Varactor Diode

Definition: Varactor diode is the one which works on the principle of variation in capacitance by changing the width of the depletion region of P-N junction. The P-N Junction diode creates capacitor effect. The capacitance is controlled by applied voltage. It works on reverse biased mode.

Varacter word is formed from words Variable reactance or variable resistor. Thus, it provides variable resistance or reactance or capacitance thus it is named as a varactor diode. The symbol of the varactor diode is same as conventional diode except the symbol of the capacitor is merged with the symbol of the diode to show the capacitance effect.

It is also called voltage-variable capacitor (VVC)  or varicap diode.

Construction of Varactor Diode

It is formed of P-type and N-type semiconductor and reverse biasing is applied to it. The majority carriers in an N-type semiconductor are electrons and the majority carriers in a P-type semiconductor are holes. At the junction, the electrons and holes recombine. Due to which immobile ions accumulate at the junction. And no more current can flow due to majority carriers.

Thus, the depletion region is formed. The depletion region is called so because it is depleted of charge carriers i.e. the majority carriers are absent in depletion region. This works as a dielectric layer and P and N-type semiconductor works as plates of a capacitor.

Working of Varactor Diode

When the reverse bias is applied to P-N junction, the width of depletion layer increases. And with the increase of reverse voltage gradually the depletion layer increases even more. Thus, the depletion region creates Transition capacitance C_T.
                                                C_T =  ɛA/W

Here, C_T  is Transition capacitance, ɛ is dielectric constant, A is the area of plates of the capacitor and W is the width of the depletion layer.

It is evident from the above relation that transition capacitance is inversely proportional to width of the depletion layer. Thus, if we want the high magnitude of capacitance the width should be small. And the width will be small if we will apply low reverse voltage.

Similarly, if we require low capacitance the width should be large and to increase the width the reverse voltage applied should be high.Thus, this width can be controlled with applied reverse voltage.

Volt-ampere Characteristics of Varactor Diode

It can be seen in the characteristic curve that as reverse voltage increase from 0 V the transition capacitance is decreasing exponentially.

The relation between the transition capacitance, V_R and V_{k .}

                                                             C_T  = K/ (V_k + V_R)ⁿ

Here, C_T is junction capacitance, Vk is the knee voltage and V_R is the reverse voltage and the value of n for alloyed junctions is 1/2 and diffused junctions it is 1/3. Thus, the voltage is inversely proportional to capacitance.

The tuning range of the capacitor varies with the doping level of the diode.  For abrupt doping junction the doping will be uniform, but for the hyper abrupt junction, the doping profile will be non-uniform.

Advantages of Varactor Diode

1. Low Noise: It generates less noise as compared to the other P-N junction diode. Thus, the power loss due to noise is low in varactor diodes.
2. Portability: It is portable due to the small size and lightweight.
3. Reliability: It is more reliable than other P-N junction diodes.
4. Economical: It is a low-cost diode thus, it is economical to use in various applications.

Disadvantages of Varactor Diode

These are specially designed to work in the reverse biased mode, it possesses the least significance when operated in forward biasing.

Applications of Varactor Diode

1. Television receivers: Varactor diodes are used as tuned capacitors and have replaced mechanically tuned capacitors in various applications. It is used in television in the resonant tank circuit.
2. Radio receivers: Radio receivers also use this diode for tuning purposes.
3. Frequency Multiplier: It is also used as a frequency multiplier in various electronic circuits.
4. Phase Locked Loops: It is used in Phase locked loop for frequency modulation. Varactor diodes help in achieving frequency modulation. Thus, in communication devices varactor diodes are significant.
5. Voltage controlled oscillators: Voltage control oscillators are used extensively in transmission and receiving circuits in communication. And varactor diode plays a significant role in construction of voltage controlled oscillator.
6. Parametric Amplifiers: It is used in parametric amplifier as a significant component.

Varactor diode produces capacitance effect, this is because it cathode and anode terminals acts as the plate of the capacitor and region between them acts as dielectric medium.