Difficulty understanding "donor" and "acceptor" energy levels.

  1. Hi all,

    I think I understand the concept of doping a group IV semiconductor material with a group V atom which has one extra valence electron to "donate" to the conduction band, or doping with a group III atom with one too few electrons which will "accept" an electron from the lattice and leave behind a hole.

    As I understood it, donor atoms introduce shallow energy levels just below the conduction band edge, and acceptor atoms introduce levels just above the valence band edge.

    However I have been reading about defects within the silicon lattice and have seen several references to "donor-like" energy levels just above the valence band edge, and "acceptor-like" defect levels close to the conduction band edge. Is there something simple that I am missing here?

    I would appreciate any help to shed some light on this.
     
  2. jcsd
  3. ZapperZ

    ZapperZ 29,765
    Staff Emeritus
    Science Advisor
    Gold Member

    It is the policy of this forum that whenever there is a phrase similar to this, you must cite what you read, i.e. give us the full citation. While there may be instances where what is being referred to might be obvious, in many cases they aren't! Therefore, in general, all referrals to external sources must be cited in full.

    Zz.
     
  4. Apologies for this.
    For example, the paper "Negative-U Properties for Point Defects in Silicon" by Watkins (Phys. Rev Letters, Vol. 44, no.9) shows in its first figure vacancy states just above the valence band edge which are labelled donor states.
     
  5. Any help would be greatly appreciated.
     
  6. chopper13,
    These are called deep levels. Usually they do not contribute to inducing electronic conductivity in the semiconductors.
    A big part of research in semiconductors (especially in compound semiconductors such as ZnO) focuses on determining whether certain defect or dopant would introduce shallow or deep levels. Shallow levels (close to band edges) contribute to electronic conductivity, while deep levels do not.
     
  7. Ah ok that's great, I figured it might have something to do with shallow and deep levels but was a little confused by the fact that some of the "deep" levels were actually close to band edges. It was only on further reading that I discovered the words deep and shallow can be a little misleading as they actually refer to which component of the defect potential is responsible for the particular level. Thanks a lot for the response.
     
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