When a diode is reverse biased, the width of the depletion region increases.
So  there  are  more  positive  and  negative  charges  present  in  the  depletion region.

Due  to  this, 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, junction capacitance, space charge capacitance, barrier capacitance or depletion region capacitance. It is denoted as CT.

Mathematically it is given by the expression,

= permittivity of semiconductor =   o
o= 1/(36  ×10  9)8.849x10-12 F/m
Cr = relative permittivity of semiconductor = 16 for Ge, 12 for Si
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 CT decreases.

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

As reverse voltage is negative, graph is shown in the second quadrant. For a particular diode shown, CT varies from 80 pF to less than 5 pF as VR changes from 2V to 15 V.

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