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Where does the energy in LED come from?

  1. Aug 19, 2015 #1
    I know that the electrons in the N-type are in the conduction energy level, while the holes in the P-type are in the valence energy level, so when they recombine light is emitted.

    What I don't understand is where that energy comes from. The electrons in a copper wire can freely flow from the valence to conduction level (and vice versa) anytime, right? So how is it that the electrons can just enter into the N-type at the conduction band and exit the P-type at the valence band? The .7V drop is to do with the depletion region, right? It has nothing to do with the light that's emitted?

    Thanks
     
  2. jcsd
  3. Aug 19, 2015 #2

    davenn

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    it comes from the power supply
     
  4. Aug 19, 2015 #3
    Yes, there is a very brief spontaneous movement of charge. Where the p-type and n-type material meet some of the free electrons on the n-type side will jump across and combine with the hole on the p-type side. They do this because full electron shells have lower potential energy (same reason why chemical bonding happens)[EDIT: I mean full octet]. But the charges in the nuclei stay behind. The result is that an electric field develops that opposes further spontaneous movement of charge. Why can't you hook up a wire and drain the charge imbalance back to where it belongs so that the current can go round and round? Because that would require applying some energy to pull the electrons back out of the valence holes. Most of physics comes down to nature's desire for lower potential energy ("entropy tends to increase") and the ultimate conservation of energy (and momentum, and a few other things).

    Search YouTube for how diodes (light emitting or otherwise) work.
     
    Last edited: Aug 19, 2015
  5. Aug 21, 2015 #4

    Henryk

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    Yes, you are correct. Semiconductors are material that have (at zero absolute temperature) a fully occupied valence band separated by an energy gap from the empty conduction band. At room temperature some electrons get thermally excited into the conduction band leaving holes in the valence band. Semiconductors can also be doped either with electron-donor atoms (N type) or electron-acceptor atoms (P type).
    The LED light is created near a N-P junction and comes from N region electrons pushed into the P region in the conduction band recombining with the holes in the valence band there and vice versa. The energy of the photon is equal to the energy gap of the given semiconductor +/- thermal energy of the electrons and typically is more than 2 eV.

    The 0.7 V is a typical forward voltage of a silicon diode. For most LED material, the forward voltage is larger but not quite equal to the photon energy. Extra energy comes actually at the ohmic contact with the leads.

    Metals can't produce light because they have a different electronic structure. Simple metals, like sodium, copper, gold, have the one, half-occupied energy band. The atoms of these elements only one valence electron and when they form a solid, the valence level splits into a band with room for twice as many electrons as there is atoms in the solid. That's why it is only half occupied. You can accelerate electrons there by field, but they quickly lose the energy due to collisions. They can never gain enough energy to be able to emit a photon of a visible light.
     
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