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Although the different atomic orbitals combine with each other they do not mix with each other. Instead they form a layer. The same atomic orbital "n" combines to form "n" molecular orbit forming a layer, since there is certain different difference of energy in the same orbital of another atom. These continuous layers of energy levels are called continuous band. The gap between the energy levels are called energy gap or band gap. The energy band are close to each, the two uppermost bands are valence band and conduction band.
Figure 2.Shows the different energy level orbital(energy levels)
The electrons at the valence band are at lower energy levels they are called valence electrons and the innermost electrons in an atom are much less attracted by neighbouring atoms and occupy discrete energy levels. The conduction band is empty in semiconductor.If sufficient energy is applied then the electron at valence band jump to conduction band since the energy gap between the valence and conduction band in semiconductor is less. This phenomenon of electrons jumping to conduction band can be increased by doping.
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