Silicon dioxide energy bands

The energy which is required to send an electron from the valence band to the conduction band is indicated by Eg and is relatively large in an insulator. Some examples of insulators are porcelain (which contains approximately 50% clay, 25% silicon dioxide and 25% feldspar), forsterite (Mg2Si04) and aluminium oxide. 

Eckelt, P. (1967). Energy band structures of cubic ZnS, ZnSe, ZnTe and CdTe (Korringa-Kohn-Rostoker method). Phys. Status Solidi 23, 307-12. Edwards, A. H., and W. B. Fowler (1985). Semiempirical molecular orbital techniques applied to silicon dioxide MIND03. J. Phys. Chem. Solids 46, 841-57. 

Crystals of quartz (silicon dioxide) and sapphire (aluminum oxide), to name two very good insulators, have large band gaps (both around 8 electron volts, or "ev"), so they do not conduct electricity at room temperature, for the same reason that pure silicon does not. However, if they are heated to about 750°C (red hot), a few electrons from the valence band will randomly have the energy required to put them up in the conduction band, and the materials will become slightly conductive. In the figure, this is represented by the "intrinsic" arrow. Silicon, with an energy gap of about 1 ev, only needs to be heated to about 200°C in order to conduct via the intrinsic mode. 

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