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Why does the width of the depletion region decrease,when the diode is forward biased?Is it because the ions have moved to p-junction,or n-junction? Please clarify.

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First of all note, that ions do not move in the semiconductor. Only electrons and holes move there.

The depletion results from diffusion of the free electrons from the n-region into the p-region and the diffusion of holes from the p-region into the n-region.

In either case it is a thermodynamical process. The electrons form something like a dense gas in the n-region and the p-region is like an electron-vacuum. If the density of electrons at the pn-junction would be higher in the n-region than in the p-region then Brown's motion would drive more electrons from the n-region to the p-region than from the p-region to the n-region. Therefore, in the thermodynamical equilibrium there must be some mean density at the junction. The electrostatic field force counteracts this.

If electrons are moving from the n-domain into the p-domain and holes from the p-domain into the n-domain the n-domain becomes positively charged and the p-domain negatively. These space-charges create an electrical field which drives electrons back into the n-region and the holes back into the p-region.

The forward bias-voltage supports the field-force driving the electrons and holes back. Thus the depletion zone decreases.

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