Varactor Diode

Varactor Diode

• In a normal diode, the depletion region exists between p‒region and n‒region as shown in the Fig. 3.3.1.

• The p‒region and n‒region act like the plates of capacitor while the depletion region acts like dielectric,

• Thus there exists a capacitance at the p‒n junction called transition capacitance, space charge capacitance, barrier capacitance or depletion region capacitance. It is denoted as C_T.

• Mathematically it is given by the expression,

 Ст = εΑ / W

where

ε = Permittivity of semiconductor,

A = Area of cross section

W = Width of depletion region

• As the reverse biased applied to the diode increases, the width of the depletion region (W) increases. Thus the transition capacitance C_T decreases.

• In short, the capacitance can be controlled by the applied voltage. The variation of C_T with respect to the applied reverse bias voltage is shown in Fig. 3.3.2.

• As reverse voltage is negative, graph is shown in the second quadrant.

• For a particular diode shown, C_T varies from 80 pF to less than 5 pF as V_R changes from 2 V to 15 V.

• In practice, special type of diodes are manufactured which shows the transition capacitance property more predominantly as compared to the normal diodes. Such diodes are called varactor diodes, varicap, (VVC (Voltage Variable Capacitance)), or tuning diodes.

• The doping profile of some varactor diodes is hyperabrupt. It means that the doping level increases towards the junction. This produces very narrow depletion region and large capacitive effect.

• The capacitance vary more dominantly with reverse voltage in such diodes. The doping profiles for abrupt and hyperabrupt junction are shown in Fig. 3.3.3.

1. Symbol and Equivalent Circuit

• Fig. 3.3.4 (a) shows the symbol of varactor diode while Fig. 3.3.4 (b) shows the first approximation for its equivalent circuit in the reverse bias region.

• The R_R is the reverse resistance which is very large while R_S is the geometric resistance of diode which is very small. The inductance L_S indicates that there is a high frequency limit associated with the use of varactor diodes.

2. Applications

• Fig. 3.3.5 shows how varactor diode can be connected in a LC tuned circuits.

• The resonance frequency for a parallel LC tuned circuit is given by,

 f_r = 1 / 2π√(LC)

• As varactor diode is connected in parallel, the resultant capacitance becomes C₁+C₂.

• Hence the resonance frequency becomes,

 f_r= 1 / 2π/√[L(C₁+C₂)]

where C₂ is transistion capacitance of varactor diode

• The value of C₂ can be changed by controlling the voltage applied to the circuit. Hence circuit can be tuned by changing voltage applied at a resonance frequency, this is called electrical tuning.

• The various other applications of varactor diodes are,

1. Tuned circuits

2. FM modulators

3. Automatic frequency control devices

4. Adjustable bandpass filters

5. Parametric amplifiers

6. Television receivers

Review Questions

1. What is varactor diode? How capacitance of a diode varies with reverse voltage?

2. Explain the principle behind the varactor diode and list out its application.

3. Write a note on varactor diode.