METALS, INSULATORS AND SEMICONDUCTORS

• The free electrons that exist in the conduction band of any material are capable of carrying charge from one point to another and so they are known as charge carriers. 
•  Depending on their concentration, materials classified as metals, insulators and semiconductors. 
• Metals (eg; copper or silver)- very large number of free electrons at room temperature. In fact in the energy band diagram, the valence and conduction bands are overlapping and so there is no forbidden energy gap. It is very easy for a valence electron to become a conduction (free) electron. Hence even in the absence of additional energy such as light or heat, they behave as a good conductor. 
• Insulating materials (eg: rubber) on the other hand, have a very wide forbidden energy gap (Eg=5eV). Because of this it is practically impossible for an electron in the valence band to jump to the conduction band at room temperatures. It may conduct if its temperature is very high or if a high voltage is applied across it
• . For a semiconductor, the forbidden energy gap. is lesser than that of insulators but more than that of metals (Silicon Eg=1.12eV, Germanium Eg=O.72eV). The heat at room, temperature is sufficient to lift the electrons across and hence they are capable of conducting some electric current