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What makes the change of sign in Hall?

  1. Apr 26, 2015 #1
    Hi!

    I have learned that the Hall-coefficient changes sign according to different metals.

    Copper for instance has a positive sign, while Zinc has a negative sign.

    Why?

    To me an ordinary non-doped metal should always have electrons as beariers of charge.

    Further more, I have learned that charge density have no impact on Hall voltage.

    Another lerndome, heat is not an issue.

    Can anyone explain this?

    I have read the Wikipedia article but I did not get any wiser.

    Best regards, Roger
     
  2. jcsd
  3. Apr 27, 2015 #2

    ShayanJ

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    Gold Member

    The sign of the Hall coefficient indicates the nature of the majority carriers in a material. For all metals, electrons are the majority carriers(I think we can safely say the only carriers) and so the Hall coefficient for all metals is negative(see here, copper has a negative Hall coefficient too).
    But for semiconductors, sometimes holes(the absence of an electron) are the majority carriers. For these materials, the Hall coefficient is positive.
     
  4. Apr 27, 2015 #3
    Hi Shyan!

    First I must confess that I was wrong.

    Copper does have a negative sign, Zinc on the other hand seems to have a positive sign.

    And as you now confirm my belief that pure metals only have electrons as majority carriers, how can there be a change of sign for Zinc?

    Best regards, Roger
    PS
    Here is proof: http://www.science.com.tw/company/index.php?route=product/download/download&download_id=429
     
  5. Apr 27, 2015 #4

    ShayanJ

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    Gold Member

  6. Apr 27, 2015 #5
    Thank you for that link.

    I have read alot by clicking around.

    By heart I seem to know that Zinc is not attracted to a magnet (to keep it simple while thinking it is called "diamagnetic").

    I think the Wikipedia article expalins nothing about the fact that the Hall coefficient is positive for Zinc.

    It tells something about anomalous Hall Effect but actually nothing about my question.

    So the question remains, why has Zinc a positive Hall coefficient?

    Best regards, Roger
    PS
    I did some editing in that poorly written article :)
     
  7. Apr 30, 2015 #6
    In a semiconductor, you can have either negative charge carriers (conduction electrons) or positive charge carriers (holes - absence of charge carriers).

    If you know about energy bands, then you'll know that when an electron is given enough energy, it jumps from the valance band to the conduction band. This however, leaves a gap behind, which is called a hole.

    You can also introduce holes by introducing a Group 3 element into the semiconductor. These elements (such as Boron) only have 3 valance electrons, so they cannot have 4 bonds with silicon unless they steal an electron from a silicon atom. Then that silicon atom only has 3 electrons, unless they steal one from somewhere else, and thus, we see how we can have a positive charge moving in a semiconductor.

    If the holes make more of an impact on the conductivity of the semiconductor, then they are the majority charge carrier, and the Hall Resistance will be positive.

    Now, the reason why Zinc has a positive Hall coefficent is because its majority charge carrier is holes.

    The reason? Don't know. Might have to do with the effective mass of an electron vs the effective mass of a hole in Zinc.

    If anyone sees any mistakes, please let me know, I have my semiconductors exam in 3 hours and would really like if I'm spouting out nonsense.
     
  8. Apr 30, 2015 #7
    I thank you Tweej for your nice reply!

    Shyan above has however already confirmed my belief that the majority charge carriers in pure metal such as Zinc are electrons.

    In other words, semiconductor theory is not relevant.

    So my question remains, why has Zinc a positive Hall coefficient?

    Best regards, Roger
    PS
    Hope your exam went well :)
     
  9. May 26, 2015 #8

    DrDu

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