When two identical atoms are brought closer, then outer most orbits of these atoms overlap and interact. The energy levels corresponding to these two split into two energy levels. Similarly, for a solid of 'n' number of atoms, each energy level splits into 'n' energy levels. These levels are so close to each other such that they form an almost continuous band.
- The band corresponding to the outer most orbit is called Conduction band. It may be empty or partially filled.
- The band filled with valance electrons is called Valance band.
- The gap between these two allowed bands is called Forbidden gap.
- Basing on the width of the gap between the bands, solids can be classified into three groups they are Conductors, Semiconductors and Insulators.
Classification of solids:
- In the case of insulators the forbidden energy band is very wide. Due to to this fact electrons can not jump from valance band to conduction band.
- In insulators electrons are bounded tightly to their parent atom.
- When very large energy is supplied, an electron may be able to jump and cross the forbidden gap.
- For example Glass have an energy gap of 10 eV and Diamond has an energy gap of 5.5eV.
- In the case of semiconductors, forbidden gap is small. Germanium, Silicon are the best examples.
- Actually at zero kelvin semiconductors acts as insulators when a small amount of energy is supplied, electrons easily jumps from valance band to conduction band.
- For example Germanium has an energy gap of 0.7 eV and Silicon has an energy gap of 1.1 eV.
- In conductors there is no forbidden gap, both valance band and conduction band overlap on each other.
- In conduction band plenty of electrons are available for the process of electrical conduction.
- Best examples are all metals.