Hi, all.
As a semiconductor, the Fermi energy level is in the forbidden band between the conduction and valance band.
After doped by other atoms, such as Al doped into ZnO, the Fermi energy level will shift upward into the conduction band.
So, my question is:
1. if the Fermi energy level shifted into the conduction band, is this crystal (e.g. Al-doped ZnO) still a kind of semiconductor?
2. How to interpret this occurrence?
The basic rule is that:
[A] “As long as the % of the dopant is up to a certain limit, the semiconductor remain a semiconductor”. There may occur some change in its conductivity on doping .
[B]We are simply explaining the n- or p- type concuction on the basis of Fermi Level while keeping in mind that conduction band is analogous to LUMO and the valence band is analogous to HOMO .The Fermi level E(F) is chemical potential for electrons. It is the energy level at which the probability of electron occupancy is 50%.{ Pls.look at the MIDDLE Figure of attached file to see position of Fermi Level{Ef}
[C] In the file attached along with[ taking a general example of both n-type ( P as dopant in Si)[ RHS] and p-type ( B as dopant in Si)[LHS], there is shifting Fermi Level toward the conduction band in n-type semiconductors[ RHS] while the Fermi level is shifted towards the valence band while in the p- type semiconductors[LHS],
A point left from the above answer:
Supping we add a dopant to make it n-type semiconductor [ Add P to Si]. It means there is now excess of electrons .i.e.there is more electron density in whole of the bulk of such a n-type conductor.Accordingly, the Fermi Level energy is also is raised in its enegy because now the 50% of this increased electron density will be more. That is why, in n-type semiconductors,the Fermi Level enegy is raised in energy and go closer to the Concuction band of n- type conductor{See RHS of the attached figure].
Now suppose we add a dopant to make it p-type conductor[ Add B to Si]. It means that now the numbers of electrons are decreased.i.e.there is less electron density in whole of the bulk of such a p-type conductor.Accordingly, the Fermi Level energy is also decreased in its enegy because now the 50% of this decreased electron density will be less. That is why, in p-type semiconductors,the Fermi Level enegy is decreased and go closer to the Valence band of p- type conductor{See LHS of the attached figure].
1 Recommendation
7th Jul, 2014
Mohammad Kazem Moravvej-Farshi
Tarbiat Modares University
To add a couple of points to rge anssweres by Monhar Seghai::
1- In a moderately doped (non-degenerate) semiconductor Ec-3kT
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