Varactor Diode

A junction diode which acts as a variable capacitor under changing reverse bias is known as avaractor diode.When a pn junction is formed, depletion layer is created in the junction area. Since there are nocharge carriers within the depletion zone, the zone acts as an insulator. varactor diode application The p-type material with holes(considered positive) as majority carriers and n-type material with electrons (−ve charge) as majoritycarriers act as charged plates. Thus the diode may be consider as a capacitor with n-region and p-region forming oppositely charged varactor diode application plates and with depletion zone between them acting as a dielec-tric. This is illustrate in Fig.1 (i).

A varactor diode is specially construct to have high capaci-tance under reverse bias. Fig.1 (ii) shows the symbol of varactor diode. The values of capaci-tance of varactor diodes are in the picofarad (10−12 F) range.

Theory

For normal operation, a varactor diode is always*reverse bias.

The capacitance ofvaractor diode is find as :

where,

C_T= Total capacitance of the junction;

ε= Permittivity of the semiconductormaterial;

A= Cross-sectional area of the junction;

W_d= Width of the depletion layer.

When reverse voltage across a varactor diode isincreased, the width W_d of the depletion layer increases. Therefore, the total junction capacitance C_T of thejunction decreases. On the other hand, if the reversevoltage across the diode is lowered, the width Wd of thedepletion layer decreases. Consequently, the total junctioncapacitance C_T increases.*A forward biased varactor diode would serve no useful purpose.

Fig.2 shows the curve between reverse bias voltage V_R across varactor diode and total junction capacitance C_T.Note that C_T can be change simply by changing the voltage V_R. For this reason, a varactor diode is sometimes calledvoltage-control capacitor.

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