Effect of temperature on semiconductors' conductivity

  1. I've known that the conductivity of semiconductors increase with temperature because more electrons are freed,
    But why doesn't the scattering and vibrations of the lattice affect the conductivity in semiconductors as in metals?
     
  2. jcsd
  3. Does the effect of additional free electrons outweighs the effect of the greater vibrations of the lattice so the overall resistance will decrease ??
     
  4. UltrafastPED

    UltrafastPED 1,919
    Science Advisor
    Gold Member

  5. In metals the number of free charge carriers is constant. As the temperature goes up their mobility goes down. Because the carriers can't move as much there is higher resistance.

    In semiconductors the number of charge carriers increase exponentially with temperature and this overrides the decrease in mobility.

    It turns out that we're lucky. Semiconductors have a complex behavior over a wide range of temperatures. It just so happens that they behave like useful semiconductors at temperatures that we're accustomed to on Earth.

    EDIT: I should have said that conductivity is a function of the number of free charge carriers multiplied by their mobility. If the number of carriers increases faster than the decrease in mobility then conductivity can increase with temperature.

    If a device's resistance goes down with temperature then we say that it has a negative temperature coefficient. This is not necessarily directly related to the conductivity of the device's material makeup. Other physical processes can affect the temperature coefficient.

    Some things that have negative coefficients:
    -light bulbs
    -diodes
    -bipolar junction transistors

    Some things that have positive coefficients:
    -metal wires
    -Field effect transistors
     
    Last edited: Jul 17, 2014
  6. NascentOxygen

    Staff: Mentor

    Incandescent bulb filaments have a positive temperature coefficient.

    I believe a carbon rod will show a negative coefficient.
     

  7. An Excellent reply, thanks !!!!! :smile:
     
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