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V. simple question about diodes i-v graph.

  1. Oct 18, 2009 #1
    so i've seen this graph all over the internet, but nowhere seems to explain why the current after +0.6v rises exponentially and not just straight up. is it mainly due to the internal resistance of the diode or is there significant inductance/capacitance in there as well. whats the dominating effect(s)?

    I'm guessing at that voltages significantly higher than Vf its just the internal resistance (r=1/gradient), but the curve leading up to Vf, whats causing it?

    http://www.yegopto.co.uk/media_yegOpto/image/400%20x%20400%20images/volt_current%20graph.jpg [Broken]

    also, whats a typical internal resistance for a power diode?

    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Oct 18, 2009 #2


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    No, it is a microscopic effect. It has to do with the shape of the bands (the density of states) in the semiconductor; you can't really think of it in terms of "macroscopic" variables like capacitance etc.
    It is not too difficult to derive the correct expression using a band model and you should be able to find that derivation in just about any book on semiconductor physics.

    The fact that is rises exponentially is essentially just a consequence of the electrons obeying Fermi-Dirac statistics.
  4. Oct 18, 2009 #3


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    No the exponential relationship is not due to series resistance. It's true that bulk resistivity in the neutral regions can have a significant effect in some diodes at high current levels (making the I/V characteristic less steep than predicted by the exponential relationship) but no the exponential relation is not due to resistance. It's a lot more complicated than that. Start with this for example : http://books.google.com.au/books?id...resnum=2&ved=0CA8Q6AEwAQ#v=onepage&q=&f=false
  5. Oct 18, 2009 #4


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    The 0.6 volts is called the built-in potential and depends on the type of semiconductor material in the diode.
    And as f95toli implies, you would need to study the junction theory to see why the curve is exponential.
    Last edited by a moderator: Apr 24, 2017
  6. Oct 18, 2009 #5
    thanks guys.

    not so simple question after all then, it's going to take me a long while to understand all that.
  7. Oct 18, 2009 #6
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