Energy band diagram for insulator, conductor and semiconductor
1. Insulator: Through these materials electricity cannot pass is called insulators. Plastic, glass, wood etc are the examples of insulators. The valence band of those material remains full of electrons. The conduction band of those material remains empty. The forbidden energy gap between the conduction band and the valence band is widest. The difference is more than 10ev. Crossing the forbidden energy gap from valence band to conduction band large amount of energy is needed.
In a diamond crystal the value of forbidden energy is as high as 7ev and so it is impossible cross forbidden gap to conduction band even at high temperature. So, the conduction band of diamond remains empty. As there is no flow of current. Hence diamond is a very good insulator.
2. Conductors: In figure b shows the energy band of good conductor or metal. The valence band and the conduction band is attached here on overlap each other. There is no forbidden energy gap here so Eg=0. At absolute zero temperature large number of electrons remain in the conduction band. The resistance of conductor is very low, a large number charge carriers are available here. So, the electricity can pass easily through the conductors.
3. Semiconductors: Semiconductors are those materials whose electrical conductivity is between conductors and insulators. The forbidden energy gap of a semiconductor is nearly same as insulator. The energy gap is narrower. The value of Eg =1.1eV for silicon crystal and Eg =0.7eV for germanium at ok. It can easily overcome due to thermal agitation or light. A semiconductor remains partially full valence band and partially full conduction band at the room temperature. The conduction band remains full empty of a semiconductor where the valence band remains full of electrons at absolute zero temperature. So, silicon and germanium are insulators at absolute zero temperature. On the other hand with the increasing of temperature the electrical conductivity of semiconductors increase.
In figure the band diagram of silicon has shown at absolute zero and at room temperature.