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Pn junction diode

  1. Dec 10, 2008 #1
    folks, when a pn junction diode is formed, electrons in the n type and holes in the p type diffuse. agreed. then how immobile ions are formed at the vicinity of the junction leading to depletion region? how these immobile ions prevent further diffusion?
    it is being said electrons and holes recombine after crossing the junction? actually what is recombination mean? what is space charge? please help.
     
  2. jcsd
  3. Dec 10, 2008 #2
  4. Dec 11, 2008 #3
    The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer .The electric field created by the space charge region opposes the diffusion process for both electrons and holes
    mr. naty1, i dont understand these two lines mentioned in wikipedia article suggested by you. space charge contains negative ions on p side and positive ions on n side. if this being the case naturally attraction between holes and negative ions should occur on the p side and same being the case between electrons and positive ions on the n side. then how electric field opposes further diffusion.?
    The space charge region has the same charge on both sides of the PN interfaces, thus it extends farther on the less doped side

    i dont understand this line too because space charge has negative charge and positive charge but how the statement says 'same charge on both sides " please help sir
     
  5. Dec 11, 2008 #4
    The regions nearby the PN interfaces lose their neutrality and become charged, forming the space charge region or depletion layer .The electric field created by the space charge region opposes the diffusion process for both electrons and holes
    mr. naty1, i dont understand these two lines mentioned in wikipedia article suggested by you. space charge contains negative ions on p side and positive ions on n side. if this being the case naturally attraction between holes and negative ions should occur on the p side and same being the case between electrons and positive ions on the n side. then how electric field opposes further diffusion.?
    The space charge region has the same charge on both sides of the PN interfaces, thus it extends farther on the less doped side i dont understand this line too because space charge has negative charge and positive charge but how the statement says 'same charge on both sides " please help sir
     
  6. Dec 11, 2008 #5

    edguy99

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    Gold Member

    Heres a couple of thoughts that may or may not help out in some manner.

    On one side is a bunch of extra electrons not bound very tightly. On the otherside is a bunch of extra holes (spots in the 3d structure that are very likely to attract and hold an electron). Both sides are neutral since the number/charge of the electrons and protons balance as long as they are kept apart...

    This situation changes when the 2 materials are placed together. Some of the close by electrons jump into the big holes (perhaps leaving a smaller hole behind). Pretty soon you have a region of space in the middle where it looks like there are no holes or electrons (actually the electrons are there, but since they are stuck in a hole, they do not easily move around or do much of anything - give them a good hit with another electron and you will see an electron/hole pair pop back out at you).

    It is difficult for electrons to diffuse through this region as there are no big holes to jump into and electrons are held too tightly to be bumped along from hole to hole inside the material.
     
  7. Dec 11, 2008 #6

    Defennder

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    The process begins by electrons on the n-side and holes on the p-side crossing the pn-junction because there exists a concentration gradient causing diffusion. The space-charged region formed opposes this diffusion because the negative ions on the p-side opposes the diffusion of electrons from the n-side. Similarly for the positive ions on the p-side. This occurs until equilibrium has been reached.

    What this means is that magnitude of the net negative charge on the p-side is the same as the magnitude of the net positive charge on the n-side, which makes complete sense because for every positive/negative ion which is exposed after an electron/hole pair recombines, overall charge neutrality of the device is preserved.
     
  8. Dec 12, 2008 #7
    many thanks mr.edguy99 and mr. defennder for your replies.
    what is concentration gradient sir?
     
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