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P-n junction & transistors: depletion zone

  1. Aug 18, 2011 #1
    For p-n junctions (connected as a junction rectifier), the forward-bias connection (battery's positive terminal connected at the p side) results in a narrowed depletion zone, because: the p side becomes more positive and the n side becomes more negative, thus decreasing the height of the potential barrier


    So why in transistors, does the negative gate which repel electrons from the n channel down into the substrate widen the depletion zone, instead of narrowing it, as in p-n junctions?


    Thank you very much! I hope my question is adequately clear :)
    Last edited: Aug 18, 2011
  2. jcsd
  3. Aug 18, 2011 #2
    The conduction mechanism is different for the two devices. For the diode, obviously there is still a gap of non-conductive material, its just that that gap is notably thinner when the diode is forward-biased. But there is still a gap and that's why you'll need at least about .6V (for silicon, anyway) of forward voltage to get the electrons to jump across that gap. The FET does not suffer from this because its conduction mechanism is different. When the FET is 'on', there is no gap electrons must be shoved across, so there is no extra voltage necessary to push electrons through.
  4. Aug 18, 2011 #3
    Thanks fleem! :)
    can you please explain why the depletion zone is widened in the FET case?
  5. Aug 19, 2011 #4
    The diagram doesn't show it, but the depletion region in the FET's n-channel is lacking in electrons because the nearby P substrate stole them. When a field is applied, it helps the P-substrate steal more electrons deeper into the n-channel, so the region of the n-channel that contains carriers (electrons) is narrower. In the diode diagram you can see how the P steals some electrons from the nearby N material.
  6. Aug 19, 2011 #5
    Thanks again!
    I'm still a little confused why the FET depletion region becomes wider when P steals electrons, but the diode depletion region becomes narrower when P steals electrons?
  7. Aug 19, 2011 #6
    A depletion region is a region in which carriers have been removed so that there is less conductivity. For both the FET and the diode, the depletion region becomes wider when the the P steals electrons (and N steals holes).
  8. Aug 19, 2011 #7
    but according to the first diagram, the depletion region becomes narrower when the P steals electrons in the diode?
  9. Aug 19, 2011 #8
    The depletion region is the light-blue area down the center of the crystal between the N and P. the diagrams show charges in that region (pluses and minuses). The region is wider on the right diagram because there the P material has more electrons and N has more holes. On the left, the depletion region is thinner because the P has fewer electrons and the N has fewer holes.
  10. Aug 19, 2011 #9
    thanks again! but-
    in the left diagram, the negative terminal is connected to the N material and depletion zone is NARROWER
    in the MOSFET the negative terminal is connected to the N material and depletion zone is WIDER
  11. Aug 20, 2011 #10
    Ah I think I see the confusion now. Maybe your question can be reworded: Why does the P region steal MORE electrons from the N region when the P region has a negative charge (and vice versa for the N region)--it seems its negative charge would cause it to steal fewer electrons? The answer is, I think, that I shouldn't have stressed so much that the P "steals electrons" from the N. It does, and it is especially apparent when there is no battery connected, but the battery (when connected as shown in the right diagram) steals even more carriers from the both the P and N. The negative terminal on the battery shoves electrons into the P, filling the holes and thus removing those carriers, and vice versa for the N region.
  12. Aug 20, 2011 #11
    haha this is confusing...thanks for you help!
  13. Oct 28, 2011 #12
    Hi everyone,

    I was wondering where i could find information relating to depletion zone thickness for silicon p-n junctions. Am doing it for a project on solar cells.

    Also, what actually happens to the thickness of the depletion zone when the solar cells are active in closed or open circuit condition,would it be different?

    btw would anyone know where to find values for diffusion lengths also?

    Thanks lots
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