| 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
[2] https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chen...
[3] https://www.iue.tuwien.ac.at/phd/wessner/node31.html |