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(a) Why is an intrinsic semiconductor deliberately converted into an extrinsic semiconductor by adding impurity atoms ? (b) Explain briefly the two processes that occur in p-n junction region to create a potential barrier. |
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Answer» (b) During formation of a p-n junction .diffusion. and .drift. are two important processes taking place in it. Whenever a p-n junction is formed, holes diffuse from p-SIDE to n-side and electrons diffuse from n-side to p-side. Diffusion of an electron from n to p-side leaves BEHIND an immobile ionised donor (positive charge). Similarly, diffusion of a hole from p-to n-side leaves behind an immobile acceptor (negative charge). Thus, due to diffusion of electrons and holes a layer of positive charge (or positive space charge region) is developed on n-side of junction and a layer of negative charge (or negative space charge region) is developed on p-side of junction. Therefore, a. depletion layer. is formed on either side of junction consisting of immobile ion cores devoid of their electrons or holes. Due to positive space charge on n-side and negative space charge on p-side of the junction an electric field is developed at the junction which is directed from n-side to p-side. Due to this electric field electrons drift from p-side to n-side and holes drift from n-side to p-side thus giving rise to a drift current. The direction of drift current is opposite to that of diffusion current and in equilibrium state the two currents just balance each other. Hence, no net current flows in p-n junction. The loss of electrons from n-region and gain of electrons by p-region causes a potential difference across the junction. This potential opposes further FLOW of charge CARRIERS across the junction and is CALLED the potential barrier. |
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