NEET 2020 Physics Question 153 Solution

During the formation of a p-n junction, holes diffuse from the p-side to the n-side and electrons from the n-side to the p-side. This uncovers immobile positively charged donor ions on the n-side and immobile negatively charged acceptor ions on the p-side, establishing a space-charge depletion layer devoid of free charge carriers and setting up an internal barrier electric field directed from n to p.

In reverse bias, the p-type region is connected to the negative terminal of the external battery and the n-type region to the positive terminal. The applied reverse voltage produces an external electric field in the SAME direction as the internal barrier field. Consequently, holes in the p-side are pulled away toward the negative terminal, and electrons in the n-side are attracted toward the positive terminal.

Because majority carriers are pulled away from the junction interface, an additional layer of ionized dopant impurity atoms is exposed on both sides of the junction. Therefore, the width of the depletion region increases, and the effective barrier height increases from $$V_0$$ to $$(V_0 + V)$$.

Under forward bias, the external field opposes the barrier field, pushing majority carriers into the depletion region and dramatically reducing its width.