Question: Explain the formation of depletion region in a p-n junction.

Step 1 (Governing Principle & Mechanism): [0.5 Mark]

When a p-n junction is formed, a high concentration gradient exists for charge carriers across the junction. Due to this, diffusion of holes from the p-side to the n-side and electrons from the n-side to the p-side occurs. This movement creates a region near the junction interface where mobile charge carriers are depleted.

Step 2 (Step-by-Step Physical Process): [0.25 Mark]

• As electrons diffuse from the n-side to the p-side, they leave behind ionized donor atoms ($N_D^+$) on the n-side.
• As holes diffuse from the p-side to the n-side, they leave behind ionized acceptor atoms ($N_A^-$) on the p-side.
• These immobile ions create an electric field ($\vec{E}$) directed from the n-side to the p-side, which opposes further diffusion.

Step 3 (Final Concluded Answer): [0.25 Mark]

The region on either side of the junction interface that becomes devoid of free mobile charge carriers (holes and electrons) due to the recombination and the resulting space-charge layer is called the depletion region. The width of this region is typically of the order of $10^{-6} \, \text{m}$.

💡 EL10 Concept Insight: Think of the p-n junction like two crowded rooms with a door in the middle; people (electrons and holes) rush to the other side until they get tired and settle down, leaving a "no-man's land" near the door where no one is moving anymore—that empty space is the depletion region.