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indirect band gap semiconductor doubt ....

Discussion in 'General Electronics Discussion' started by munna007, Sep 18, 2010.

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  1. munna007


    Jul 28, 2010
    its said that indirect band gap semiconductors like silicon and germanium are not used as a light sources because they have more non-radiative recombinations rather than radiative ,which inturn decreases efficiency of optical source.

    but i want to understand how indirect band gap materials produce non-radiative photons.

    i have studied many books like keiser , senior on optical fibre .those all are not explanatory on this ....also googled for this ,but not helped from there.....

    hoping for help to all....

    Attached Files:

  2. Militoy


    Aug 24, 2010
    It sounds like you are researching Dark-Emitting Diodes. What level of the physics involved are you trying to unravel? It seems from your question like you already understand the difference between direct and indirect band gap materials. It follows that it’s the difference in K-vectors in the Brillouin zone of indirect band gap materials that prevent low-energy release of photons – since electrons are not able to shift from the conduction band to the high-energy valence band without a change in momentum. The photons start out the same in each case – but the change in momentum necessary to shift K-vectors “snubs” the photons in the indirect band gap material – so they are unable to escape the valence band. They’re not “special” photons – just ones that were prevented from escaping as light.

    If you are looking for a more in-depth explanation of LED physics – it’s way above my pay grade – and I suspect you’ll have to ask on a university physics BBS. Or – maybe there is an LED design engineer on this site that will be able to shed more light on the subject.
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Stranger things have happened ;)
  4. shrtrnd


    Jan 15, 2010
    Germanium originally used because it conducts at .3v
    Silicon conducts at .7v
    Silicon more likely to be damaged by static discharge (or EMP), when they talk about
    'hardended' electronics for military, they're talking about germanium devices, which are less prone to such damage.
    Doesn't help with question on light sources, but helps broaden the explaination of why
    germaniun or silicon used in electronics devices.
  5. NickS


    Apr 6, 2010
    Its been a few years on this but your question sounds to me like your asking how does a semiconductor like silicon conduct(excluding radiative). That can't be right? It seems you already have a firm grasp on the basics. If that is correct then the answer is that non-direct conducts the same as direct it just requires more energy to do so because of the indirect gap. Sorry if I underestimated the question.

    I was taught that the reason silicon has remained so popular despite its inherent conduction shortcoming is due to the superior properties of its oxide layer as an insulator.
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    My understanding (and take this with an appropriately large grain of salt) is that pretty much all semiconductors are able to very efficiently create photons with energy (and therefore frequency) determined by the bandgap.

    The problem is that most of these never make it outside the junction and are absorbed leading to heating effects and no light.

    The trick in getting more efficient LEDs is not so much in making them more efficient creators of photons, but to either create the photons where they can escape, or to make the material more transparent to the photons.
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