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Here are a few band structure examples for different element types that can be found in nature. The shown structures are valid at a temperature of 0 K.
 Electronic state densities with respect to energy (shell filling at 0 K) : Full level
Metals
Very good conductors. When the temperature increases, the very mobile electrons are moving over the Fermi level and become available for electrical conduction. 
Semi-metals
In a similar way when the temperature increases the very mobile electrons are moving. 
Semiconductors
The temperature increase is sufficient to cause electrons to cross the energy gap. These electrons are then in the conduction band. 
Insulators
The temperature increase is not sufficient to cause electrons to cross the energy gap. They move to higher energy states but cannot reach the conduction band energy level.  |