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Valence band electrons in semiconductor may jump to conduction band if appropriate energy is applied in form of (I found the point from Samares' answer in this link and it also makes sense):

  1. Thermal energy
  2. Photon energy
  3. Electric field

According to this link, "Red photons of light carry about 1.8 electron volts (eV) of energy, while each blue photon transmits about 3.1 eV.".

So, if I place a silicon solid (that has a Silicon-Silicon lattice with each Silicon sharing its valence electrons with 4 other Silicon) in a well lit room, then can I say that majority of the valence band electrons (that can) will jump to conduction band?

Given that from this link, Silicon has a band gap of 1.1eV, it seems to me that the able electrons would get promoted to conduction band in a well lit room (natural sunlight thru windows or an electrical lamp). This is because the minimum photon energy from visible light is more than the band gap.

My confusion comes from certain points

  1. The band gap mentioned above might be for one isolated atom and not for an atom in a lattice.
  2. If the point above is wrong and the band gap given is indeed for an atom in a lattice then is it for all 4 valence electrons? I am inclined to think that when one electron will jump, the band gap for other 3 may increase.

So, is it correct to say that in a well lit room we will have conduction band filled as much as possible with valence band electrons, even at a fixed temperature (temperature variance won't matter then)?

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  • $\begingroup$ If all valence electrons move to the conduction band the crystal instantly vaporizes. Not very likely. Do you mean any valence electron? $\endgroup$ – my2cts Nov 20 '20 at 10:20
  • $\begingroup$ Yes, I mean the majority (not all) that can make a jump. I edited the question. Thanks a lot for pointing it out. $\endgroup$ – Prasanjit Rath Nov 20 '20 at 10:34
  • $\begingroup$ If the majority of valence electrons is kicked out of their orbit, the silicon will still instantly vaporize. This will take something like, let's see for 28g of Si 2 valence electrons per atom 1.1 eV, about 1.3x10$^24$ eV = 100.000J. Enough to bring one cubic meter of water from 0C to boil. This you want to dump on 28 g of Si just by exposing it to daylight? This question needs more focus and clarity. $\endgroup$ – my2cts Nov 20 '20 at 13:05
  • $\begingroup$ That is why I mentioned majority "that are able to", I mean if we give sufficient energy, valence electrons will go to conduction band. Right? Let this be statement 1. Now, my statement 2 is if kept in a well lit room, then will those electron that can go to the conduction band always stay in the conduction band. If statement 1 is wrong then I think my whole question falls. But if it is true and some electrons can go to that band, then will they stay in conduction band in a well lit room due to photons? $\endgroup$ – Prasanjit Rath Nov 20 '20 at 13:24
  • $\begingroup$ I apologize that the question is not clear because your comments and answer below gave more clarity than that at posting question time. So, my question is meant to ask that in a well lit room, due to photon energy, will the electrons (that can jump if excited) jump? $\endgroup$ – Prasanjit Rath Nov 20 '20 at 13:28
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Firstly not all photons of the required wavelength will generate a carrier pair, this also depends on things like the penetration of the absorber material by the specific photon, determined by the absorption coefficient. I would recommend the website pveducation.org which is a good resource for this sort of thing. Also I'm not sure whether this is what you mean, but the electron lifetime is quite short, so it would be impossible to excite all electrons at once.

To address your points of confusion:

  1. The band gap varies throughout the lattice based on the momentum - the commonly used band gap is the shortest energetic distance between the valence and conduction bands. Hence in an indirect band gap semiconductor a phonon (lattice vibration) is also required for absorption. An atom doesn't technically have a 'band gap', as it is just between the HOMO and LUMO.

  2. As an electron is moved into the valence band, the proton charge is spread over fewer electrons, so the remaining ones from that atom would be harder to remove (this is easier to visualise in a single atom). Also the silicon electrons are involved in bonding, so removing all 4 would collapse the structure.

I hope this helps a bit.

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It is not clear what you ask but if you ask if any valence electron can be excited to the conduction band by exposure to say daylight, the answer is no. Blue light has an energy of 3 eV, so it can only excite valence electrons up to 2 eV below the top of the valence band. The valence band is much wider than 2 eV.

To excite all valence electrons at the same time requires quite a large amount of energy and any attempt will vaporise your Si crystal instantly.

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