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| Conductors, Insulators and Semiconductors |
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| The electrons in the valence band are not capable of gaining energy from external electric field and hence do not contribute to the current. This band is never empty but may be partially or completely with electrons. On the contrary in the conduction band, electrons are rarely present. But it is possible for electrons to gain energy from external field and so the electrons in these bands contribute to the electric current. The forbidden energy gap is devoid of any electrons and this much energy is required by electrons to jump from valence band to the conduction band. |
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| In other words, in the case of conductors and semiconductors, as the temperature increases, the valence electrons in the valence energy move from the valence band to conductance band. As the electron (negatively charged) jumps from valence band to conductance band, in the valence band there is a left out deficiency of electron that is called Hole (positively charged). |
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| Depending on the value of Egap, i.e., energy gap solids can be classified as metals (conductors), insulators and semi conductors. |
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| In metals, either the conduction band is partially filled or overlaps with valence band. There is no forbidden energy gap in between. Even if a small electric field is applied, free electrons start moving in a direction opposite to field and hence a good conductor of electricity. |
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| Here the valency bands are completely filled and conduction band is empty and the forbidden gap is quite large. For example in diamond Egap is 6eV. Even if an electric field is applied, no electron is able to go from valence band to conduction band. |
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| The valence band is completely filled and conduction band is empty. The Egap is also less i.e., of the order of few eV. At zero kelvin, electrons are not able to cross this forbidden gap and so behave like insulators. But as temperature is increased, electrons in valence band (VB) gain thermal energy and jump to conduction band (CB) and acquire small conductivity at room temperature and so behave like conductors. Hence they are called semiconductors. |
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| At high temperature, electrons move from valance band to conduction band and as a result a vacancy is created in the valence band at a place where an electron was present before shifting to conduction band. The valency is a hole and is seat of positive charge having the same value of electron. Therefore the electrical conduction in semiconductors is due to motion of electrons in conduction band and also due to motion of holes in valence band. |
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