Why do doped semiconductors have a decreased band gap?

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SUMMARY

Doped semiconductors exhibit a decreased band gap compared to pure semiconductors due to high doping levels. When impurity atoms are closely packed, they interact, causing the doping energy levels to split and form a doping band. This doping band can merge with the valence or conduction band, resulting in a narrower band gap. For further details, refer to the resource provided in the discussion.

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Goodver
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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.
 
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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 .
 
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