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Electrical mobility definition confusion

  1. May 26, 2015 #1
    Hi! According to this http://en.wikipedia.org/wiki/Electrical_mobility, the definition of electrical mobility ##\mu## is:

    ##\vec{v} = \mu \vec{E}##. But since electrical mobility is always positive, this means that the velocity is always parallel to the E-field regardless of charge. How can this be?

    thanks for all replies.
  2. jcsd
  3. May 26, 2015 #2
    Actually its not necessarily always positive. It can be negative in semiconductors.

    Wikipedia is not a very rigorous source. It is good to get the idea of something and should not be taken too seriously. The mobility is defined as
    v = +uE for 'holes.' And v = -uE for 'electrons.' Where v and E are vector quantities
  4. May 26, 2015 #3
    In the Wiki article they don't use vectors. The text explicitly mention that E is the magnitude of the electric field. So it is a scalar relation between the magnitudes of the drift velocity and of the electric field.
    And this is how mobility is defined in Kittel's reference book: as ratio between magnitudes. The values of mobility are given as positive for both electrons and holes.
  5. May 26, 2015 #4
    Thanks guys
  6. May 26, 2015 #5
    I apologise I should have been more specific that the minus sign cancels due to the relative direction of [tex] \vec v \ and\ \vec E [/tex]]
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