Explain, with a labelled diagram, the formation of the depletion region and the barrier potential in a p-n junction.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Start with the two sides. The p-side has many holes; the n-side has many free electrons.
2 · Explain diffusion. Holes diffuse p to n and electrons n to p because of the concentration difference: diffusion current.
3 · Explain the depletion region. Fixed − acceptor ions remain on the p-side and fixed + donor ions on the n-side; this layer has no free carriers.
4 · Explain drift and equilibrium. The ions set up a field from n to p, which drives a drift current of minority carriers. When drift equals diffusion, the barrier potential V₀ is set up.
5 · Draw the diagram. [Diagram: p and n regions, depletion region with − and + ions, field E from n to p; below it, potential rising by V₀ across the junction]
6 · Real life. In a solar cell, this field separates the charges made by sunlight.
7 · Close in one line. So, diffusion builds the barrier, and drift balances it at equilibrium.
Scoring tip: Use the four words diffusion, majority, drift and minority in the right places. Show the field direction from n to p.
