Depletion Capacitance - an introduction

Introduction: Mainly, there are two types of capacitance related with a junction:
  1. The capacitance of junction because of the dipole in the transition region.
  2. The charge storage capacitance rising from the lagging behind of voltage as the current changes with the effects of charge storage.
Depletion Capacitance:

The junction capacitance is overriding under the conditions of reverse-bias and the charge storage capacitance is overriding if the junction is forward biased. In several applications of p-n junctions, for usefulness of the device the capacitance is a limiting factor. In place of the common expression C=Q/V, which is implemented on capacitors where charge is linear function of voltage, we should use the definition which is more general

As the charge Q varies on each side of the transition region in non linear manner,

we should use the changed value of the electrostatic potential barrier (V0 – V). Then, the right expression for transition region's width is

We can write value of Q in terms of width of transition region and the concentration of doping on each side of the junction.

Connecting the total width of the transition region W to the individual widths xp0 and xn0

and thus the charge on dipole's each side is

After that the junction capacitance Cj can be written as 

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