Mumbai University > EXTC > Sem 3 > Analog Electronics 1

Marks: 7 M

Year: May 2014

• Varactor or varicap(variable capacitance) is diode operated in reverse bias region.
• It act as variable capacitor whose capacitance change with respect to voltage applied across its terminal.
• We can generalize this variable capacitor as parallel capacitor.

1. Construction:

• The junction in the varactor diode is formed by joining two semiconductors one p-type material and other n-type material.
• At the base we have heatsink to moderate the temperature of the diode and for cooling purpose.
• Glass passivation is provided to create a shell that protects from corrosion.
• Metal contact is provided for the connection with battery.

Fig1 Internal structure of Varactor diode

2. Working:

• Varactor diode is used as voltage controlled capacitor
• All the diode exhibit capacitance up to some extent in reverse biased region but varactor diode is specially designed to exhibit this phenomenon.
• A reverse biased semiconductor has two adjacent microscopic space charge or depletion region which functions like plates of capacitor.
• As we increase reverse bias voltage the width of depletion region increases. The relationship between voltage and depletion region is sublinear.
• Depletion region width depends on doping profile of p-n junction. If the doping profile is uniform then the depletion region width is directly proportional to square root of applied voltage. Capacitance is inversely proportional to the width between two plates of capacitor here depletion layer thickness thus capacitance is inversely proportional to square root of applied voltage.

Fig2 Formation of depletion layer and characteristics of diode