I am wondering what actually happens when an electron moves in the valence band. As I understand an electron needs energy to free itself from an atom. Is it so that the electron gets energy to free itself from the atom, then moves to a new atom, and the loses the energy when it recombines with the new atom? If so, how does it initially get the energy?
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$\begingroup$ It can be supplied via electrical excitation (run a current through semiconductor) or by absorbing a photon for example. $\endgroup$– DakkVaderCommented Jan 27, 2021 at 9:48
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$\begingroup$ @DakkVader Thank you! So it is indeed so that when an electron moves in the valence band in a semi conductor, each time it moves it must "leave the valence band" to move to a new atom, and then fall down in the valence band in a new atom? $\endgroup$– user394334Commented Jan 27, 2021 at 9:51
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$\begingroup$ Both conduction and valence bands are states that extend across the crystal - electrons and holes are not tied to specific atoms. So, ignore the atoms and only consider the electrons and holes as 'free' (well, not free, but they have a whole band to play in without needing any help). $\endgroup$– Jon CusterCommented Jan 27, 2021 at 15:13
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$\begingroup$ @JonCuster Thank you!, so we can not say that when a hole move in the valence band, a covalent bond is first broken and then another covalent bond is formed? $\endgroup$– user394334Commented Jan 27, 2021 at 20:39
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1$\begingroup$ No, not at all. The valence band also extends throughout the crystal and is not associated with any specific atoms. $\endgroup$– Jon CusterCommented Jan 27, 2021 at 21:37
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3 Answers
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First consider a semiconductor. Electrons in the full valence band do not move . Only if one or more electrons are missing, that is, reside in the conduction band, the remaining ones can move. It is convenient to describe this as hole conduction. The conduction electron can also move until it recombines with a hole.
Indeed it takes energy to excite an electron from the valence to the conduction band, at least the band gap energy. This energy can be thermal. At room temperature there will always be a few electrons and holes around causing intrinsic conductivity. Optical excitation is also possible, as happens in solar cells. Electron-hole recombination can also be thermal or radiative, as in LEDs. Semiconductors are usually doped with impurity atoms causing them to have permanent conduction electrons or holes.
In metals it does not take energy to move, or rather, to activate an electron.
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$\begingroup$ Thank you for your answer. What I am wondering is in relation to your first paragraph. When we have hole conduction, do we then need to excite the electron to move? When a hole moves does the corresponding electron first get excited, then move, and then fall down to the valence band again? $\endgroup$ Commented Jan 27, 2021 at 10:08
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$\begingroup$ @user394334 No energy is needed as the total energy is independent of where the hole is located. $\endgroup$– my2ctsCommented Jan 27, 2021 at 10:23
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$\begingroup$ Isn't that something moving need energy to be accelerated? I am afraid your answer is not to the question, as for I takes "moves" as current. $\endgroup$ Commented Jan 27, 2021 at 10:40
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$\begingroup$ @Alchimsta If you have a question yourself feel free to ask it. $\endgroup$– my2ctsCommented Jan 27, 2021 at 11:15
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Electron travel as waves across the lattice in semiconductors. This is a quantum mechanical effect and to all purposes the image of the electron being stuck and one atom and jumping out is not correct.
When the electron is in an atom it has an amplitude to leak to neighbouring atoms. The fact that amplitudes are complex numbers do place a key role in this description and when you model the system this way, the result is that electrons travel like wavelet.
For more details see https://www.feynmanlectures.caltech.edu/III_13.html
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A band is an (allowed) energy state within which an electron can exist. As long as it remains within the valence band, it absorbs and emmits no energy. Energy is absorbed or emitted when there is transition between states. Therefore, to knock an electron off the valence band to a higher one (conduction band) energy must supplied to the electron.
