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In a gaz, the ionisation creates pairs of electron and ion. Why in the semiconductor, during an ionisation, there is no creation of an electron and an ion ? Thank you

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First, because in the crystal, electron states (either in the conduction band or the valence band) aren't associated with individual nuclei. So when an electron is promoted to the conduction band, we can't say that any specific atom within the crystal is the one that "lost" that electron.

Second, we can't say an atom is ionized, because the electron didn't escape from the crystal, it only moved to a higher energy band within the crystal. The material as a whole remains neutrally charged, so there are no ions. It's analogous to the scenario where a hydrogen atom has its electron excited to the p orbital instead of the s orbital. The atom is still neutral, so it's not considered an ion, just an atom with an electron in an excited state.

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  • $\begingroup$ All right. Thank you for these useful explanations. So when the incoming particle ionized the material in the depleted region, where is coming the electron : from the neutral atoms or from the ions, or from both ? And is it coming from the doped atoms or from the non doped atoms ? Thank you $\endgroup$ – Mathieu Krisztian Sep 13 at 17:03
  • $\begingroup$ In the case of a photodiode, the electron is coming from the valence band. It wasn't and isn't associated with any particular lattice site. It isn't coming from doping sites (remember for photodiodes we often use a p-i-n structure with no doping in the active region). $\endgroup$ – The Photon Sep 13 at 17:10
  • $\begingroup$ Thank you so much ThePhoton : every thing is clear now. $\endgroup$ – Mathieu Krisztian Sep 13 at 18:42
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A hole is just a model for an ion surrounded by non-ionised atoms. That means that an electron from a neighbouring atom can jump in and turn the ion back into a neutral atom, but the neighbouring atom then becomes an ion. So it looks like a "hole" has moved from the original atom to the neighbouring atom.

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  • $\begingroup$ Very interesting. Ok, so if I understand: this means that for the ionization of a semiconductor junction, the incoming particle takes an electron from a neutral atom in the depleted region. In particular, the incoming particles does not take an electron from the ions of the depleted region. But actually, it looks like this expression of electron/hole is playing with words : when the incoming particle takes an electron from a neutral atom, the neutral atom transforms to a ion, so there is really a electron/ion creation. It is only playing with words, isn't it ?. Do you agree with my summary ? $\endgroup$ – Mathieu Krisztian Sep 13 at 15:40
  • $\begingroup$ thank you. All is clear now. $\endgroup$ – Mathieu Krisztian Sep 13 at 18:42

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