Why the holes are not created when donar electrons go to conduction band in case of doping in n-type semiconductor?
Answers
Explanation:
For an n-type semiconductor, there are more electrons in the conduction band than there are holes in the valence band. This is due to the doping of donor impurity to form n-type semiconductor.
Answer:
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.
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