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From what I know, electrons that are excited can move into the conduction band, but how and what causes electrons to be excited? Heat/light seem to make sense as to something that can 'excite' the electron, but I am not completely sure.

Does the location of the electron matter at all, or can an electron at the lowest energy of the valence band be excited to the conduction band (even if there is an acceptor defect above it, could it possibly skip that and move straight to conduction)?

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There are many processes that may excite electrons into the conduction band:

  • light (even if we do not illuminate the semicodnuctor, there is usually some background radiation)
  • strong electric field (Zener tunneling)
  • collisions with other electrons, in which they may exchange energy
  • scattering particles, such as neutrons against the semiconductor

and others.

If nothing special is done in terms of forcing excitations, it is usually background radiation that causes the excitation. In statistical mechanics terms the semiconductor can be thought of as a subsystem in thermal equilibrium with its surroundings, which enables us to use the Boltzmann/Fermi distribution without thinking of the exact processes that cause the thermal equilibrium (which are usually neglected in statistical physics, but whose existence is always implied).

Let me note that heat is not the same as light, but rather a general measure of energy exchange. In this case it is indeed light (meaning generally electromagnetic radiation, not necessarily visible spectrum).

The probability of transition from this or that place in the valence band is governed by the energy conservation and the selection rules (i.e., the magnitude of the matrix element for the specific type of excitation mechanism).

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  • $\begingroup$ I have never heard of this background radiation theory for electron/hole generation. It doesnt sound correct. Thermal energy most certainly can excite electron/hole pairs. In cryo systems you can substantially reduce the background radiation to effectively eliminate 1.1 eV and up photons then heat your substrate and you will generate all the electrons you would expect from heat alone. Do you have a reference for the background radiation claim? How would you generate carriers deep within the material? $\endgroup$
    – Matt
    Commented Jun 1, 2021 at 19:28
  • $\begingroup$ @Matt Thetmal energy and heat are general thermodynamic terms for energy that could be of different nature, including radiation. It seems to me that I have rather carefully described the conditions when radiation would play the main role - I don't see any contradiction with you proposed experiment (and even in this experiment heat might be radiation). You can look up black body radiation for more information. $\endgroup$
    – Roger V.
    Commented Jun 1, 2021 at 20:06
  • $\begingroup$ Yes, Im familiar with blackbody radiation, thats why I was talking about cooling your environment. I still dont see how you can claim its electromagnetic radiation generating electrons and holes. You can construct situations where you dont have photons with sufficient energy to do so. But you will still get your electrons and holes. Do you have any sources about this specifically? Everything I have ever read about this indicated it was kinetic energy. Of course photons can generate carriers, but typically that gives you more carriers than in the thermal case. $\endgroup$
    – Matt
    Commented Jun 1, 2021 at 20:14
  • $\begingroup$ You have not carefully described the conditions where radiation plays the main role. You said "in this case" and this case seems to be the normal, nothing special, everyday block of semiconductor case. $\endgroup$
    – Matt
    Commented Jun 1, 2021 at 20:16
  • $\begingroup$ Note that semiconductors do not behave remotely close to a blackbody. Also, if you are about to say the material itself is generating these photons which go on to excite other carriers I would ask where the energy came from to do that in the first place. $\endgroup$
    – Matt
    Commented Jun 1, 2021 at 20:18

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