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| Defects in Crystals |
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| According to third law of thermodynamic it is only at OK that true crystals passes perfect order of arrangement of atoms in pure crystals. But with rise in temperature some derivations from complete order takes place. Presence of impurities also adds to disorder. |
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| "Any departure from perfectly ordered arrangement of atoms/ions in crystal is called imperfection or crystal defect". |
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| These imperfections modify the properties of the crystal and also sometimes impart new properties to the solids. |
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| There are two types of imperfections: |
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 Electronic imperfections |
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Atomic or point imperfection |
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| At zero K (absolute zero) electrons in covalent or ionic crystals (e.g., Si or NaCl) occupy the lowest energy level. |
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| Above zero K in pure silicon or germanium electrons are released thermally from the covalent bonds. This leads to the presence of free electrons and electron deficient bonds called holes, which are responsible for intrinsic conduction. |
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| These free electrons and the holes in solid give rise to electronic imperfections. |
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