Introduction
Band theory is based on quantum mechanics and comes from the theory of molecular orbitals.If multiple atoms are brought together into a molecule their atomic orbitals will combine to form a layer of molecular orbitals each with a different energy . For the giant molecule the energy levels are so close that they can be considered to form a continuum. Fiugure 1 shows the different layer of the band theory such as conduction band, band gap and valance band.
Figure 1. Show the different gaps
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 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.
Doping is the process of adding impirities like intrinsic material at the junction or gap between conduction band and valence band. It decreases the gap by making holes and electrons available.
References
- About Band theory[Online]
URL: https://www.boundless.com/chemistry/textbooks/boundless-chemistry-textbook/metals-20/band-theory-of-electrical-conductivity- 143/semiconductors-562-6136/ - Information abut the band structure.[Online]
URL: http://en.wikipedia.org/wiki/Electronic_band_structure - Information about band gaps and atomic structure.[Online]
URL:http://www.physics.iisc.ernet.in/~shenoy/mr301/WWW/band.pdf