Classification of Solids into Insulators, Semiconductor and Conductors
A solid can conduct electric current if the electrons can move in it. This is possible only when an energy band is either partially filled or electrons can be easily excited to a vacant band. If a band is completely filled with electrons, the electrons are not free to move. If a band is completely vacant, there are no electrons to move and conduct electric current. Based on energy band theory the solids can be classified as under-
Insulators
Insulators are solids, which do not conduct electric current. In insulators valence band is completely filled and conduction band is completely vacant. They are separated by a very wide energy gap of the order of 6 e V, as shown in figure. Since the valence band is completely filled, the electrons cannot move, so they cannot conduct electricity. The conduction band is completely vacant having no electrons to move. To excite electrons from valence band valence band to conduction band we require an electric field of the order of 6*108 volt/m. Hence an insulator does not conduct electric current under normal condition.
Semiconductors
These solids have conductivity less than that of conductors but more than that of insulators. In semiconductors the valence band and conduction band are separated by a comparatively narrow forbidden energy gap of the order of <3 e V . Silicon has a crystal structure similar to that of diamond which is a typical insulator. At 0 K the valence band is band is completely filled and conduction band is completely vacant as in diamond. Hence in acts as an insulator. At room temperature some electrons are excited to the conduction band due to their thermal energy. Now the conduction band has some electrons and at the same time the valence band is no9 more completely filled. When an external potential difference is applied these electrons move constituting electric current. The vacancies formed in the valence band are considered as positive charge carriers moving in opposite direction to that electron in the conduction band.
Conductors
These are solids which readily conduct electricity. There are two types of energy bands in conductors depending on the electronic configuration of atoms. In alkali metals and other metals having configuration ns1 or ns2 np1 etc. having unpaired electrons in the outermost orbit of their atoms the valence band is partially filled. As the valence band is partially filled the electrons are easily excited to the higher levels in the same band. As a very large number of vacant levels exist, a large current can flow in conductors. In conductors having paired electrons in their outermost orbit the valence band is completely filled. So they should not conduct electric current. But it is observed that they also conduct electric current. This is because conduction band overlaps with the valence band forming a composite band which is also partially filled. In these conductors the forbidden energy gap E g = 0.