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What is the polarity of a Pn junction diode

  1. Feb 24, 2015 #1
    Hello all

    Textbooks teaches us that in the formation of Pn junction in between P and N type semiconductor a region is formed where free electrons of N diffused in P side and exhaust by the holes of P near the junction. Such a region becomes devoit of free charge careers and their are only ions in these region viz negative ions in P side positive ions in N side of junction. This forms the depletion region. And due to negative ions on P side and positive ions on N side a fictious battery is formed with negative terminal on P side and positive terminal on N side. And this the inbuilt voltage and will remain so no matter we use diode in forward biasing or in reverse biasing.

    This direction of polarity is in compliance with theoretical details we deal but while dealing with circuit problems texts are giving solutions by considering P side to be positive and N side negative.

    I am attaching the pictures attempting to use kirchoffs law in diode problems assuming P as positive.

    Any help will be highly appreciated.
    Thanks a bunch :-)

    Attached Files:

  2. jcsd
  3. Feb 25, 2015 #2
    Do I need to be more clear?
  4. Feb 25, 2015 #3


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    Staff: Mentor

    Yes, just the small matter: what is your question??

    Your attachments make this appear to be in the nature of homework exercises. So you need to provide your attempt at the exercise in question.

    In order for an ordinary diode to conduct current, an external voltage must make the P side more positive than the N side, by approximately 0.5v for a silicon diode.
  5. Feb 25, 2015 #4
    My question is that the P side should be negative and N side should be positive that is the potential barrier of diode should be such that P side(being negative) should be at lower potential and N side (being positive) should be at higher potential. In other words potential must increase as one goes from P to N. And this makes kirchoffs law to say that under forward bias the external battery and potential barrier both are acting in same direction, this is something I am finding not good.

    Yes such should be the case but that is coming possible only if we take P side of junction diode at higher potential and N side at lower potential only then will kirchoffs law will condition the current to be flow only when applied voltage is more than a minimum value.

    I request you to actually apply kirchoffs law on the problems I gave, you will find they took P at higher potential and N at lower potential.
    What I understand from text is that P should be at lower potential and N must be at higher. And here is the contradiction.

    Attached Files:

  6. Feb 25, 2015 #5


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    Staff: Mentor

    The barrier potential has a polarity with P negative and N positive. To cause forward conduction, the applied voltage needs to oppose this and overcome it, this means making the P material more positive than the N.
  7. Feb 25, 2015 #6
    Yes this should be but the workout problems(I attached pics in first post) use the reverse assumption( P side Positive and N side negative) in applying kirchoffs laws. Just try to actually apply the law please.
  8. Feb 25, 2015 #7
    Why they make diode equivalent circuit with P side positive(at higher potential i. e replace it with bigger plate of battery) and N side negative(at lower potential I. e replace it with small plate of battery)

    Attached Files:

  9. Feb 25, 2015 #8


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    Staff: Mentor

    The barrier potential is not a voltage you can measure by connecting a voltmeter to a floating diode. Think of it as a hill that electrons must overcome in order to pass through. So 0.5v of the external voltage drops across the diode when it forward conducts.

    So when it is conducting current you will measure 0.5v across it (with P more positive than N), in addition to a voltage of similar polarity due to bulk resistance of the material. The 0.5v you see with a conducting diode can be modelled as a voltage source, with P more positive than N because that's the polarity you measure with a voltmeter across the conducting diode.
  10. Feb 25, 2015 #9
    So should I say as one goes from P to N potential decrease? As the examples of the book is assuming.
  11. Feb 25, 2015 #10


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    Staff: Mentor

    That is what the voltmeter shows for a forward-biased conducting diode.
  12. Feb 25, 2015 #11
    How can be P side more positive than than N side. When the P side near the junction has negative ions and N side has positive ions.

    Attached Files:

  13. Feb 25, 2015 #12


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    Staff: Mentor

    That has been your question from the start.

    See those negative ions in the depletion region? To get electrons past them you need to give the electrons some energy (from an external source) to overcome the repulsion.

    Perhaps do some reading about the depletion region on wikipedia.
  14. Feb 25, 2015 #13
    My earlier concepts(like [1]positive charge cause positive potential and negative charge cause negative potential [2] potential decreases in the direction of electric field) and the theory of textbook(which is saying [1] their are negative ions near P side and positive ions near N side, [2] their is electric field from N side to side) make me difficult to accept that potential decreases as one goes from P to N.

    I know I must be missing something and I am feeling really good that I am being helped. It will more helpful if you give answer in more open way so that a poor fellow like me can know it.

    Regards :-)
  15. Feb 25, 2015 #14
    Yes electrons do need some extra energy to past them but it depends from which side(N to P or P to N) you want electrons to pass. In our circuit we want electrons to move from N side to P side. According to you the polarity of junction is positive at P side and negative at N side. And it is well known fact that in low to high potential direction electrons movement is favoured.

    So it will difficult for electrons(if P side has more potential than N side) to pass from P side to N side as electrons have tendency to move from higher to lower potential. But this potential(P more positive than N) will aid the electrons in passing from N to P side as our case is. And one can notice in forward bias current(positive charge) moves from P to N and electrons are moving from N to P side. So your assumption that P is more positive tha N side should in fact aid the current.

    Kindly make your own circuits and give direction of current.

    I read in Wikipedia this

    Departure of an electron from the N-side to the P-side leaves a positive donor ion behind on the N-side, and likewise the hole leaves a negative acceptor ion on the P-side.

    The uncompensated ions are positive on the N side and negative on the P side.

    Last edited: Feb 25, 2015
  16. Feb 26, 2015 #15
    Those ions cannot move, so they form the embedded electric field across the junction; that field is applied to free carriers ("electromotive force") and directed from N to P - therefore P becomes the positive terminal of the "battery".

    But, without forward current or pair generation near the junction, the "battery" is "disconnected" by the gap separating holes and electrons.
    Last edited: Feb 26, 2015
  17. Feb 26, 2015 #16
    I am also saying that field is from N to P, but the fundamental concept is that potential decreases in the direction of field which clearly means potential decreases from N to P, so P must be lower potential.
    I don't know what is happening with me that why I can't grasp it or what is the matter how can people here at famous physicsforums can say that potential in the direction of field attaing higher values. :-(
  18. Feb 26, 2015 #17
    The thing is, potential is somewhat tricky in a semiconductor... When electrons go from N to P, their potential energy increases as they "climb" over the junction - that's according to your potential. But in the P domain, they have to "fall down", across the energy gap, and all that additional energy is completely lost...
  19. Feb 26, 2015 #18
    I would like to know it in the simplest way, linking the issue in hand.

    When electrons go from N to P, what happens to potential in reality? Will like to keep in mind that the term potential is defined as the work done(by external agent without change in kinetic energy or negative of the work done by field) in moving POSITIVE charge between two points. So potential is conveniently defined for positive (test) charge, if it difficult for positive charge to move from side A to side B we say side B has higher potential. But if charge is negative(like electrons) if it is difficult to move from side A to side B, it would mean side A has higher potential.

    I do not understand what you are saying here. Sorry for my dump mind. :-)

  20. Feb 26, 2015 #19
    I fear that it may be taken as arguments or at least considered repeating my own(false) misconceptions but my heart knows it is only due to hunt for the truth. I am not understanding how can potential be more at P than it is at N side when it is clear the field of depletion region is from N to P.

    If this(P higher potential) is true I need a clear comprehending explanation for this. For I know electrons(negative charge) only feel difficulty in going to a region of low potential do the explanation that since electrons have to work hard in going from N to P will mean P has more potential is wrong for obvious reason(electrons require hard work to ho to rather low potential). Further how can we forget the fact that potential decreases on the direction of field an field is from N side to P side this further let me not accept that potential of P is higher than that of N.

  21. Feb 27, 2015 #20

    jim hardy

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    There's the rub.

    P and N describing material refer to doping.

    Positive and Negative in circuits refer to externally applied potential across a device.

    A diode whose P side is made positive with.respect to its N side by more than its barrier potential might conduct.

    A diode by itself held in your fingers has no positive or negative side.

    That unfortunate terminology confuses a lot of beginners. Maybe we shoulda called semiconductor doping D and R for Donor and Receptor, instead of P and N.
    P and N almost rhyme with Positive and Negative,
    but one applies to an internal property
    and the other to an external connection..

    its that simple, i think.
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