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  4. In a PN junction :-

Q. In a PN junction :-

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Solution:

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A p-n junction is shown in the figure. On account of difference in concentration of charge carriers in the two sections of p-n junction, the electrons from H-region diffuse through the junction into p-region and the holes from p-region diffuse into n region.
Since the hole is a vacancy of an electron, when an electron from n region diffuses into the p-region, the electron falls into the vacancy, i.e. it completes the covalent bond.
Due to migration of charge carriers across the junction, the n-region of the junction will have its electrons neutralized by holes from the p-region, leaving only ionised donor atoms (positive charges) which are bound and cannot move. Similarly, the p region of the junction will have ionised acceptor atoms (negative charges) which are immobile.
The accumulation of electric charges of opposite polarities in the two regions of the junction gives rise to an electric field between these regions as if a fictitious battery is connected across the junction with its positive terminal connected to n region and negative terminal connected to p region. Therefore, in a p-n junction high potential is at N side and low potential is at P side.