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Question
(i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.
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Solution
(i) In a p−n junction, a p-type and an n-type material are joined together. The concentration of holes is higher in p-type material as compared to that in n-type material. Therefore, there is a concentration gradient between the p-type and n-type materials. As a result of this concentration gradient, holes move from p-side to n-side (p → n) by the process of diffusion. Similarly, electrons move from n-side to p-side (n → p).
As the holes diffuse from p-side, they leave ionised spaces (negatively charged) on p-side near the junction. These ionised spaces are immobile. Hence, a negative space-charge region is formed on the p-side near the junction. Similarly, a positive space-charge region is formed on the n-side. These two space-charge regions on either sides of the junction constitute what is called a depletion layer.
Since the n-side loses electrons and p-side gains electrons, a potential difference is developed across the junction of the two regions. This potential difference tends to oppose further motion of electrons from the n-region into the p-region. The same happens for holes too. The reverse polarity of this potential opposes further flow of carriers and is thus called the barrier potential
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