Varactor diode that is used to enable the oscillator to be tuned. C1 prevents the reverse bias for the Varactor or varicap diode being shorted to ground through the inductor, and R1 is a series isolating resistor through which the Varactor diode tuning voltage or bias is applied.

1.5.          Construction & Working:

When any diode is reverse biased, a depletion region is formed, as seen below:

The larger the reverse bias applied across the diode, the width of the depletion layer W becomes wider. Conversely, by decreasing the reverse bias voltage the depletion region width W becomes narrower. This depletion region is devoid of majority carriers and acts like an insulator preventing conduction between the N and P region of the diode, just like a dielectric, which separates the two plates of a capacitor.

When Varactor diode is reverse biased than the neutral region between P and N layers increases and when the reverse biasing decreases then this neutral region is also decreased. From this it is concluded that diode also has the capacity like a capacitor the difference is only that capacity in the capacitor varies due to dielectric between the two plates and in the diode capacity varies with neutral region thus dielectric region of the capacitor can be considered as neutral region of the diode and in this way diode can be considered as capacitor whose capacity changes with the reverse voltage. All the diodes change their capacity with the reverse voltage but some of them are manufactured specially which changes their capacity with reverse voltage of a definite capacity range.

1.6.          Characteristics of Varactor Diode

Varactor diodes, also known as varicap diodes, are a simple electronic component. A type of simple semiconductor diode commonly used in electronics such as parametric amplifiers, filters, oscillators and frequency synthesizers, Varactor diodes have a variable capacitance, which is a

function of the voltage impressed on its terminals. In electronics, Varactor diodes are mostly utilized as voltage-controlled capacitors.

1.6.1.      Operation:

Varactor diodes are constructed in the same way as a capacitor and operate under reverse bias conditions, which gives rise to three current-conducting regions. Currents conduct through positive (P) and negative (N) regions, located at either end of the diode. Near the junction of the P and N regions, a depletion region ensures that no current carriers are available, thus acting as an insulator. Due to this arrangement, a Varactor diode's conductive plates are separated by an insulator like dielectric, much like a capacitor.

1.6.2.   Capacitance:

In electronics, capacitance is the ratio of charge impressed on a given conductor. This characteristic determines a diode's frequency of operation. Any capacitor or conductor's capacitance depends on varying factors such as the area of its conductive plates, the dielectric constant of the insulator between the plates and the distance between the two plates. The width of a Varactor diode's depletion region increases and decreases via changing the level of the diode's reverse bias. In effect, changing this level alters the distance between the capacitor's plates. As the capacitance range of Varactor diodes are controlled by adjusting the gradient and junction width, range changes are applied using reverse voltage. Commonly, Varactor diodes operate at a four-to-one capacitance range.

1.6.3.   Reverse Breakdown:

Varactor diodes are designed to provide voltage-controlled capacitance operation under reverse bias. A diode's reverse breakdown is defined by the minimum reverse voltage required to make the diode conduct in reverse. As reverse bias increases, capacitance decreases; the

maximum voltage that a Varactor diode can withstand is determined by its maximum capacitance level. The reverse bias of most Varactor diodes operates from around a few volts up to about 20 volts, with some rare exceptions operating up to 60 volts. As a Varactor diode's voltage increases, specific energy supplies must be provided for the circuits driving the diode.

The Varactor diodes have the following some other significant characteristics:

• Varactor diodes produces considerably less noise compared to other conventional diodes.
• These diodes are available at low costs.
• Varactor diodes are more reliable.
• The Varactor diodes are small in size and hence, they are very light weight.
• There is no useful purpose of Varactor diode operated when it is operated in forward bias.
• Increase in reverse bias of Varactor diode increases the capacitance as shown in the figure below.

1.6.4.       Important Criteria:

·         Capacitance: Capacitance of the device. Capacitance from a few Pico Farads to hundreds of Pico Farads is provided. ¢

·         Capacitance range: Range of capacitance produced when voltage is varied. ¢ Voltage range: The minimum and maximum voltage that can be applied to the device.

·         Bias current: The bias is always reverse. This means that the Varactor diode does not conduct electricity. If the bias is turned positive, then the device will start conducting.

Other criteria to be considered include:

·         Reverse and breakdown voltage,

·         Leakage current,

·         Junction Temperature.

·         Voltage and other transients must be avoided.

·         Transients can occur if the DC voltage is put off.

1.7.          Application of Varactor Diode:

1.7.1.      Voltage controlled oscillators, VCOs:

 Voltage controlled oscillators are used for a variety of applications. One major area is for the oscillator within a phase locked loop - this are used in almost all radio, cellular and wireless receivers. A Varactor diode is a key component within a VCO.

1.7.2.      RF filters:

Using Varactor diodes it is possible to tune filters. Tracking filters may be needed in receiver front end circuits where they enable the filters to track the incoming received signal frequency. Again this can be controlled using a control voltage. Typically, this might be provided under microprocessor control via a digital to analogue converter

Some other applications are:

2.         It is used in variable resonant tank LC circuit. Here C part is varied using Varactor diode.

3.         AFC (Automatic Frequency Control) where in Varactor diode is used to set LO signal.

4.         Varactor is used as frequency modulator.

5.         It is used as frequency multiplier in microwave receiver LO.

6.         It is used as RF phase shifter.