physical meaning of energy band, why it creat and how conductor and semiconductor define

every one on that stage has been introduced with MOT(molecular orbital theory). the result comes from there.

suppose there is si atom which electronic configuration is 1s2 2s2 2p6 3s2 3p2. it means it have four electrons in outer most cell. when two atom have a sufficient distance from each other that there is no interection force between them than atom behaves as a free atom. but as well as two atoms comes closer  then nucleus of both atoms attract valance electron of each other.

according to bohr's theory every isolated atom has finite energy but when two atmos comes closer then they influence eachother. so the distribution of energy change and level define. following example can explain it .


si has four electrons in their outer most cell.suppose there is n atoms in a crystal then there will be 2n electron filling 2n possible  energy level in 3s and 6n possible levels in 3p in which 2n filled.in simple mean when n atoms comes closer then energy level get slightely changed and there will be large no. of closely packed levels thats called energy band which decide the conductor and semiconductor.in that condition 3s and 3p completely disappeared and 8n energy level continuouly distributed (2n of 3s and 6n of 3p).in which 4n are completely filled and 4n are empty. the gap between filled and unfilled is called forbidden gap  ,filled level is called valance band and unfilled is called conduction band. (see dia in basic books of electronics)