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Why is the electric field zero outside of depletion region?

  1. Aug 18, 2015 #1
    why do electric field is zero out side of depletion region?
     
  2. jcsd
  3. Aug 20, 2015 #2

    mfb

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    In an area with charge carriers, how could there be a permanent electric field without current flow?
     
  4. Aug 20, 2015 #3
    can you explain it with using of coulomb rule?
     
  5. Aug 20, 2015 #4

    mfb

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    This is simple U=RI with negligible I and finite R. The Coulomb rule is not useful here.
     
  6. Aug 20, 2015 #5
    in depletion layer there are uncovered charges that generate electric field , how cancel their field?
     
  7. Aug 20, 2015 #6

    mfb

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    The remaining atoms have a charge as well. There is nothing "uncovered". A doped semiconductor has free charges of one type with zero overall charge density. If you remove those free charges the volume gets charged.
     
  8. Aug 20, 2015 #7
    the remaining atom dont generate any fied, then who cancel the field of depletion layer?
     
  9. Aug 20, 2015 #8

    mfb

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    Sure they do.
    Replace a silicon atom with boron. It has one electron and one proton less, so the overall charge is zero - but now you have a hole because another electron can bind there easily. If you fill the hole you have a negative charge.
    Replace a silicon atom with phosphorus. It fits in the crystal structure but has a 5th valence electron, while the overall charge is still zero - but now you have an electron that will easily leave its spot. If you remove the free electron you have a positive charge.
     
  10. Aug 20, 2015 #9
    if for each atom the overall charge is zero, then they dont generate any fied.
     
  11. Aug 20, 2015 #10

    mfb

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    And that's exactly what you have outside the depletion region.
     
  12. Aug 21, 2015 #11
    but who cancel the field of depletion layer?
     
  13. Aug 22, 2015 #12

    mfb

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    The other half of the depletion layer.
     
  14. Aug 22, 2015 #13
    it is not correct,
    the other half is more far
    then it can not cancel the field of opposite half
     
  15. Aug 22, 2015 #14

    mfb

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    Distance does not matter in a one-dimensional problem. Sheets of uniform charge density lead to uniform fields in all space (outside those sheets).
     
  16. Aug 22, 2015 #15
    sheets of uniform charge must have infinite dimension(infinite plane) to lead uniform fields
    in pn junction there is not infinite sheet
    then distance matter
     
  17. Aug 22, 2015 #16

    mfb

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    Compare the typical thickness of a depletion region with the typical dimension of a semiconductor device.
    The infinite sheet is a good approximation unless you consider modern microprocessors, and then things are much more complicated anyway.
     
  18. Aug 22, 2015 #17
    i dont talk about approximation
    i talk about exact situation
     
  19. Aug 22, 2015 #18

    mfb

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    Your statement in post 1 is an approximation.

    Actually, every description you will ever see is an approximation. Just the quality is different.
     
  20. Aug 22, 2015 #19
    actually, in non ideal diode electric field is zero outside of depletion region.
    this is exact not approximation.
     
  21. Aug 23, 2015 #20

    mfb

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    - The electric field is never exactly zero anywhere.
    - Charge distribution is never exactly uniform in space
    - the depletion layer does not have an exact boundary
    - ...
    All approximations.
     
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