Lecture12

Just under carbon 4e 9 si crystal silicon crystals

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Unformatted text preview: have a large band gap around the Fermi Level.  Thermal excitaLons to the conducLon band have an exponenLally reduced probability. 8 Intrinsic Semiconductors   Intrinsic semiconductors have a small band gap so that some electrons from below the Fermi Level can be excited thermally into the conducLon band and some holes will be le` behind in the valence band.   The most common crystals used as semiconductors are Silicon and Germanium. (Just under Carbon, 4e) 9 Si Crystal   Silicon crystals have strong covalent bonds.  Same tetrahedral structure as diamond.  Large boules are sliced to make wafers. diamond cubic, 0.543 nm 2 m X 0.3 m 10 N ­Type Semiconductors   N ­Type semiconductors have smallish impuriLes put into the crystal. These impuriLes have extra electrons (5 valence) compared to Si.   These give an extra electron which is in the conducLon band.   So, by doping the crystal with the impurity, we can choose the density of charge carriers.   With extra electrons, the charge carriers are negaLve and don’t leave behind posiLve holes. 11 P ­Type Semiconductors   By doping the semiconductor crystal with an atom with just 3 electrons (Boron), we have too few electrons to fill the valence band.   So there are states in the valence band which are not...
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