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.