# Why does Fermi Level change due to change in donor atom concentration?

Suppose I have a n-type semiconductor whose fermi-level lies (say) 0.2 eV below the conduction band. Why would this level change if I changed the doping by making the donor concentration (say) 4 times the original value?

And what does it even mean? Does this mean that chemical potential also changes ? (I guess it, probably, does but what it means?)

Also could we estimate how would it change in some approximate way?

• The impurities can shift $E_c$ and $E_v$ therefore shifting $E_F$ by its definition. I think it probably has to do with the fact that electrons are fermions and can't occupy the same level, aka Pauli Exclusion. – M Barbosa Jun 12 '16 at 12:57
• @MBarbosa shifting of $E_C$ and $E_V$ happens, but the main point is the increased carrier concentration in the conduction band due to the donor states. As the donor states are close to the conduction band, some of them will be ionized. As a result the CB carrier concentration is higher as one would expect just from the band gap and the temperature. Therefore the band gap has to be shifted by $E_F$ to describe the electron distribution. Maybe this documents helps to clear up things: uwyo.edu/cpac/_files/docs/kasia_lectures/… – user_na Jun 12 '16 at 14:35