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Compensated semiconductors

  1. May 18, 2012 #1
    Hi all,

    In a compensated n-type semiconductor where there are both acceptors and donors in similar densities, the acceptors are filled with electrons (and therefore have a negative charge) even at very low temperature, close to 0 K.

    Where do they get these electrons from ?

    Whether they get them from the material valence band or from the donor impurities these electrons don't they have a given ionization energy to be emitted and that at these very low temperatures those electrons don't have enough energy and therefore they can not be emitted and hence not be attracted by the acceptors ?
     
  2. jcsd
  3. Jul 9, 2012 #2
    The shallow acceptors become filled with electrons from donors and deep levels that are above the dark Fermi level. The electrons can jump by tunneling, by emission of a photon, or by a more complicated process.
    Electrons and holes can jump from one impurity atom to a nearby impurity atom without entering either the conduction band or the valence band. For a compensated semiconductor at very low temperatures which isn't illuminated, the majority of transitions will occur between impurity or defect levels.
    Electrons can tunnel from one state to another even without thermal excitation. An electron can tunnel from a donor to a nearby acceptor, often with the emission of photons and phonons. When examining the photoluminescence of a material, the brightest emission lines often comes from the transitions between shallow donors and holes.
    A neutral donor has a bound conduction electron. A neutral acceptor has a bound hole. If the conduction electron in a neutral donor jumps to a neutral acceptor, then one has a positively charged donor and a negatively charged acceptor. The jump does not have to involve thermal excitation of donor electron into the conduction band. As long as the dark Fermi level is between donor and acceptor, the electron will make eventually make the jump. The energy difference may be made up for by the emission of a photon. Donor-acceptor transitions have a well characterized emission spectrum.
    Compensated semiconductors are photosensitive. When illuminated, all sorts of transitions can occur. The impurities can absorb and emit photons.
    References:
    "Optical Properties in Semiconductors" by Jacques I. Pankove (Dover, 1971). ISBN: 0-46-60275-0.
    I recommend Chapter 6, Radiative Transitions, especially 6-F (Donor-Acceptor Transitions).
    Also, a very nice study of one compensated semiconductor is
    "Native Defects in undoped semi-insulating CdSe studied by photoluminescence and absorption," by David L. Rosen, Q. X. Li, and R. R. Alfano. Physical Review B 31(4), 2396-2404 (1985).
     
    Last edited: Jul 9, 2012
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