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Energy bands: conductors, semiconductors and insulators sheet
3-mark answer frame

On the basis of energy bands, distinguish between a conductor, a semiconductor and an insulator. Draw their energy band diagrams.

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

  1. 1 · Explain bands in one line. In a crystal, atomic levels spread into a filled valence band and a higher conduction band, separated by a gap Eg.
  2. 2 · Describe a conductor. The bands overlap or the conduction band is partly filled, so many electrons are free even at low temperature.
  3. 3 · Describe a semiconductor. Small gap, Eg < 3 eV (Si 1.1 eV): a few electrons cross at room temperature.
  4. 4 · Describe an insulator. Large gap, Eg > 3 eV (diamond 5.4 eV): almost no electrons cross.
  5. 5 · Draw the three diagrams. [Diagram: three panels with electron energy upward; overlapping bands; small gap Eg < 3 eV; large gap Eg > 3 eV; valence band, conduction band and Eg labelled]
  6. 6 · Close in one line. So, the size of the energy gap decides how well a solid conducts.

Scoring tip: Draw all three panels at the same scale so the difference in gap size is obvious, and label every band.