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Highery energy photon on lower band gap semiconductor

  1. Sep 12, 2008 #1
    Can a semiconductor absorb a higher energy photon than its Band gap Eg and make a transition from valence band to conduction band? For a example commonly used IR detector is InAs (semiconductor) whose Eg is 0.354 eV, if a blue light falls on it, will it able to absorve that energy and make a transition from valence band conduction band? If it is able to do that then how will we be sure that it is detecting only IR not any other EM waves? And if will not absorve higher energies will it be transparent to visible portion of light?
     
  2. jcsd
  3. Sep 12, 2008 #2
    What you're really asking goes deep into the device physics of a photodetector. For example, the image sensors on most cell phone are made of silicon but clearly they're designed to recognize the entire visible spectrum. I'll just deal with the basics for now....

    To answer your question, yes, if you hit a semiconductor with with a photon of energy greater than that of the semiconductor, it will pump an electron to the conduction band. It's important to note that the electron will initially be pumped above the conduction band edge due to the extra energy. Usually the electron will then thermalize back down to the conduction band edge.

    I initially meant the above description for a direct band semiconductor, but it easily generalizes to the indirect case.
     
  4. Sep 12, 2008 #3

    ZapperZ

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    Actually, this is not necessarily the only case. One can get photoemission from semiconductors as well, whereby the excited electrons escape to the vacuum level and leave the bulk material. In fact, in general, photocathodes with the highest quantum efficiency (certainly higher than metals) are semiconductors.

    Zz.
     
  5. Sep 19, 2008 #4
    I also have that kind of question.
    I think it really does. But absorbtion efficiency may be depend on the light. The absorbtion efficiency is larger when the wavelength of the light is close to the bad gap of the semiconductor.
     
  6. Sep 19, 2008 #5

    ZapperZ

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    Why?

    Zz.
     
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