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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