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Donor Impurities and the Forbidden region

  1. Sep 26, 2015 #1
    Hello,

    I am doing my prep work for the 'Hall Effect' experiment that I will be doing next week in labs and have a question regarding the forbidden region between the valence band and the conduction band. I haven't taken any formal solid state physics, but this is 2nd year lab, if that helps.

    The description of band theory for conductors and insulators is easy to understand, with the fermi level being in the conduction band for conductors, allowing electrons to be in the conduction band when in the ground state, and the fermi level being in the valence band for insulators.

    The energy gap between the conduction and valence band is small for semiconductors, which makes their properties highly temperature dependent. We dope the material in order to make it more useful.

    N-Type Extrinsic semiconductors are doped with donor impurities that contribute electron energy levels high in the band gap and shifts the fermi level. It is now easy to elevate these electrons into the conduction band with an applied voltage, but I thought this 'band gap' was the forbidden region.

    What is missing/wrong with this understanding?
     
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  3. Sep 26, 2015 #2

    ZapperZ

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    The band gap is a "forbidden" region only for the parent semiconductor. The dopant has a separate energy level.

    Zz.
     
  4. Sep 26, 2015 #3
    Thanks.

    So, am I correct in thinking that the adding of an impurity effectively changes the forbidden region? Electrons from the parent semi conductor can now be 'promoted' into the energy levels provided by an acceptor, so this part of the band gap no longer seems to be 'forbidden' in the sense that electrons are not allowed to populate it.
     
  5. Sep 26, 2015 #4

    ZapperZ

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    I'm not exactly sure what you mean by "changes the forbidden region". The band gap remains the same (i.e. as in the size). The dopant simply has an energy state that happens to fall somewhere in the gap region, and the non-zero temperature allows it to add charge carrier to the semiconductor.

    Zz.
     
  6. Sep 26, 2015 #5
    I was imagining that this new energy level became the boundary for the valence band, but I can see that this is mistaken.

    The gap stays the same size, and the dopant adds an energy level somewhere in the gap, making the energy required to reach the conduction band (in a donors case) less, and provides an easy route for electrons to move from the valence band to the new energy level (in an acceptor case) creating holes, since it no longer has to jump all the way to the conduction band.
     
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