DIFFUSION CAPACITANCE



During  forward  biased  condition,  another  capacitance  comes  into  existence called diffusion capacitance or storage capacitance, denoted as CD.

In forward biased condition, the width of the depletion region decreases and holes from p side get diffused in n side while electrons from n side move into the p-side. As the applied voltage increases, concentration of injected charged particles increases.

This rate of change of the injected charge with applied voltage is defined as a capacitance called diffusion capacitance.

The diffusion capacitance can be determined by the expression





where r = mean life time for holes.

So  diffusion  capacitance  is  proportional  to  the  current.  For  forward  biased condition, the value of diffusion capacitance is of the order of nano farads to  micro farads while transition capacitance is of the order of pico farads. So CD
is much larger than CT.
Diffusion capacitance versus applied forward biased voltage

However in forward biased condition, CD appears in parallel with the forward resistance  which  is  very  small  hence  the  time  constant  which  is  function  of product  of  the  forward  resistance  and  CD  is  also  very  small  for  ordinary signals.



Hence for normal signals CD has no practical significance but for fast signals CD must be considered.The graph of CD against the applied forward voltage is shown in the Fig

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