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I thought this was a really well-produced video! It's difficult to communicate science to the public in an accessible way at the right level, and I think Derek

I thought this was a really well-produced video! It's difficult to communicate science to the public in an accessible way at the right level, and I think Derek does a commendable job. I really liked the LED explanation at the 4:00 mark. Can anyone who is familiar with semiconductor physics opine on how well this explanation models the reality?

quenix 3y ago
Yeah, all I can say is that is tracked my undergraduate semiconductor theory classes pretty well. More confirmation needed.
- ace2358 3y ago
  I would agree. Having said that I still think it was a bit wishy washy. The whole treatment of band gap energies I think is quite complicated beyond the simple diagram shown.
  - pfdietz 3y ago
    Also, I don't think the explanation for silicon was correct. As I understand it, the problem with Si is not that the bandgap is in the infrared, it's that it's an indirect bandgap semiconductor which suppresses emission of photons. Instead, the energy of recombination goes into heat.
    - ace2358 3y ago
      Thanks yes I think I remember my whole semiconductor education to be wishy washy (according to the lecturer) even at the undergrad level. Pretty complicated stuff and I think the ‘band gap’ model is a bit of a stretch of the quantum mechanics that is occurring.
empath-nirvana 3y ago
My 5 and 7 year old watched the whole thing with me and had lots of follow up questions for me.
- whycome 3y ago
  You're the dream mom/dad. A cool chance to show the importance of determination!
archontes 3y ago
Bachelor's in engineering physics (condensed matter experimental)/EE specializing in semiconductors here. The explanation starting at 4:00 is very accurate. When he talks about the electrons "feeling" the neighboring atoms, he's talking specifically about a result that follows from the materials being crystalline, that is, having regular ordered structure. The regular structure gives rise to a periodic potential. You plug that periodic potential into the Schrodinger equation and apply continuity conditions and translational symmetry to the wavefunction. Computing the solutions to the Schrodinger equation with those conditions reveals that there are allowed and disallowed energy levels, and also reveals the relationship between energy and momentum in the crystal lattice. You can step through this by reading the wikipedia page on the Kronig-Penney Model. This depends on the periodicity, which obviously can change depending on direction in a crystal. His explanation, and the result that "the" band gap is a single number, isn't dishonest because when we grow semiconductor devices, we grow them such that the crystal is oriented such that current flows in the desired direction, so that simple result holds true. Even his portrayal of the bands leaning down as potential/voltage is applied mirrors how potential change is shown in diagrams of semiconductor devices, see Streetman and Banerjee - Solid State Electronic Devices.
- cypherpunks01 3y ago
  That's great! Much appreciated, thanks :)
  - archontes 3y ago
    Do be careful, though. Some other folks here are saying, correctly, that this glosses over the "direct" or "indirect" nature of a semiconductor. I only very slightly alluded to this when mentioning the relationship between energy and momentum. Trying to make a long story short, it can be the case that in order to transition to another energy level, an electron also has to exchange momentum with something, usually the lattice in the form of quantized vibrations. Photons carry energy but almost no momentum, so an indirect semiconductor (one that requires both energy and momentum exchange for a transition to the conduction band) is usually an abysmal choice for optoelectronics.
01HNNWZ0MV43FF 3y ago
I liked it, though it bugs me a little when people equate infrared and heat. Infrared is light. Light can heat things, and hot things can glow, but "infrared is heat" isn't exactly right.
- Smoosh 3y ago
  I agree with you, but I think that for the general public you have to relate to what they experience, and thus intuitively know, and that is that heat “seems” to be different from light.
- amarant 3y ago
  I know basically nothing about physics, so sorry if this is a dumb question. The existence of infrared LEDs seems to indicate to me that infrared light can exist without heat. The existence of infrared thermometers seems to imply that hot stuff radiates infrared light, at least usually. So my question is, is there any case where heat does not cause infrared radiation? What are those cases? Some special materials? Special colours(perhaps outside the visible spectrum)?
  - elevatedastalt 3y ago
    That's a good question. All bodies above the temperature of absolute zero emit electromagnetic radiation across the whole spectrum (this is called Black-body radiation). Think of it as a mixture of different amounts of light of every possible wavelength. However, what the exact mixture is depends on the temperature of the body. As the body gets hotter, the 'peak' wavelength, i.e. the wavelength whose "amount" is highest in the mixture decreases. Objects at room temperature emit most of their energy outside of the visible spectrum, so they are not 'visible' in the dark. However, as you heat them up, the radiation mixture moves towards lower wavelengths, closer to infra-red. Heat it up further and things become red hot, yellow, blue hot and so on. Infra-red LEDs produce light of the the specific infrared wavelength through semi-conductors. They have nothing to do with the black-body radiation one associates with 'hot' objects.
  - mensetmanusman 3y ago
JohnFen 3y ago
I agree. Setting how LEDs work aside, I never really got how semiconductors worked, despite reading about it and talking with experts for years, until this video. I mean, I could explain how they worked in the same ways that they were explained to me, but I couldn't connect those explanations to a true physical understanding. But thanks to this, I finally actually understand. Also, the LED story was fascinating.
thirdhaf 3y ago
The explanation is really well done, it captures the essence of the Pauli exclusion principle without delving too deeply into the weeds. In my opinion the best part of the video is the explanation of the "hole" quasiparticle at 6:10 (I learned this as a pseudo-particle but will defer to Wikipedia [1]). While a great introduction to semiconductor behavior this does gloss over a very important detail namely direct vs indirect semicondoctors as some others have mentioned. In the video the detail that's glossed over relates to the nature of crystals, namely that they're highly ordered repeating structures but that they don't look the same when viewed from every direction. This means that there isn't a single band-gap but multiple ones depending on the direction of the crystal you're contemplating. At this point you may reasonably ask why the direction matters and now we unfortunately get deep into the weeds with quantum mechanics again. When a single photon is absorbed in the semiconductor system both momentum and energy must be conserved. The momentum of the photon for something like the Silicon bandgap is quite small (something like the equivalent of an electron traveling at 1500m/s) while the momentum of room-temperature conduction electrons is substantially faster [2] so as a very slight simplification transitions due to the absorption of photons are not accompanied by a change in momentum and so we only care about the band structure (and the accompanying free carriers) associated with a particular crystal direction. In particular in Silicon you have what's called an indirect bandgap, namely the minimum energy conduction band electrons have a different momentum from the valence band holes ([3]) and as a consequence while you can _absorb_ a photon in order to make a detector you cannot make it efficiently _emit_ a photon as an LED should (something the video got wrong). None of this matters for the heart of the video, which focuses blue LEDs in the GaN materials system which is definitely a direct bandgap material, however if someone does manage to create a manufacturable light emitter in pure Silicon expect an absolute revolution with regards to optical computing and photonics. (Not for lack of trying, this has been the holy grail for at least 20 years, possibly longer) [1] https://en.wikipedia.org/wiki/Quasiparticle https://en.wikipedia.org/wiki/Quasiparticle [2] https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch2-2.pdf https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chen... [3] https://www.iue.tuwien.ac.at/phd/wessner/node31.html https://www.iue.tuwien.ac.at/phd/wessner/node31.html
diedyesterday 3y ago
That explanation closely follows the outline of the equally good and very accessible explanation offered in the Halliday/Resnick/Walker's Fundamental's physics (11 edition, chap. 41)

quenix 3y ago

Yeah, all I can say is that is tracked my undergraduate semiconductor theory classes pretty well. More confirmation needed.

ace2358 3y ago
I would agree. Having said that I still think it was a bit wishy washy. The whole treatment of band gap energies I think is quite complicated beyond the simple diagram shown.
- pfdietz 3y ago
  Also, I don't think the explanation for silicon was correct. As I understand it, the problem with Si is not that the bandgap is in the infrared, it's that it's an indirect bandgap semiconductor which suppresses emission of photons. Instead, the energy of recombination goes into heat.
  - ace2358 3y ago
    Thanks yes I think I remember my whole semiconductor education to be wishy washy (according to the lecturer) even at the undergrad level. Pretty complicated stuff and I think the ‘band gap’ model is a bit of a stretch of the quantum mechanics that is occurring.

empath-nirvana 3y ago

My 5 and 7 year old watched the whole thing with me and had lots of follow up questions for me.

whycome 3y ago
You're the dream mom/dad. A cool chance to show the importance of determination!

archontes 3y ago

Bachelor's in engineering physics (condensed matter experimental)/EE specializing in semiconductors here. The explanation starting at 4:00 is very accurate. When he talks about the electrons "feeling" the neighboring atoms, he's talking specifically about a result that follows from the materials being crystalline, that is, having regular ordered structure. The regular structure gives rise to a periodic potential. You plug that periodic potential into the Schrodinger equation and apply continuity conditions and translational symmetry to the wavefunction. Computing the solutions to the Schrodinger equation with those conditions reveals that there are allowed and disallowed energy levels, and also reveals the relationship between energy and momentum in the crystal lattice. You can step through this by reading the wikipedia page on the Kronig-Penney Model. This depends on the periodicity, which obviously can change depending on direction in a crystal. His explanation, and the result that "the" band gap is a single number, isn't dishonest because when we grow semiconductor devices, we grow them such that the crystal is oriented such that current flows in the desired direction, so that simple result holds true. Even his portrayal of the bands leaning down as potential/voltage is applied mirrors how potential change is shown in diagrams of semiconductor devices, see Streetman and Banerjee - Solid State Electronic Devices.

cypherpunks01 3y ago
That's great! Much appreciated, thanks :)
- archontes 3y ago
  Do be careful, though. Some other folks here are saying, correctly, that this glosses over the "direct" or "indirect" nature of a semiconductor. I only very slightly alluded to this when mentioning the relationship between energy and momentum. Trying to make a long story short, it can be the case that in order to transition to another energy level, an electron also has to exchange momentum with something, usually the lattice in the form of quantized vibrations. Photons carry energy but almost no momentum, so an indirect semiconductor (one that requires both energy and momentum exchange for a transition to the conduction band) is usually an abysmal choice for optoelectronics.

01HNNWZ0MV43FF 3y ago

I liked it, though it bugs me a little when people equate infrared and heat. Infrared is light. Light can heat things, and hot things can glow, but "infrared is heat" isn't exactly right.

Smoosh 3y ago
I agree with you, but I think that for the general public you have to relate to what they experience, and thus intuitively know, and that is that heat “seems” to be different from light.
amarant 3y ago
I know basically nothing about physics, so sorry if this is a dumb question. The existence of infrared LEDs seems to indicate to me that infrared light can exist without heat. The existence of infrared thermometers seems to imply that hot stuff radiates infrared light, at least usually. So my question is, is there any case where heat does not cause infrared radiation? What are those cases? Some special materials? Special colours(perhaps outside the visible spectrum)?
- elevatedastalt 3y ago
  That's a good question. All bodies above the temperature of absolute zero emit electromagnetic radiation across the whole spectrum (this is called Black-body radiation). Think of it as a mixture of different amounts of light of every possible wavelength. However, what the exact mixture is depends on the temperature of the body. As the body gets hotter, the 'peak' wavelength, i.e. the wavelength whose "amount" is highest in the mixture decreases. Objects at room temperature emit most of their energy outside of the visible spectrum, so they are not 'visible' in the dark. However, as you heat them up, the radiation mixture moves towards lower wavelengths, closer to infra-red. Heat it up further and things become red hot, yellow, blue hot and so on. Infra-red LEDs produce light of the the specific infrared wavelength through semi-conductors. They have nothing to do with the black-body radiation one associates with 'hot' objects.
- mensetmanusman 3y ago

JohnFen 3y ago

I agree. Setting how LEDs work aside, I never really got how semiconductors worked, despite reading about it and talking with experts for years, until this video. I mean, I could explain how they worked in the same ways that they were explained to me, but I couldn't connect those explanations to a true physical understanding. But thanks to this, I finally actually understand. Also, the LED story was fascinating.

thirdhaf 3y ago

The explanation is really well done, it captures the essence of the Pauli exclusion principle without delving too deeply into the weeds. In my opinion the best part of the video is the explanation of the "hole" quasiparticle at 6:10 (I learned this as a pseudo-particle but will defer to Wikipedia [1]). While a great introduction to semiconductor behavior this does gloss over a very important detail namely direct vs indirect semicondoctors as some others have mentioned. In the video the detail that's glossed over relates to the nature of crystals, namely that they're highly ordered repeating structures but that they don't look the same when viewed from every direction. This means that there isn't a single band-gap but multiple ones depending on the direction of the crystal you're contemplating. At this point you may reasonably ask why the direction matters and now we unfortunately get deep into the weeds with quantum mechanics again. When a single photon is absorbed in the semiconductor system both momentum and energy must be conserved. The momentum of the photon for something like the Silicon bandgap is quite small (something like the equivalent of an electron traveling at 1500m/s) while the momentum of room-temperature conduction electrons is substantially faster [2] so as a very slight simplification transitions due to the absorption of photons are not accompanied by a change in momentum and so we only care about the band structure (and the accompanying free carriers) associated with a particular crystal direction. In particular in Silicon you have what's called an indirect bandgap, namely the minimum energy conduction band electrons have a different momentum from the valence band holes ([3]) and as a consequence while you can _absorb_ a photon in order to make a detector you cannot make it efficiently _emit_ a photon as an LED should (something the video got wrong). None of this matters for the heart of the video, which focuses blue LEDs in the GaN materials system which is definitely a direct bandgap material, however if someone does manage to create a manufacturable light emitter in pure Silicon expect an absolute revolution with regards to optical computing and photonics. (Not for lack of trying, this has been the holy grail for at least 20 years, possibly longer) [1] https://en.wikipedia.org/wiki/Quasiparticle https://en.wikipedia.org/wiki/Quasiparticle [2] https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch2-2.pdf https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chen... [3] https://www.iue.tuwien.ac.at/phd/wessner/node31.html https://www.iue.tuwien.ac.at/phd/wessner/node31.html

diedyesterday 3y ago

That explanation closely follows the outline of the equally good and very accessible explanation offered in the Halliday/Resnick/Walker's Fundamental's physics (11 edition, chap. 41)