When a voltage is applied to a metal crystal, electrons are excited to the unoccupied orbitals in the same band and move towards the positive terminal. Thus, a material with a partly filled energy band is a conductor. A conductor conducts electricity as well as heat through movement of electrons. All metals are conductors because they have partly filled energy bands. The electrical conductivity of group II metals, which have fully filled valence s-subshells, can be explained on the basis of overlapping of s and p bands as shown in figure 1.22. The overlapping energy bands are only partly filled, so that there are many closely spaced energy levels available within the band.
fig 1.22 - Overlapping of s and p bands in group II elements
Metals are extreme examples of delocalized electrons. In a metallic crystal the electrons are free to move throughout the crystal. However, the metal ions are fixed in position and can only vibrate. When a metal is heated the metal ions start vibrating more vigorously and as a result motion of the valence electrons through the crystal is hindered. Thus, electrical conductivity of metals decreases with increase in temperature.In case of insulators the highest occupied band is completely filled. The forbidden band just above the highest filled band is wide. Therefore, it is not easy to excite the electron to next higher unoccupied energy level. Most of the non-metals are insulators.
In case of semiconductors the forbidden band is narrow. The thermal energy available at room temperature is enough to excite some electrons from the highest occupied band to the next permitted energy band. With increase in temperature, the number of excited electrons increases and hence conductivity of semiconductors increases with increase in temperature.