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I want to know why some semiconductors band gap decreases after doping with elements. Burstein-Moss band-filling effect can be useful to explain band gap widing in a semiconductor materials but i was unable to find any logical explanation for band narrowing effect. Can you please explain the mechanism of band gap narrowing.

This below quotes were taken from a research article.

There is general agreement that two competing phenomena are dominant in affecting the absorption edge in heavily doped semiconductors. First, the well-known Burstein-Moss band-filling effect which shifts positively the measured band-edge energy with increasing carrier concentration. In this case the measured optical gap $E_{m}$ is the sum of the optical gap of the lightly doped material $E_{0}$, plus that due to filling of the conduction band due to Is $\Delta E_{BM}$, I. E. $E_{m}=E_{0}+E_{BM}$. Thc second phenomenon which affects the optical absorption edge with increasing donor density is due to a change in the nature and strength of the interaction potentials between donors and the host crystal. This latter effect gives rise to a band-gap shrinkage and to some increased tailing of the absorption edge. In this case the measured optical gap is $E_{m}=E_{0}+E_{BM}-\Delta E_{g}$., where $\Delta E_{g}$ is the gap shrinkage.

I didnt understand the explanation given in second phenomenon. What is author meant by due to a change in the nature and strength of the interaction potentials between donors and the host crystal.

Advance thanks for your help

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I found a paper titled: "A New and Simple Model for Plasma- and Doping-Induced Band Gap Narrowing" (by Ahmed Shaker and Abelhalim Zekry in the Journal of Electron Devices, Vol. 8, 2010, pp. 293-299) which has a more intuitive explanantion of why the band gap narrows.

link to the paper for the lazy: A new and simple Model for Plasma and Doping and Doping induced Band Gap Narrowing [pdf]

I hope it helps.

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    $\begingroup$ Can you explain the important bits of the paper? Link-only answers are frowned upon because link rot can be an issue. $\endgroup$ – HDE 226868 Apr 11 '15 at 20:23

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