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Diode's Built-In Potential

  1. Oct 20, 2011 #1
    Hello,

    i was wondering for 1N4001 diode or zener diode has some built in potential, why cant they be used as batteries? I have some diodes lying around and I tried connecting to an LED, but it doesnt light up.

    I know this question seem dumb, any help will be appreciated!

    thanks!
    Yinx
     
  2. jcsd
  3. Oct 20, 2011 #2
    Why do you believe they have "built in potential"?
     
  4. Oct 20, 2011 #3
    doesn't diodes have built in potential (aka contact potential)??

    yinx
     
  5. Oct 22, 2011 #4
    The built in potential of a diode is a result of thermal energy. At a temp above absolute zero, 0K, the silicon crystal lattice vibrates. These vibrations have energy which separates electrons from the parent atoms leaving holes behind. This is thermal generation of electron-hole pairs, ehp.

    The electrons, e-, & holes, h+, move through the Si & cross the junction. On the n-side of the junction, holes accumulate & recombine w/ electrons, vice versa on the p side. This accumulation of charges results in a local E field near the junction, & the integral of E over the distance is the potential.

    In order to forward bias the diode, i.e. establish forward current, the built in potential must be considered. It has a polarity which tends to oppose current flow. The source powering the network must supply a potential large enough to overcome this potential.

    A good semiconductor physics text covers this in detail. It has diagrams which make it clear. That should be where you look for your answers. If you need clarification on what I've said, that would be fine.

    Claude
     
  6. Oct 23, 2011 #5
    Oh, contact potential -- that's a whole different thing. That refers to extremely low energy levels.

    Here is a good, simplified description of http://www.answers.com/topic/contact-potential-difference".
     
    Last edited by a moderator: Apr 26, 2017
  7. Oct 23, 2011 #6
    The term built-in potential is frequently used in electronics and isn't contact potential.
    Even though there is such a potential difference between the P and the N side, when we close a circuit no current flows. We feel disappointed but Thermodynamics feels very happy (if it were a person) because we haven't been able to get energy out of nothing.
    When we consider a closed circuit we realize the built-in potential isn't the only potential difference to account for. When you do it properly, you end up with zero electro motive force around the circuit and no current flows.
    However, we can build a thermal engine. Simply heat a thermocouple with a flame and close the circuit with another thermocouple submerged in ice water. You'll notice current flows and you get work as in any thermal engine.
     
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