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L.E.Ds producing voltage without power supply except for white light incident on it

  1. May 4, 2005 #1
    hello - i feel confident that i understand the principles of how L.E.D's work. I can grasp the concept of the depletion zone and how this leads to conduction in one direction.

    But how does light from an ordinary filament lamp cause a voltage in an L.E.D to be produced when the L.E.D has no obvious power supply?

    I know of the principles of the photoelectric effect and realise it has something to do with this but need a concise explanation of how the voltage is produced.

    i have been discussing this with sum1 else and have come up with this explanation:

    light from the lamp when incident on the semiconductor in the L.E.D causes valence electrons in the n type semiconductor to be released into the conduction/ depletion layer. these are attracted to the p type semiconductor and they move into the 'holes'. this causes a drop in energy level for the electron which is called relaxation and causes light to be emitted.
    this process means that a current is flowing from the n to the p type semiconductor which means that a voltage is present in this mini - circuit.

    now the fact that white light has a low photon energy means that if it were the photoelectric efect that causes this process - the material in the L.E.D would need to have a low work function or sum other proerty that allows this to happen.

    anyone who can correct any details - add more information - or completely change the explanation to the question - please feel free

    thanks
     
  2. jcsd
  3. May 4, 2005 #2

    chroot

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    If passing a current through a diode creates light, then shining light on a diode should create current. Most physical processes, like this one, are symmetric.

    When you shine light on a diode, the photons kick electrons out of atoms, creating an electron-hole pair (this is the photoelectric effect). When this occurs in the depletion region (the area arround the p-n junction which is normally devoid of charge carriers), the hole is rapidly swept away to the p-type semiconductor, and the electron to the n-type semiconductor. This is a current.

    - Warren
     
  4. May 4, 2005 #3
    brilliant - will light be emitted during this process though?
     
  5. May 4, 2005 #4

    chroot

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    I would guess that the light-induced current would, indeed, cause the diode to produce some additional light, but it would be a very tiny fraction of the incident light and probably would be negligible.

    - Warren
     
  6. May 4, 2005 #5
    well the thing is - i have seen this experiment done and the light level produced by the diode seemed to be at a relatively high intensity that u would expect from an L.E.D in a circuit. so this would mean that the voltage is being created by a process that has a large amount of relaxation occurring. Does ur explanation allow for this - do u mean that the electrons released would move to the holes in the p type material.
     
  7. May 4, 2005 #6
    So ideally you would want monocromatic light of the same frequency as that that is normally emitted from the LED. That way, when you shine the light onto the LED it hits the gas contained inside and the atoms absorb the photons and the electrons transition up to higher orbitals. They then cascade down, and release a photon of equivalent wavelength which is what induces a current. Just trying to see if thats how they work.. I never really thought about it, and I've done very little work with electricty, only work funtions and general magnetism and ac circuits.

    Edit: the way my explanation goes, beacues the photons are quantized in energy, you would see light emitted from the LED as the electrons make their transitions.
     
  8. May 4, 2005 #7

    chroot

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    I don't see anything wrong with my explanation. No, the released electrons move toward the n-type, and the holes move toward the p-type. Remember that in a forward-biased pn junction, the p-type has the more positive potential, and the electrons are moving due to the externally-applied electric field.

    - Warren
     
  9. May 4, 2005 #8
    gases arent involved. it is a semiconductor consisting of two doped materials that form a p - n junction.
     
  10. May 4, 2005 #9

    chroot

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    There's no gas in a diode, and the rest of your explanation is pretty much entirely wrong. Please don't post on topics with which you are unfamiliar.

    - Warren
     
  11. May 4, 2005 #10
    no chroot. there is no electric field supplied. It is simply an L.E.D connected to a voltmeter. then a normal filament lamp is shone onto the L.E.D and a voltage is read on the meter - this increases with decreasing distance of the filament lamp to the L.E.D ie. increasing light intensity. i know that this will follow an inversely proportional relationship by the Intensity equation of a point source.

    so no power supply. just the movement of the released electrons causes the current flow and hence voltage created. how do u explain that?
     
  12. May 4, 2005 #11

    chroot

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    Sorry, I misread the question. If the pn junction is in equilibrium, then yes, the electrons will "roll down-hill" into the n-type semiconductor, while the holes will "roll up-hill" into the p-type semiconductor.

    - Warren
     
  13. May 4, 2005 #12
    Take it easy there man, I was asking a question not trying to prove someone wrong. Ya know ask, so then maybe I will understand..
     
  14. May 4, 2005 #13

    chroot

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    There wasn't a single question mark in your entire post. It sounded as though you were making a statement.

    - Warren
     
  15. May 4, 2005 #14
    wot explains the low work funciont if the material in the diode
     
  16. May 4, 2005 #15

    chroot

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    I'm not sure I understand why you think anything is special about the work function.

    - Warren
     
  17. May 4, 2005 #16

    the work function would be low because this preocess occurs in a l.e.d with visible light - how dus the material hav sucj a low work function
     
    Last edited: May 4, 2005
  18. May 4, 2005 #17

    ZapperZ

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    Because what you described is not the photoelectric effect, but rather a "photodiode" characteristics. What's the difference? A photoelectric effect is defined as the process whereby electron emission INTO THE VACUUM STATE occurs due to absorption of light. The problem you described is the formation of a CURRENT or a potential different IN THE MATERIAL due to absorption of light. The electrons are not released into the vacuum state, but rather went from the valence to the conduction band. This process simply overcomes the band gap, and has nothing to do with the work function, the latter is defined as the energy difference between the top of the valence band and the vacuum state.

    Zz.
     
  19. May 4, 2005 #18
    ok - u seemed pretty clued up about this subject: please could u give a concise account to this question - it would be greatly appreciated - even if it means repeating some of the details that have been said on this thread already - pls:

    Light from a filament lamp is incident on a l.e.d that is connected to a voltmeter only.

    1. explain how the voltage is generated in the l.e.d

    could u give as detailed response as possible pls - sorry to be so demanding but its just that i am to be examined on this topic on friday. thankyou so much
     
  20. May 4, 2005 #19

    chroot

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    Aha! Now I understand what ryan750 was asking. I was wondering how in the world the work function was relevant in the first place.

    - Warren
     
  21. May 4, 2005 #20
    its ok i have just emailed a source that is expert on the subject - it is a diode company and they should be able to explain all - thanks for all ur help
     
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