Energy Band model of Metal, Semiconductor and Insulators

Energy Band: 

  Modern electronics categorized Metal, Semiconductor and Insulator in terms of energy bands.  Energy bands are a band of different energy levels. According to quantum mechanics each particles (electron, proton etc..) of any elements possess a discrete energy level. So if the no of particles increases then the energy levels  are very close to each other. Thus for very very large no of particles the energy levels are found to be very close to each other, such that they form a continuous range of Energy called  Band of energy. 

 In any element there are two types of electrons namely Valence electron and Conduction electrons. Valence electrons are the electrons which  take part in the bonding of the materials and Conduction electrons are the electrons which take part in the conduction process. In a Metal there are abandon  electrons for conduction called free electron of Conduction electrons. While the formation of Metal all these conduction electrons forms Conduction Band and bonding of the metal is hold by Valence electron which forms Valence Band i.e. Band of Valence electron is called Valence Band. Thus There are two types of band exist in any material called

1. Conduction Band
2. Valence Band   

In between  this two Energy bands there is a gape called Energy gape or Forbidden Energy gape.

Depending upon this energy gape at temperature 0K the materials are categorized in terms of Metal Insulator and Semiconductor.

Energy Band of Metal: 

Metals like Cu, Fe Au, ;Si are very good conductor of electricity due to abandon amount of free electrons present in the Valence Band. Let us Discuss about the Band Formation of any Metal.

when atoms of metal are heated with very high temperature and mixed together and cooled down to form a metal rod or metal strip, the atoms are ionized and outer cell electron is loosen out from the atom and it moves freely around. Thus the metal atom becomes electropositive . When more amount of elecropositive atoms come closer then they forms bond.  During the bond formation the inner cell electron takes part in the  bonding and free electrons moves around the metal. Some of the free electron also take part in the bond formation. All these free electrons have different discrete Energy level  according to the Pauli's Exclusion Principal. All the energy levels of these free electrons are closely spaced to each other forming Band of Energy called Conduction Band. 
     

                                

          Similarly the valence Electrons forms Valence Band. Since some of the conduction electron also takes part in the Bonding  thus there is no Band gape in between Conduction Band and Valence Band. i.e. Conduction Band And Valence band is overlap to each Other and No forbidden Energy gape at 0K or More.