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Semiconductor doping

  1. Nov 30, 2017 #1
    1. The problem statement, all variables and given/known data
    Which one of the following describes the result of doping in semiconductors?
    a. The impurity provudes additional energy states within the conduction band
    b. More electron - hole pairs are introduced as a result of doping
    c. For n - type semiconductor, the number of "holes" in the valence band remains small
    d. For p - type semiconductor, there are more "holes" in the conduction band
    e. None of the above

    2. Relevant equations
    None

    3. The attempt at a solution
    Option (a) is wrong because energy states stay the same

    Option (b) is wrong because doping only adds only electron or hole, not electron - hole pair

    Option (c) I think is correct because for n - type the number of free electron in conduction band increases and number of hole in valence band stays the same

    Option (d) I am not sure

    Is my reasoning for (a) - (c) correct? And what about (d)?

    Thanks
     
  2. jcsd
  3. Nov 30, 2017 #2
    For C: the number of holes remains small - that is, zero. But I don't think the author is taking that to be an answer.
    Actually, the number of holes in the valence band isn't really small. The valence band of every atom has holes that share electrons with neighboring atoms. What is small (zero) for P-type is the number of holes in the conduction band.
     
  4. Dec 1, 2017 #3
    I learn that valence band is the highest populated band, filled completely with electrons. How can valence the number of holes in the valence band not really small?

    So, option D should be: For p - type semiconductor, there are more "holes" in the VALENCE band. Am I correct?

    And why for p - type the number of holes in conduction band zero? Conduction band is the first band that is not fully occupied by electrons but why the holes are zero?

    The answer of this question should be (e)?

    Thanks
     
  5. Dec 1, 2017 #4
    There are more holes in the CONDUCTION band for p-type semiconductors.
    I am taking "valence band" to be an attribute of the atoms. Although I am not sure how the author intends it to be interpreted in this context.
    I believe the answer is D.
     
  6. Dec 1, 2017 #5
    English is not my strong point but I feel that the two sentences above contradict each other. Maybe I misinterpret your intention?

    Is it not correct if I say in p - type semiconductor, the addition of holes make the electrons in valence band be able to move to fill the holes so there are more holes in valence band?

    Also, I think in n - type semiconductor, the addition of electrons will result in more electrons in conduction band. Am I correct?

    Thanks
     
  7. Dec 1, 2017 #6
    In post number 2, I mistyped. I was referring to option C so I should have said: "What is small (zero) for N-type is the number of holes in the conduction band.".
    Since the author seems to be differentiating between the valence band and the conduction band, I am not sure that is correct.[/QUOTE]
    Yes
     
  8. Dec 1, 2017 #7
    You mean valence band and conduction band should be the same?
     
  9. Dec 1, 2017 #8

    Delta²

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    Gold Member

    semiconductor physics not my strong point but I THINK the only option that seems reasonable here is C, for the reason you stated in your first post.

    Doping increases electrons in the conduction band (for n type) or holes in the valence band (for p-type).
     
  10. Dec 1, 2017 #9
    I think Delta is right.
    I just read up on bands.
    In n-type semiconductors, charge moves as electrons through the conduction band.
    In p-type semiconductors, charge moves as holes through the valence band.

    So D cannot be right because there are never holes in the conduction band.
    C is right because doping an n-type affects the conduction band, not the valence band.
     
  11. Dec 13, 2017 at 5:25 AM #10
    thank you very much for the help
     
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