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Why there are so many positive and negative ions in N and P types?

  1. Jun 8, 2013 #1
    The image bellow is the PN junction under equilibrium.
    I wonder why there are so many positive and negative ions in N and P types respectively.
    For me, I think that these ions should only exist in the depeletion region not outside the region.
    attachment.php?attachmentid=59368&stc=1&d=1370674622.jpg
    Thanks for help.
     

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  3. Jun 8, 2013 #2

    ehild

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    Do you know what we call n type and p type semiconductor?

    ehild
     
  4. Jun 8, 2013 #3
    Yes, I do.
    I have just watched it carefully now I see. For example in N type right next to each positive ion is an electron.
    I know that the electron and positive ion are separated but I usually consider that they is an unit beacuse its net charge still zero.
     
  5. Jun 8, 2013 #4

    ehild

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    The p and n type regions are neutral, but the electrons and holes are free to move in the whole crystal, like the conduction electrons in a metal. The pure (intrinsic) semiconductor has only few free charge carriers, as the forbidden band, the gap between valence and conduction band is broader than the thermal energy. But adding impurity atoms which have one electron more than the host atoms (for example phosphorus in silicon) these extra electrons are very loosely connected to their atoms and the thermal energy (kT) is enough to remove them from the atom. We say that the impurities make a donor level just under the conduction band. Similarly, atoms less than one electron than silicon capture an electron from an other atom, so the "hole", the place of the missing electron moves as free positive charge carrier in the crystal. The number of doping atoms can be about 1015-1020 atoms/cm3, so they are not so many with respect to the silicon atoms.

    In the depletion region, the free carriers move to the other side of the junction by diffusion, electrons to the p part and holes to the n part, leaving the ions alone. The depletion region is free from free carriers, but the concentration of the ions is the same as in the bulk of the semiconductor.

    ehild
     
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