Question

Briefly explain the two important processes that occur during the formation of a p-n junction.

Hint:

Remember the sequence of events during the formation of a p-n junction: \[ \text{Diffusion} \rightarrow \text{Formation of depletion layer} \rightarrow \text{Electric field} \rightarrow \text{Drift} \rightarrow \text{Equilibrium} \] At equilibrium, \[ \boxed{ I_{\text{diffusion}} = I_{\text{drift}} } \] and the net current through the junction is zero.

Answer 1

Concept: A p-n junction is formed when a p-type semiconductor and an n-type semiconductor are joined together. Immediately after the formation of the junction, the charge carriers on the two sides are not uniformly distributed.
• The p-region contains a large concentration of holes and a very small concentration of electrons.
• The n-region contains a large concentration of electrons and a very small concentration of holes. Because of this concentration difference, charge carriers start moving across the junction. During the formation of the p-n junction, two important processes take place:
• Diffusion
• Drift

Step 1: Diffusion Process
Diffusion is the movement of charge carriers from the region of higher concentration to the region of lower concentration. Immediately after the p-type and n-type semiconductors are joined:
• Electrons diffuse from the n-side to the p-side because the concentration of electrons is much higher in the n-region.
• Holes diffuse from the p-side to the n-side because the concentration of holes is much higher in the p-region. This movement of majority charge carriers across the junction constitutes the diffusion current. As electrons leave the n-region, positively charged donor ions are left behind. Similarly, as holes leave the p-region, negatively charged acceptor ions are left behind. As a result, a region around the junction becomes depleted of mobile charge carriers. This region is called the depletion region or depletion layer. \[ \boxed{ \text{Diffusion}= \text{Movement of majority carriers from high concentration to low concentration.} } \]

Step 2: Drift Process
The immobile ions left behind near the junction create an electric field directed from the n-side towards the p-side. This electric field opposes further diffusion of majority carriers and exerts a force on the charge carriers. Due to this electric field:
• Electrons in the p-region are driven towards the n-region.
• Holes in the n-region are driven towards the p-region. This motion of minority carriers under the influence of the electric field gives rise to the drift current. The process of drift continues until the drift current becomes equal in magnitude and opposite in direction to the diffusion current. At this stage, the p-n junction attains equilibrium. \[ \boxed{ \text{Drift}= \text{Motion of minority carriers due to the electric field of the depletion region.} } \] Conclusion: Thus, the two important processes that occur during the formation of a p-n junction are:
• Diffusion – movement of majority charge carriers from a region of higher concentration to a region of lower concentration.
• Drift – movement of minority charge carriers under the influence of the electric field developed across the depletion region. The equilibrium condition is reached when \[ \boxed{ I_{\text{diffusion}} = I_{\text{drift}} } \] and therefore the net current across the junction becomes zero.