What is a p-n junction diode? Explain the formation of the depletion region and potential barrier in a p-n junction diode. Also, explain its V-I characteristics under forward bias and reverse bias conditions with the help of a characteristic curve.

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Definition of p-n Junction Diode
A p-n junction diode is a two-terminal semiconductor device formed by joining a p-type semiconductor and an n-type semiconductor together. The contact boundary between these two regions is known as the p-n junction. It allows electric current to pass easily in one direction while restricting it in the opposite direction, making it fundamental for rectification and switching. Formation of Depletion Region and Potential Barrier Diffusion of Charge Carriers: Immediately after the formation of the p-n junction, holes from the p-side diffuse into the n-side due to the concentration gradient, and electrons from the n-side diffuse into the p-side. Recombination: Near the junction, electrons and holes recombine, neutralizing each other. Creation of Immobile Ions: As electrons leave the n-side, they leave behind positively charged donor ions. Similarly, as holes leave the p-side (or accept electrons), negatively charged acceptor ions are left behind on the p-side. Depletion Region: This space-charge region on either side of the junction becomes depleted of free mobile charge carriers and is called the depletion region. Potential Barrier: The accumulation of positive ions on the n-side and negative ions on the p-side sets up an internal electric field directed from the n-side to the p-side. This electric field creates a potential barrier across the junction that prevents further diffusion of majority charge carriers. The height of this barrier depends on the semiconductor material (approx. 0.7 V for Silicon, 0.3 V for Germanium). V-I Characteristics Forward Bias: When the positive terminal of an external battery is connected to the p-type and the negative terminal to the n-type, the diode is forward-biased. The external field opposes the potential barrier, narrowing the depletion region. Current starts flowing at a small threshold voltage (cut-in voltage) and rises sharply with an increase in forward voltage. Reverse Bias: When the positive terminal is connected to the n-type and the negative terminal to the p-type, the diode is reverse-biased. The external field aids the potential barrier, widening the depletion region. Only a very small reverse saturation current (due to minority carriers) flows, which remains almost constant until the breakdown voltage is reached, where the diode current increases dramatically.