Question for mechanism of depletion zone reduction and current in forward bias of PN junction.

Discussion in 'General Electronics Chat' started by Dong-gyu Jang, Jan 31, 2016.

  1. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Hello.

    In forward bias on PN junction, it is said that depletion zone width is reduced. What is detailed mechanism of this reduction? In this bias, holes in P side and electrons in N side are pushed toward the junction and these carriers are recombined with opposite carriers existed in the zone so width is reduced?

    And in sufficient forward bias, current can flow. What is this current? Is this diffusion current by carrier diffusion due to carrier imbalance around the junction or drift current by pushed carriers due to external bias field?

    I feed hard to accept 2nd scenario as external field has double plays; the depletion zone reduction and making current.

    Please give me details of what is really going on in the biasing.
     
  2. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Look at the definition of drift current: its due to the external force applied. So the current that is allowed to flow in forward biasing must be drift current.

    Why do you find it hard to believe that the external field can do two things? Just think of a power supply: it provides a voltage and will supply current when connected to a circuit. As long as the current asked of it is within its rating it will maintain its voltage. So hook it up to a diode (with current limiting resistor) and the voltage will provide the forward biasing to shrink the depletion zone, and thus current will flow from the power supply.
     
  3. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Am...I actually thought forward-biasing make depletion zone reduction in "someshow" and I want to know what actually this "somehow" is. And I've read several PN junction articles telling that depletion zone is shrunk to be very thin with sufficient forward biasing current can diffuse this thin barrier quite easily. In addition, I guess drift current is linearly proportional to the applied voltage, however, diode current is has very high nonlinearity so it may not be considered as drift current.

    In this view, I don't see drift current, I only see diffusion current in forward bias current. That's the point I have this question. Is there drift current contribution in forward bias current?
     
  4. Brownout

    Well-Known Member

    Jan 10, 2012
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    The depletion zone is a result of minority carriers crossing the junction, P carriers move to the N side, and visa-versa. This sets up an internal E field across the junction, from the N <P minority carriers> side to the P <N minority carriers> side. Once the E field grows enough to prevent further migration of minority charges, the PN junction is in equilibrium. The bigger the E field, the bigger the depletion region. Now, when you apply an external forward voltage, majority charge in injected into the spaces near the junction where the depletion originally occurred, pushing out the minority carrier and filling the space. Also, an E field is set up which opposes the E field that originally resulted from the minority charge migration. This lowers the E field across the junction, which establishes a new equilibrium supporting a more narrow region, and thus narrows the depletion region.

    Current across the junction is drift current.
     
    Last edited: Jan 31, 2016
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  5. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Sorry, I flipped it. Current in forward bias is diffusion current.

    When you apply an external force (voltage) you're providing a supply of additional electrons to the N-type side, causing some of the neutrally-charged depletion region to become negatively charged. Same with holes on the P-type side if you want to look at it that way. You then have a more narrow depletion region.
     
  6. Veracohr

    Well-Known Member

    Jan 3, 2011
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    Well that's weird. The book I looked in says the overall circuit current is mostly diffusion current, while most internet sources I'm finding calls it drift current.
     
  7. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    What do you mean "neutrally-charged depletion region"? Is it actually meant to be zone near depletion zone? (For PN junction where P is left, you mean zone right to depletion zone?)
    9713, member: 109352"]Well that's weird. The book I looked in says the overall circuit current is mostly diffusion current, while most internet sources I'm finding calls it drift current.[/QUOTE]

    Yes
     
  8. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
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    Your explanation seems too complicated. I'm confused your use of minority and majority. I guess minority means for example, P-side electrons in depletion zone as electrons are minority in P-type semiconductor.

    And what do you mean "majority charge injected into the spaces near the junction where the depletion originally occurred, pushing out the minority carrier and filling the space"? In forward-biasing, electrons in N-side (majority carriers) are pushed by external field not minority carriers that are holes are pushed.
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    Current in the depletion region of a forward biased PN junction is diffusion current. I had that wrong in the last sentence of post #4.
     
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  10. Brownout

    Well-Known Member

    Jan 10, 2012
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    It only gets more complicated from here.

    If you don't know what majority and minority carriers are, I suggest you look it up. It's very simple.

    We need a place to start. Maybe after doing a some research, you can tell me what you know about PN junctions, then I can start from there.
     
  11. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Hello.

    I've several articles of PN junctions and as you can note, at least I know what majority and minority carriers are. Without knowledge, I should have not been able to reply your comments.

    It seems your comments has point to be noted; in forward-biasing, for example in N-type semiconductor, electrons (majority carriers) are pushed toward the junction by external biasing field and you said minority carriers (holes here) are pushed out. Which direction you mean? pushed out to the opposite side to junction (cathode terminal)? If you said this, this minority carriers looks more spread out so making depletion zone more wider? the majority is toward junction whole minority goes opposite and summation of these two effect eventually results in depletion zone shrinking in forward biasing...but...how this result comes from this summation?
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    You told me you were confused by minority carriers:

    Anyway...

    No, minority carriers don't spread out. The are concentrated near the junction. But "pushed out" was the wrong way to describe it. When majority carriers are injected (electrons into the N-type side of the junction) They neutralize the charge that was created by the minority carrier. for example, if there is a "hole" in the N-type, an electron "fills" the hole, lowering the barrier E field and narrowing the depletion region.

    I shouldn't have said the minority carriers was pushed out, I should have said they were neutralized by combining with injected majority carriers.
     
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  13. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
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    Thanks!

    I've gotten more clear and clear except for one thing; could you tell me why the minority carriers are not spread out? I guess they're also under external biasing field so they can move..
     
  14. Brownout

    Well-Known Member

    Jan 10, 2012
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    Where would they spread out? The depletion region is defined by charged regions where minority charges create an E field. Any free majority carriers that enter the depletion are swept away by this E field. In the bulk silicon, the E field is zero, so no minority charges are found there. The width of the depletion region is dependent on doping levels and temperature (and of course, external applied voltage)
     
  15. Dong-gyu Jang

    Thread Starter Member

    Jun 26, 2015
    100
    4
    Hello.

    I got very clear answer of why charges of depletion zone are not spread out in forward-biasing according to the field. It is simply because that the charges in the depletion zone are not mobile carriers. they are not either majority and minority carriers, they're just negatively or positively charged ions left behind when majority carriers diffuses and eventually gone by recombination during depletion zone establishment.

    For N-type semiconductor side in forward bias, electrons are pushed toward the junction (in energy, they get some energy to move into the zone ) so they screens electric field of original depletion zone . 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 (Fermi energy is very close to the 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 in the conduction band have so much change to cross over this "very weak barrier" and it is true nature of current of forward-bias diode (diffuse current). The same but opposite things happens in P-type side.

    In reverse bias, majority carriers are pushed away from the junction leaving behind charged ions so depletion zone is wider and its field becomes stronger.

    I guess this is accurate picture of PN junction:)
     
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