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Thread: electron hole concept

  1. #1
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    electron hole concept

    Hi everybody. How can electron have negative effective mass? I'm a 3rd year mechanical engineering student and I do of course know basic science. But I never could understand this thing. Does anybody here really know the answer to my question? I have when people simply copy-paste from wikipedia thinking I haven't already looked there and 100 other pages. My research into this also revealed (you might be surprised) that in those cases when electron behaves as if it has negative mass, it behaves so only to electrical field but not to magnetic field. why would electron move towards negative terminal of battery instead of positive terminal in p-type semiconductors? Thank you all very much

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    Was this negative mass idea presented by a textbook or instructor in one of your college courses, or is it just someone or his uncle posting some hooey online? In the courses on solid state electronics in my college days I don't think the mass was even a factor in analyzing the behavior of electrons in a semiconductor. If I remember correctly about p-type semiconductors, they had "holes" where there was a shortage of electrons. An electron moving toward the positive terminal of the battery would jump into a hole and leave another hole behind it. We analyzed it mathematically as if the holes were positively charged particles moving toward the negative terminal.

    Don't take this as gospel. My memory of this topic has been rusting up for over 40 years. Perhaps others in this forum who are active in solid state electronics can help.

  3. #3
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    I don't see that we can add much to the Wiki article here:

    http://en.m.wikipedia.org/wiki/Electron_hole

    It's just a convenient quasi particle. It's not a real particle
    I'm not a hardnosed mainstreamer; I just like the observations, theories, predictions, and results to match.

    "Mainstream isnít a faith system. It is a verified body of work that must be taken into account if you wish to add to that body of work, or if you want to change the conclusions of that body of work." - korjik

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    Quote Originally Posted by Hornblower View Post
    If I remember correctly about p-type semiconductors, they had "holes" where there was a shortage of electrons. An electron moving toward the positive terminal of the battery would jump into a hole and leave another hole behind it. We analyzed it mathematically as if the holes were positively charged particles moving toward the negative terminal.
    This is what I think of when I hear "electron hole". In a sea of electrons, a hole acts like a positive charge, but it's really just all the electrons reconfiguring. It's a nice mathematics trick, not evidence of a new particle.

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    IIRC, the electrons were described as flowing, and the holes were described as migrating. This nicely distinguishs electrons, which are real particles, from holes which are non-particles. Also, be careful that you are not confusing conventional current and electron flow.

    Ol' Ben Franklin had a 50/50 chance of guessing the direction of current flow correctly, but I think he used up all his good luck when he didn't electrocute himself with that kite experiment.

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    For those who don't already know this, the electrons in p-type semiconductors move towards negative terminal. Teachers and prfessors who can't explain the reason for this (or don't want to bother) simply teach students to think of electron holes as positive charges. But after all it's electrons that are actually moving, so Hall-effect should reveal same polarity for all materials, but it doesn't. For p-type semis Hall probe reveals opposite polarity of that for metals, indicating that electrons actually moving towards negative terminal in p-type semiconductors. And here arises my qestion, WHY? Based on info I gathered so far it's the reault quantum stuff happening in crystal lattice, but so far I was not able to clearly understand all those stuff and I hope somebody can explain me by simplifying certain hard things to understand.

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    Quote Originally Posted by roboticmhd View Post
    Hi everybody. How can electron have negative effective mass? I'm a 3rd year mechanical engineering student and I do of course know basic science. But I never could understand this thing. Does anybody here really know the answer to my question? I have when people simply copy-paste from wikipedia thinking I haven't already looked there and 100 other pages. My research into this also revealed (you might be surprised) that in those cases when electron behaves as if it has negative mass, it behaves so only to electrical field but not to magnetic field. why would electron move towards negative terminal of battery instead of positive terminal in p-type semiconductors? Thank you all very much
    The negative effective mass comes from equating the crystal momentum of an electron with its free-space momentum. Basically you're pretending that an electron in a periodic lattice is the same as an electron in free-space, and responds in the same way to externally applied electric and magnetic fields, at the cost of using an "effective mass" for the electron rather than its real mass. That this effective mass can be negative means nothing, and you shouldn't read too much into it, it's just a way to be able to retain the same "simple" equations for an electron in free space to an electron in a crystal.

    ETA: It's the equivalence between the behaviour of an electron in a periodic lattice and one in free space that has varying mass. It only behaves as if it has negative mass when you disregard the fact that it's in a crystal rather than in free space. For an easier way of seeing this, let's use an easier example of how we could use this. Suppose we have a newtonian gravitational point source and a test mass falling towards it. Normally we would say that we have a gravitational field and the test mass has constant mass, and the equation of motion that we get is from having the interacting between the test mass and the gravitational field. But we can also get the same equation of motion by pretending that there is no gravitational field but that the test mass has decreasing effective mass over its trajectory.
    Last edited by caveman1917; 2014-Jun-01 at 01:53 AM.

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    Going entirely from memory (so that this is "my own words" rather than a copy-and-paste), semiconductor material (geranium at first, subsequently replaced by silicon) has a lattice arrangement of 4 due to covalent bonds. Imagine each silicon atom has four arms and each hand is holding hands with one other silicon atom.

    In N-type semiconductor material, the silicon is doped with impurities - atoms that effectively have five arms, but due to the overall lattice structure, each atom can hold hands with only four silicon atoms, so one hand isn't holding anything. Leaving the analogy behind, this means an extra atom drifting freely. Meanwhile, in the P-type material, there are impurities with only three arms. In real terms, this provides a slot for extra electrons to drop into. These slots are called holes. They are effectively the absence of an electron.

    Holes can move around - that is to say, they appear to travel in the opposite direction to the electrons that fill them. Holes being filled and emptied is a bit less straightforward than an electron simply moving around, so the movement of holes is relatively slow.

    In semiconductor material that has had one side doped with N-type and the other doped with P-type (e.g. a diode), there is a PN boundary. Surplus electrons from the N-type drift over the boundary and fill some of the nearby holes in the P-type. Prior to this happening, the material was all electrically neutral; now, with electrons in the holes, the P-type near the boundary has a negative charge, and the N-type, having lost some electrons, now has a positive charge. With this potential difference across the boundary, there is no tendency for any more electrons to drift across.

    I don't know if this answers any of the OP's questions. I certainly don't remember negative mass being discussed at all.

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    Is there a p-type conductor, rather than p-type semiconductor? For example, there are n-type conductors(metals) and n-type semiconductors? Or, p-type material can only be a semiconductor? And What are easiest obtainable p-type conductor( either semi or plain)?

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    A semiconductor can either be n-type or p-type. Is there a p-type conductor, just like there is n-type conductor(metals)? And, what is the easiest obtainable p-type conductor(either semi or plain)?

  11. #11
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    P-type is only meaningful for semiconductors. We don't think of conduction as the flow of holes for ordinary conductors. And therefore there is no point in calling copper (for instance) an N-type conductor.

    It would be helpful if you responded to other people's posts, letting people know whether the explanations are clear, helpful, relevant or whatever.

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