Monday, May 31, 2010

Lasers and LEDs


Comments, questions and discussion are encouraged.

Why n+ to p to p+ ?
Where is the p-n junction?
Where is the light emitted?

What role(s) does the (GaAl)As play?
How does it help?
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4 comments:

  1. UnknownMay 31, 2010 at 9:46 PM

    Is the graded transition from n+ to p+ for confinement? It also needs to support inversion and internal refraction.

    Inversion appears to be taken care of, since apparently there is a significant population of electrons in the CB of the n+. Is this because the material is really heavily n-doped? Like n+++? Or is the material just n-doped, but "pumped" so as to populate the CB like that?

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  2. dirtbirbJune 1, 2010 at 1:03 AM

    Ethan -
    I think n+ means n+++, like it's doped way higher than the p region. I would guess that it doesn't have to be pumped in any way.

    Also, did you mean that the heavily doped n+ material puts the p material in inversion? I think that seems likely, given the high concentration of electrons in the p-type material that the graph shows.

    -
    Anyway, in response to Zack's questions -
    I'm guessing the n+ to p to p+ is to create a sort of a trap that forces electrons to hang around in the p section, making it more likely that they'll recombine there and emit a photon of the desired wavelength. Also, it seems like it would ensure that when one electron drops down, plenty of others are around to follow suit and drop down too via stimulated emission, like in a traditional laser.

    I think both junctions there could be seen as sort of a p-n junction, right? As far as I know, it's all a relative thing. If you have to pick one, depending on how you define it, you could argue either way. You could say it's the p to p+ junction since the inverted p-type material looks like an n-type, or you could argue it's the n+ to p type simply because separately, the n+ is n-type and the p-type is, well, p-type, and putting those together is the definition of a p-n junction.

    Finally, I'm assuming that light is emitted in the p region - that is, after all, where the image shows photons coming from!

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  3. AnonymousJune 1, 2010 at 11:29 AM

    I think you guys are on the right track. An added benefit of forcing electrons to drop only in the p region is that they will all be "about" the same energy and the emitted photons will all be "about" the same wavelength. Making coherent light much easier to attain.

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  4. EmmaJune 2, 2010 at 9:25 PM

    It seems like the n+ p junction starts the boundary for the overall junction of the whole system. and the p p+ junction I think acts as the other boundary of the overall junction because relative to p+, p seems like a donor on a local scale. So most of the photons will probably be emitted from recombinations somewhere in between those two boundaries which would be in the p-type material.

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