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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?

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

    f95toli

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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

    uart

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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

    dlgoff

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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.
    http://en.wikipedia.org/wiki/Diode#Semiconductor_diodes"
    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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