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Charge carriers recombination

  1. Sep 15, 2014 #1
    In solid state physics the "holes" usually regarded as an effective positive charge carriers.
    In order to have neutral charge some object needs to have an equal amount of protons and electrons. If some material such as an intrinsic semiconductor has equal amount of protons and electrons + some amount of free holes (which are effective charge carriers) does it mean this one material suppose to be positively charged? If not, why? And which effect carrier generation/recombination does have on total charge of particular object?

    Also question: which laws govern carriers generation/recombination in metals? Do they suppose to change in metal with change in temperature? And if not, why?
     
    Last edited: Sep 15, 2014
  2. jcsd
  3. Sep 16, 2014 #2
    I suppose this statement means the bulk of the material is electrically neutral. Why do you consider protons at all? They are part of the nuclei of the lattice atoms and will not migrate (hardly).
     
  4. Sep 16, 2014 #3
    Let suppose we have an absolute dielectric with zero electrons in conduction band. Subsequently there is no holes at all. Such properties could have a gases, for example, or semiconductors/dielectrics at absolute zero Kelvin. In this case the total charge of an objet will be defined by relation between number of protons and electrons. If they are equal the charge is neutral. Then we increase temperature and some electron leave valence band for conduction band and leave positive holes. Holes are positive charge carriers. Does it mean total charge suppose to become positive? Holes electrostatically attracted to electrons. It means they are an effective charge carriers. If in some object number of protons + holes exceed number of electrons does it mean an object suppose to be positively charged in total?
     
  5. Sep 16, 2014 #4
    This violates the law of charge conservation,
    http://en.wikipedia.org/wiki/Charge_conservation
    in case it is meant to create additional positive charge
     
  6. Sep 16, 2014 #5

    marcusl

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    In an intrinsic semiconductor, a positively charged hole is created by the ejection of a negatively charged electron from one of the crystal atoms. The number of holes and electrons is identical, so the material remains electrically neutral.
     
  7. Sep 16, 2014 #6
    Yes, this is what should happen to maintain charge conservation. Each hole may be considered to have a positive charge contributed by the unbalanced positive charge of some atomic nucleus after an electron has been ejected by this same lattice atom.
     
  8. Oct 6, 2014 #7
    Does carriers injection in some material always leads to carriers recombination? Lets say we have neutrally charged semiconductor or semimetal with 100 of electrons, 99 of them are in valence band and 1 in conduction band. If we inject in this material 50 electrons and 50 holes additionally, how many of them suppose to recombine? Recombination of carriers will mean that electrons fall to valence band. So, if all 50 electrons and holes will recombine, a material will get 50 additional electrons in valence band? So, if before it had 99 electrons in its natural state at 20 centigrade, now it will got 149 electrons at valence band at the same temperature? Is it possible? Is it not going to violate chemical laws of a substance? Usually valence electrons are those according to which atoms are tight to each other.
     
  9. Oct 6, 2014 #8
    A hole is an electron vacancy; injection of 50 electrons increases the negative charge by 50e-; injecting 50 holes decreases it by 50e-
     
  10. Oct 6, 2014 #9

    So, what? I have request to user answer my question in normal way rather than in riddles. Injection of 50 electrons and 50 holes will preserve charge neutrality.
     
  11. Oct 6, 2014 #10
    Speaking of charge, it means adding 50 of each is the same as not having done anything. How are these 50 each are to be injected? What kind of material?
     
  12. Oct 6, 2014 #11
  13. Oct 7, 2014 #12

    analogdesign

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    Yes, carrier injection always leads to recombination unless you have an external field applied. Carrier inject leads to a non-equilibrium state. The material will settle with some time constant related to the recombination rate of the material.

    In your example, if you magically inject 50 electrons and 50 holes for some period of time you will have a semiconductor in non-equilibrium and you will have local diffusion of the charge carriers. Over time, depending on the temperature, doping level, etc etc the 50 electrons and 50 holes will recombine, the semiconductor will return to equilibrium and you will be right back where you started. No violations of any laws required!
     
  14. Oct 7, 2014 #13
    So you are going to tell that it will come to equilibrium even with number of electrons exceeding number of protons?
     
  15. Oct 7, 2014 #14

    analogdesign

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    What does that even mean? If you're putting order 10^22 electrons / cm^3 into the system it will be highly, highly, highly nonequilibrium until you stop doing that. The protons are fixed in nuclei of the lattice atoms. They don't come into play here.
     
  16. Oct 7, 2014 #15
    They suppose to come into play because valence electrons hold atoms together. If there becomes too many valence electrons in comparison to nuclei, I have suspicion it suppose to play some role in crystalline structure change...
     
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