Electronic States in Solids-The Fermi Distribution Function

1 Electronic States in Solids—The Fermi Distribution Function 

In a free atom the electrons are allowed to exist only in certain discrete energy states (Fig. 7.1a). In solids, the energy states widen into energy bands. Electrons can exist only in bands 1, 3, and 5, but not in bands 2 and 4 (Fig. 7.1b). An electron can move from band 1 to band 3 if 

The electron acquires the energy necessary to cross the forbidden gap 

There is an empty state in band 3, which the jumping electron can occupy  

The energy distribution of electronic states is described in terms of the following quantities  

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