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In a solid, atomic levels broaden into bands: a filled valence band and an empty conduction band separated by the energy gap Eg.
| Material | Energy gap | Behaviour |
|---|---|---|
| Conductor (Cu, Al) | Bands overlap (Eg ≈ 0) | Free electrons abound |
| Semiconductor (Si, Ge) | ~1 eV (Si 1.1 eV, Ge 0.7 eV) | Conduction rises with temperature |
| Insulator (diamond) | > 3 eV (≈ 6 eV) | Practically no conduction |
Key contrast with metals: heating a semiconductor increases conductivity (more electron–hole pairs), so its temperature coefficient of resistance is negative.
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