Recently I was studying about band theory of semiconductors and I have some questions.
I found two definitions of Fermi level
$1)$ The quantum state which has a probability of occupancy of $0.5$
(from my book)
$2)$ The Fermi level of a body is the thermodynamic work required to add one electron to the body
Everywhere the Fermi level of a semiconductor is talked about, its done on the basis of the $1$st definition and the Fermi-Dirac probability distribution.
My question is if the Fermi level of a semiconductor can be defined in terms of the $2$nd definition (also known as the chemical potential of the system) or does this definition fail when talking about semiconductors because in semiconductors (both doped and pure) the Fermi level lies in the band-gap and that confuses me because how can an electron in the highest occupied state in valence band (please assume at $0$K) have energy corresponding to somewhere in the band gap.
My second doubt is that in an extrinsic semiconductor, (both p-type and n-type), the extrinsic Fermi level is also defined in terms of the 1st definition everywhere.
(Please note I am assuming at $0$K)
But if we consider the $2$nd definition using the chemical potential, shouldn't the Fermi level (in a p-type doped) rise above the intrinsic Fermi level instead of being somewhere in the middle of the acceptor and the valence band? Because in the case of doped semiconductors, no matter p-type or n-type, we're adding dopant atoms into the system which now has more atoms, and hence more electrons in addition to that of the electrons in the valence band of the intrinsic semiconductor so we are adding more energy to the system in the process of adding those additional electrons of the dopant and it should shift the Fermi level above the middle (intrinsic level).
But it doesn't happen since Fermi level of a p-type doped semiconductor lies far below the intrinsic level near the valence band(somewhere in the middle of the acceptor and the valence band, to be precise) so it's sure that I'm missing something.
Why does nobody want to talk in terms of chemical potential?
Please try to answer in terms of chemical potential rather than redefining Fermi level in terms of the Fermi-Dirac probability distribution. That will be very helpful to me.
I tried a lot but couldn't get what's going. Please help me to understand this thing.
Thank you very much :)