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Consider an ideal crystal with three energy bands where the gap between the lowest band and the middle band is Eap, and the gap between the upper band and the middle band isEgap For such an energy band structure, draw three separate figures illustrating (a) a semiconductor, (b) a metal, and (c) an insulator; be sure to show which energy states are populated with electrons in each of the three cases. 8ap2. FOr

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The large energy gap between the valence Energy of electrons and conduction bandsgap is small In semiconductors, the band energy can bridge the gap for a small fraction of the electrons. In conductors, there is no band gap since the valence band overlaps the conduction band. in an insulator says that at ordinary Conduction Bandtemperatures, no electrons can reach the conduction band. Fermi Conduction Band level Fermi level in gap Conduction Band Overlap a. Insulator c. Conductor Explanation Energy of electrons Conduction Band Large energy gap between valence and conduction bands a. Insulator Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band) Glass is an insulating material which may be transparentto visible light for reasons closely correlated with its nature as an electrical insulator. The visible light photons do not have enough quantum energy to bridge the band gap and get the electrons up to an available energy level in the conduction band. The visible properties of glass can also give some insight into the effects of doping on the properties of solids. A very small percentage of impurity atoms in the glass can give i color by providing specific available energy levels which absorb certain colors of visible light. The ruby mineral (corundum) is aluminum oxide with a small amount (about 0.05%) of chromium which gives it its characteristic pink or red color by absorbing green and blue light While the doping of insulators can dramatically change their optical properties, it is not enough to overcome the large band gap to make them good conductors of electricity. However the doping of semiconductors has a much more dramatic effect on ther electrical conductivity and is the basis for solid state electronics

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