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A capacitor is a device that stores charge. Therefore any device that stores charges( static or dynamic) can be said to have capacitance.

When a PN diode is forward biased, a current flows due to the majority charge carriers. At a particular instant there will be charges in motion. This is dynamic charge. The capacitance due to storage of dynamic charge is called the diffusion capacitance.

We know that C = Q * V. That is capacitance is directly proportional to charge stored. Since the diode current increases exponentially with the voltage applied across it, the dynamic charge also increases exponentially . Hence the diffusion capacitance increases exponentially with the increasing diode voltage.

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Q: What is diffusion capacitance in pn junction?
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Diffusion potential in p-n junction?

The built in potential in a pn junction. Due to the difference in carrier concentration between the sides of a pn junction. Diffusion potential increases with increase in doping levels.


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Transition capacitance : A reverse biased PN-junction has a region of high resistivity (depletion layer) sandwiched in between two regions of relatively low resistivity. The P-N regions act as the plates of a capacitor and the depletion layer acts as the dielectric This is known as the transition capacitance or depletion capacitance. Diffusion capacitance : It is the capacitance due to transport of charge carriers between two terminals of a device like the forward biased PN junction. In a semiconductor device with a current flowing through it (for example, an ongoing transport of charge by diffusion) at a particular moment there is necessarily some charge in the process of transit through the device. If the applied voltage changes to a different value and the current changes to a different value, a different amount of charge will be in transit in the new circumstances. The change in the amount of transiting charge divided by the change in the voltage causing it is the diffusion capacitance. The adjective "diffusion" is used because the original use of this term was for junction http://www.answers.com/topic/diode, where the charge transport was via the diffusion mechanism.


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