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Depletion zone and current in forward-biased PN junction

  1. Jan 30, 2016 #1

    goodphy

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    Hello.

    I've learned that in PN junction, forward-biasing on it pushes holes in P-type and electrons in N-type toward the junction so depletion width is reduced. Is it due to that in N-type side(P-type side), pushed electrons (holes) are recombined with holes (electrons) in depletion zone?

    And what is true identity of current in forward-biasing? In this bias, I guess there are two kinds of current; diffusion current by charge carrier diffusion due to carrier concentration imbalance at the junction and drift current by drifted carrier pushed by external field of biasing. Is forward-biasing current consisted of both types of current or one of them? Here, I'm only thinking about majority carriers but if there are also contributions from minority carriers, I'll appreciate if you also cover this.
     
  2. jcsd
  3. Jan 31, 2016 #2
    The drift current is caused by the built-in electric field over the depletion layer. It hardly changes with biasing.
     
  4. Feb 2, 2016 #3

    Henryk

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    Let's go step by step.
    An n-type semiconductor is neutral. The charge of electrons in the conduction band is equalized (on average) by the positive charge of the donor atoms. Similarly the positive charge of the holes in the p-type semiconductor is neutralized by acceptor atoms.
    If you bring n and p type semiconductors together, the electrons from the n side will diffuse to the other side due to concentration gradient leaving unbalanced positive charge of the donor sites - depletion layer. This process will terminate when the electric field created by the charge of the donor sites will create a potential barrier counteracting the diffusion flux due to concentration gradient. A simple math can show that the width of the potential barrier is proportional to the square of the width of the unbalanced charge (that is the width of the depletion layer) times the density of the unbalance charge.
    Biasing the p-n junction means changing the potential barrier. Since the density of the unbalance charge is fixed at the manufacturing level (that's the donor or acceptor concentration), the only way to change the potential barrier is to change the width of the depletion zone. Forward biasing means reduction of the potential barrier that, in turn, implies the reduction of the width of the depletion zone.

    The answer to this question is, no, they do not recombine with anything. They just neutralize some of the donor site charges to reduce the potential barrier.
    The answer to this question is, both. In a typical semiconductor, diffusion and drift currents are usually similar in magnitude. The net current is the sum of both.
    In a p-n junction without external bias, the drift current due to the depletion layer field ( this is really a strong field) is exactly balanced by the diffusion current due to the gradient of concentration. Applying an external bias, changes that balance and you get a net current through the junction. In the case of the forward biased diode, you reduce the drift current and the net current becomes dominated by the diffusion of majority carriers. Minority carries concentration is so low their contribution is negligible.
     
  5. Feb 3, 2016 #4

    goodphy

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    I appreciate your very helpful comments!

    Yeah, the depletion zone is actually created with charged immobile ions as mobile charge carriers are eventually recombined at destination side during establishing zone.

    For N-type semiconductor side in forward biasing, electrons are pushed toward the junction so they screens original depletion zone electric field. Of course they can be recombined with ions (donor) in the zone as time goes on but thermal energy makes combined electrons go back to conduction band so we can say they're practically not recombined in the zone. As biasing becomes stronger, more screening occurs and zone is apparently reduced more. When biasing is strong enough that zone becomes very thin, thermal electrons have so much change to cross over the very weak barrier and it is true nature of current of forward-bias diode!

    Thanks again. You makes me very clear and skyrocketing happy!!!!
     
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