Can anyone tell me that why is there a donor level in the band gap and conduction band under (and not in the conduction band ) and why the effective Fermi level shifts toward the conduction band in N-type semiconductor?
Impurities create additional eigenstates or allowed energy levels in the semiconductor. These eigenstates in principle can be anywhere energy-wise - in the band gap, in the conduction band, in the valence band, etc. Only those levels which are slightly below the conduction band bottom - shallow levels - and can donate an electron to the conduction band, are useful as donors. Each donor donates an electron to the conduction band, thereby creating a free carrier of electricity. The donor impurities have generally a valence more than the host it replaces in the crystal. The donor levels have to be shallow, so that their ionisation energy is of the order of kT at room temperature, which is around 25 meV. The Fermi level is determined from charge balance, i.e. the positive charges (holes and ionised donors)= the negative charges (electrons + ionised acceptors). The Fermi level in principle can be inside the band gap (in the case of non-degerate semiconductors) or inside the bands (degenerate semiconductors, i.e. heavily doped semiconductors), depending upon the magnitude of the acceptor/donor density. In non-degenerate n-type semiconductors, the Fermi level is not above the donor level, but is below the donor level, so that most of the donors are ionised, i.e. do not have the extra electron.
Impurities create additional eigenstates or allowed energy levels in the semiconductor. These eigenstates in principle can be anywhere energy-wise - in the band gap, in the conduction band, in the valence band, etc. Only those levels which are slightly below the conduction band bottom - shallow levels - and can donate an electron to the conduction band, are useful as donors. Each donor donates an electron to the conduction band, thereby creating a free carrier of electricity. The donor impurities have generally a valence more than the host it replaces in the crystal. The donor levels have to be shallow, so that their ionisation energy is of the order of kT at room temperature, which is around 25 meV. The Fermi level is determined from charge balance, i.e. the positive charges (holes and ionised donors)= the negative charges (electrons + ionised acceptors). The Fermi level in principle can be inside the band gap (in the case of non-degerate semiconductors) or inside the bands (degenerate semiconductors, i.e. heavily doped semiconductors), depending upon the magnitude of the acceptor/donor density. In non-degenerate n-type semiconductors, the Fermi level is not above the donor level, but is below the donor level, so that most of the donors are ionised, i.e. do not have the extra electron.
Please consult Chapter 28 of the book Solid State Physics, by Ashcroft and Mermin. The explicit answer to the main question on this page is to be found on page 579 of this book.
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