What's the difference between Fermi Energy and Fermi Level?
Question:
I'm a bit confused about the difference between these two concepts. According to Wikipedia the Fermi energy and Fermi level are closely related concepts. From my understanding, the Fermi energy is the highest occupied energy level of a system in absolute zero? Is that correct? Then what's the difference between Fermi energy and Fermi level?
Answers:
1- If you consider a typical metal the highest energy band (i.e. the conduction band) is partially filled. The conduction band is effectively continuous, so thermal energy can excite electrons within this band leaving holes lower in the band.
At absolute zero there is no thermal energy, so electrons fill the band starting from the bottom and there is a sharp cutoff at the highest occupied energy level. This energy defines the Fermi energy.
At finite temperatures there is no sharply defined most energetic electron because thermal energy is continuously exciting electrons within the band. The best you can do is define the energy level with a 50% probability of occupation, and this is the Fermi level.
2- It depends on who you ask.
If you ask someone with solid-state physics background, they will probably answer along the lines of Colin McFaul or John Rennie: The fermi level is the same as chemical potential (or maybe one should say "electrochemical potential"), i.e. the energy at which a state has 50% chance of being occupied, while the fermi energy is the fermi level at absolute zero.
If you ask someone with semiconductor engineering background, they will probably give the same definition of "fermi level", but they will say that "fermi energy" means exactly the same thing as fermi level. (The obvious question is, "Then what term would a semiconductor engineer use to describe the fermi level at absolute zero? The answer is, they call it "the fermi level at absolute zero"!)
3-
A small comment: the definition of fermi energy, fermi level and chemical potential is a question of semantics and confusion, see for instance http://physics.stackexchange.com/questions/30922/whats-the-difference-between-fermi-energy-and-fermi-level Since this is primarily a physics forum, it is probably best to stay with the Fermi energy as T=0K and Fermi level = chemical potential at T > 0K. If you use TEMPS in case.in2(c) an approximation of the chemical potential is printed out. For a metal or small-gap insulator (semiconductor) it is quite good; for a large-gap insulator for numerical reasons it is not right. |
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