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hole speed in semiconductor

Discussion in 'Electronic Basics' started by steve, Feb 21, 2004.

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  1. steve

    steve Guest

    Could anyone explain why hole speed in semiconductor material is less
    than electron speed?


    TIA
    Steve
     
  2. Hi,

    The simple answer, but one that begs the question, is that the
    effective mass of a hole is greater than that of an electron and so it
    requires a greater force to accelerate it to a given velocity. Of
    course you might rightly think that a hole doesn't have any real mass.
    However, for it to move in the valence band, it has to become involved
    in the transport of an electron and it is this process that slows it
    down making it appear heavy. Up in the conduction band though,
    electrons really fly at around twice the speed.


    Cheers - Joe
     
  3. steve

    steve Guest

    However, for it to move in the valence band, it has to become involved
    Could you suggest which book which discuss this? In all of books I've
    read they are not discussing it, perhaps it is for higher level
    learning?

    I would really like to know how it is calculated though.

    Do you teach this subject? perhaps I can attend a class which teach
    more about this very interesting thing.

    Thank you very much
    Steve
     
  4. Steve,

    No, I am not an instructor and was simply quoting that bit on effective
    hole mass from memory. I've retained a reader from a university course of
    years ago, and sometimes dip into it, but don't know of a book that covers
    the subject you want in any depth. In fact, generally in engineering texts,
    it wouldn't serve much of a purpose to go too deeply into the physics of
    carrier movement beyond that needed to understand device properties. You
    could look at this site for a little more information -

    http://en.wikipedia.org/wiki/Effective_mass

    As for measuring carrier effective mass, one method was to use a form of
    cyclotron where a crystal of the material under investigation was placed in
    a magnetic field inside a very low temperature waveguide system. Swept
    frequency RF energy was fed into the waveguide at one end and, at certain
    frequencies (around 10^10Hz for the electron, I understand), the carriers
    would move in circles consuming energy from the RF field. At the other end
    the absorption peaks were measured against frequency and their effective
    mass deduced from the relationship -


    m* = B x e / 2 x pi x f(res)

    m* is the effective mass
    B is the magnetic flux density
    e is the charge on an electron
    f(res) is the absorption peak frequency


    That's it - that's all I know! In fact I had to look up the last bit :)


    Cheers - Joe
     
  5. Roy McCammon

    Roy McCammon Guest

    Steve,

    The only truly correct answer to a question about why
    a theoretical construct has the properties that it has
    is that it makes the theory work.

    The best theory we have for what is going on is quantum
    electrodynamics (QED) which is intractable (not impossible)
    for most engineers (myself included). But the results of
    that theory applied to semiconductors can be boiled down
    to a simpler theory in which there are "holes" that are
    visualized to behave as positively charged heavy electrons.

    The theory works pretty good, but that doesn't mean
    that the holes have objective existence. They don't
    need a reason to be slow, other than that's what the
    experiments suggest.

    Now the following is just hand waving speculation,
    but if it helps, you can think of it this way:
    when an electron moves, it just moves. When a
    hole moves, the lattice of electrons holding the
    crystal together are rearranged.
     
  6. Well, I'm going to quibble a bit here on the technicalities. Solid state
    physics (or condensed matter physics) doesn't really require/use QED in
    any direct sense. Quantum Mechanics on its own usually sufficient.

    Holes are simply a method of noting that certain electrons go in the
    other direction.
    Arguable, electrons suffer the same fate.

    "Physical concepts are free creations of the human mind, and are
    not, however it may seem, uniquely determined by the external world" -
    Einstein
    Again, this suggests some sort of lack of theoretical backup. Basic
    Quantum Mechanics fully and completely explains why holes have lower
    mobility. Experimental support is pretty much a trivial consequence.


    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"
    http://www.anasoft.co.uk/replicators/index.html

    "quotes with no meaning, are meaningless" - Kevin Aylward.
     
  7. Roy McCammon

    Roy McCammon Guest

    quibble noted. I won't argue that point. QM is also intractable
    for most engineers. Instead, we have a theory involving holes and
    currents and densities and numerous empirical constants. Its wonderful
    if you have the math skills and believe in QM and can calculate
    those constants, but all you've done is explain one theory by
    deriving it from another. Qm also has empirical constants (though
    not as many) and the only answer as to why those constants have
    the value that they have is that it makes the theory work.
    yes, I agree. But I didn't want to have to defend it.
    I think we be going meta-physical here. Experimental support
    is the reason for the existence of QM, QED, etc.

    But, I'm eager to see your explanation, using basic qm and
    tractable math as to why the holes are slow. I'll bet you
    an ayl that less than 1 in 10 of the readers here will be
    able to follow it.
     
  8. QM was certainly arrived at by a trial and error processe, both
    experimental and theoretical, but QED is a bit more of a pure
    theoretically based extrapolation/extension to QM.
    I agree. That's why in some of my posts I keep pushing the black box
    approach. One needs to know what something does, not why it does it.
    Ultimately, if you have no control over it as an electronics design
    engineer, it doesn't help much knowing why. The why in any case, is an
    arbitrary why. We just reduce things to other e equally unexplainable
    causes that we seem happy to accept as a self evident truth of the
    universe, even its not.

    Kevin Aylward

    http://www.anasoft.co.uk
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.

    "That which is mostly observed, is that which replicates the most"
    http://www.anasoft.co.uk/replicators/index.html

    "quotes with no meaning, are meaningless" - Kevin Aylward.
     
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