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Is there such a thing as a neutral hole?

  1. Mar 19, 2013 #1
    Most explanations of doping, coupled with the simple definition of hole as positive charge carrier, and reinforced by classic graphic illustrations of P- and N-doped regions full of little plus and minus signs respectively, makes for a common point of confusion when learning basic semiconductor theory.

    When it is explained that p-doping introduces holes and n-doping, electrons, it is easy for the unwary student to conclude that the p-doped region therefore attains a net positive charge and the n-doped region a net negative charge. This is clearly not true, as there are in each case a balanced total of electrons and protons and no net charge. Yet even the author of a textbook I have in my hand falls directly into this trap.

    So how to distinguish e.g. between the "holes" introduced by p-doping and charged holes diffusing through the material? Do we speak of "neutral" holes ("empty" holes??) vs. charged holes? I have never seen a formal distinction in my searches through books and web sources, yet the difference seems clear enough to me.

    Further, can we quantitatively distinguish between the attraction between a free electron and a "neutral" hole vs. a "charged" hole? In my amateur view it seems that a charged hole (i.e. associated with a positive ion) must surely be more attractive than a "neutral" hole (associated with an electrically-neutral formation of silicon atoms bonded to a single p-dopant atom) which, relatively, just seems like a nice place for an electron to be.

    I hope I haven't embarrassed myself too badly. I beg your forgiveness.
  2. jcsd
  3. Mar 19, 2013 #2
    The bulk of any material (in the absence of external electromagnetic field) is charge neutral. And so you are right about the overall charge neutrality of p-type or n-type material. But what really matters is the "effective charge" not the actual one. So what you called "neutral" hole is indeed charged "with respect to the original site in the lattice" for example suppose you dope silicon with Al. There is a place around the Al "ion" where you should find an electron but since Al is able to participate in bonds using three electrons only (compared to four given by silicon) this place is empty. So it has a charge +1 with respect to the original lattice site. On the other hand the counter balancing charge is not very far! Observe that you have Al3+ setting in place of Si4+, so Al has -1 charge with respect to the original ion.

    What you call "charged" hole is no different at all from what I described above.

    There is however a distinction between "localized" hole and "band" hole , but the distinction is not in the charge, but rather in its mobility.

    Regarding coulomb interactions, it is much easier to calculate repulsion/attraction forces based on the effective charges rather than "actual" ones. The ease comes from the fact that almost all the lattice (except where you have dopants/defects) will have an "effective" charge of zero and hence does not exert coulombic force.
  4. Mar 20, 2013 #3
    This is very helpful, thanks. Although my mind is not completely at rest, that is likely too much to hope for at my level. :-)
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