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p-n junction: depletion region and barrier potential sheet
3-mark answer frame

With a labelled sketch, explain what happens to the electrons and holes when a p-n junction is made, and how this sets up the barrier potential.

How to lay out the answer. Follow the frame, then write it in your own words.

  1. 1 · Start with the two sides. The p-side has many holes and the n-side many free electrons.
  2. 2 · Explain diffusion. Electrons diffuse from n to p and holes from p to n because of the concentration difference, and they recombine.
  3. 3 · Explain the depletion region. Fixed − acceptor ions stay on the p-side and fixed + donor ions on the n-side. This layer, about 0.5 μm thick, has no mobile carriers.
  4. 4 · Explain the barrier. The ions' field from n to p gives a barrier potential (Si 0.7 V, Ge 0.3 V) that stops further diffusion.
  5. 5 · Draw the diagram. [Diagram: p and n regions; depletion region with circled − ions on the p-side and circled + ions on the n-side; field E from n to p; barrier potential marked]
  6. 6 · Close in one line. So, diffusion leaves fixed ions, and their field blocks further diffusion.

Scoring tip: Put the − ions on the p-side and the + ions on the n-side. Swapping them is the commonest mistake.