Page 118 - Materials Chemistry, Second Edition

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2.3. The Crystalline State

Figure 2.69. Top: The splitting of 3s and 3p atomic orbitals of Si x into bands, showing the presence of a
bandgap at the equilibrium bond distance, a o. Reproduced with permission from Neamen, D. A.
Semiconductor Physics and Devices, 3rd ed., McGraw-Hill: New York, 2003. Copyright 2003
McGraw-Hill. Bottom: The band diagram for Mg metal, showing the formation of valence/conduction
bands and zero bandgap through overlap of 3s/3p atomic orbitals on neighboring Mg atoms.

At extremely high temperatures, electrons may be removed entirely from the
crystalline lattice, referred to as thermionic emission. The minimum energy required
to remove an electron from the Fermi level to the vacuum level (a position outside of the
solid – Figure 2.69b) is referred to as its work function, [58] and varies depending on
the composition and crystallographic orientation of the solid. For instance, some