Certain questions about diodes

Discussion in 'Homework Help' started by blazedaces, Dec 5, 2008.

  1. blazedaces

    Thread Starter Active Member

    Jul 24, 2008
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    Alright, so I'm doing a lab report that is generally about LED operation. I've accumulated some data, obtained IV-characteristics, and the emission spectrum.

    I'm having trouble answering the discussion questions though. The two discussion questions are:

    1) Are the LEDs photodectectors as well?
    2) Determine what material (compound or elemental semiconductor) are the LEDs made of? Justify your answer

    I'm having trouble finding enough information on the subject (another words, I've already tried simple google searches). Does anyone know, or can point me to somewhere where I can obtain the information on, how to answer these questions?

    -blazed
     
  2. blazedaces

    Thread Starter Active Member

    Jul 24, 2008
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    Alright, so according to this:

    (Source)

    Is it a valid conclusion to state that ALL LED's are technically also photodiodes to a degree?

    -blazed
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    As far as the material goes, you will have to rely on each manufacturers' information about every LED. That is another way of saying that many different materials may be found in LED's, although for every color, the list is fairly small.
     
  4. steveb

    Senior Member

    Jul 3, 2008
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    I'm note sure, but it sounds like the question is guiding you to think about direct bandgap semiconductors versus indirect bandgap semiconductors. I would recommend you do some searches on those terms.

    Just as an example, it is known that silicon based semiconductors are not good LED/Laser materials because they are indirect bandgap materials, and require a phonon interaction in order to generate a photon. Since this is a low probability event at room temperature, silicon is not a good optical emitter. However, silicon is just fine to use as a photodiode, as I'm sure you are aware. On the other hand GaAs semiconductors are used for both LED/Lasers and photodiodes, since this is a direct bandgap material.
     
  5. bertus

    Administrator

    Apr 5, 2008
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  6. steveb

    Senior Member

    Jul 3, 2008
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    I think that is a reasonable statement. Clearly, an LED will not perform anywhere near as well as a photodiode since it is not designed for that purpose, but the basic operation would be essentially the same.

    Very often photodiodes are doped to create a large intrinsic region between the p and n sections of the photodiode. This is called a PIN diode or PIN structure for obvious reasons. The large intrinsic layer allows a large photon collection region where efficient absorption can take place.
     
  7. blazedaces

    Thread Starter Active Member

    Jul 24, 2008
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    Thanks. I have another question about intrinsic regions. I know p refers to positive (doped to have more holes), n is negative (doped to have more free electrons)... so what is an intrinsic region exactly? Is it doped to accept more electrons and holes?

    -blazed

    Edit: and thank you everyone else for all your informative help and links...
     
  8. steveb

    Senior Member

    Jul 3, 2008
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    The intrinsic layer is basically a region of low doping that is somewhat like the pure semiconductor crystal in terms of the optical absorption properties. When a photon is absorbed in this region, a hole/electron pair is created and the reverse bias voltage produces a large electric field that sweeps most of these charges out of the region before they can recombine.
     
  9. blazedaces

    Thread Starter Active Member

    Jul 24, 2008
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    I re-read the lab theory/introduction and found this is indeed the case. Thank you.

    Thanks for the insight. I do also recall from class mention of Intrinsic AND Extrinsic silicon. This is my first electronics class (not exactly my track) so the dots are only starting to connect.

    -blazed
     
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