- In a PN junction a very thin depletion region (acts as an insulator) of theorder of 5 x 10 exists on either side of the ‘P’ and ‘N’ regions and it separate the ‘P’ and ‘N’ regions are acts as conducting plates, as a result this capacitor is formed.
- The capacitance may be varied by varying the reverse voltage applied to itthus, the varactor diodes also known as VVC (Voltage Variable Capacitance) diode. In other words the varactor diode is a semiconductor, voltage dependant, variable capacitors diode. Their mode of operation depends on the capacitance that exists at the PN junction when it is reverse biased.
- The depletion region in a PN junction forms a barrier which separates thepositive and negative charges on the opposite plates of a diode and the depletion region acts like a dielectric. Thus PN junction possesses junction capacitance. The transition capacitance CT established by the junction is determined by
where, ‘ ’ = permittivity of semiconductor materials Wd= the depletion width
As the reverse potential increases the width of the depletion region increases, which in turn reduces the transition capacitance. In terms of the applied reverse bias, the transition capacitance is given by
k = constant
VT= knee potential
VR= magnitude of the applied reverse potential
n = 1/2 for alloy junctions, & 1/3 for diffused junction
When the reverse bias voltage decrease, the depletion layer narrows down. This decreases the dielectric thickness, which increases the capacitance; figure 17 shows the variation of capacitance with the reverse voltage. This indicates that the variation of capacitance is maximum when the reverse voltage is equal to zero. It reduces in a non linear manner, as the value of reverse voltage is increased.
In this diode the variation of capacitance are controlled by the method of doping in the depletion layer or the size and geometry of diode construction. There are two types of doping used in varactor diode
(1). Abrupt doping
(2). Hyper doping.
Advantages of VVC over other variable capacitors
1.Smaller in size
2.Rugged and reliable
3.No movable part
4.Operate over wide range of frequencies
5.It has very fast response
Electrical tunning Applications
1.Because of large range of capacitance variation these diodes are used in Some of the high frequency areas like frequency modulation TV tunning and FM radio
2.In automatic-frequency control circuits
3.In adjustable band pass filters
4.In parametric amplifiers.