• In  a  PN  junction  a  very  thin  depletion  region  (acts  as  an  insulator)  of  theorder 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  itthus, 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  thepositive  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.

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