Why do doped semiconductors have a decreased band gap?

[Goodver]

Could anyone please explain why doped semiconductors show decrease of the bang gap in comparison to the pure semiconductors.

Web/literature links to relevant information will be appreciated.

Thanks.

 

[ehild]

It happens at high doping, when the impurity atoms are so close that they interact and the doping energy levels split forming a doping band. That band can merge with the valence band/conduction band making the band gap narrower. See for example http://ecee.colorado.edu/~bart/book/eband6.htm or browse impurity band, gap narrowing .