How diodes, LEDs and solar panels work

this video is a standalone explanation of how diodes LEDs and solar panels work if that's what you have to keep watching but it's also the second part of a two-part series if you want to see a video of diodes doing weird things then check out part 1 first the link is in the card there and in the description the purpose of a diode is to restrict to the flow of electricity in a circuit electricity can only flow in one direction through a diode that's the point of it and there's a couple of ways basically to make a diode the old-fashioned way with vacuum tubes or the modern way with semiconductors and this video explains the modern way of doing it because semiconductor diodes are so much more than simple one-way valves for electricity diodes are made by pushing to little bits of semiconductor together and to figure out what happens when you do that you need to understand covalent bonds so you might know that atoms like to have eight electrons in their outer shell with some exceptions and yes I am anthropomorphizing the atom atoms don't actually like to have eight electrons in the outer shell but I actually think and flipping warfighting is fine so long as everyone knows that you're doing it and that it's a shorthand for a more complex process also I'm gonna do it a lot more so strap in if you've got an atom of say fluorine that has seven electrons in its outer shell when it's neutral then it wants one more so if there's another fluorine nearby they can come together and share an electron each so look in the crossover between the two outer shells there's these two electrons so now this atom has eight electrons in the outer shell and this atom has eight electrons in the outer shell and they're both happy I'm not even gonna do air quotes so that is the covalent bond of a fluorine molecule f2 but think about silicon now silicon has four electrons in its outer shell so it needs to share the four that it has with for more silicon atoms surrounding it and those four atoms around it to have silicon atoms around them so that they can have full shells and those atoms need to have atoms around them so they can have full shells and so on and so the crystal grows outwards through covalent bonding this flat 2d representation of the crystal is just to help us see those complete shells in reality it's a three-dimensional structure so you get these tetrahedral bonds like that all these electrons are locked away in those complete shells they can't move so there's no freely moving charged particles in a silicon crystal so silicon isn't a great conductor of electricity but suppose we swap out some of these silicon atoms for phosphorus phosphorus is the next atom in the periodic table so it has an extra electron but because all of these shells are already full this extra electron has nowhere to go as in it doesn't have a shell to fill it's not locked in place it's free to move around so when you dope silicon in this way you introduce these freely moving charged particles these electrons and so this doped substance is able to conduct electricity and this is called an n-type semiconductor and standing for negative because the charge carriers are negative alternatively we can switch out silicon atoms for atoms that have one fewer electron in their outer shell like boron and when you do that you have these incomplete shells in the lattice and what you find is that nearby electrons jump into those holes and when they do that they leave a hole behind themselves and then that hole will be filled by a neighboring electron and then that hole will be filled by a neighboring electron and so the hole moves around and you can actually model the hole like it's a particle like it's a positively charged charged carrier so when you dope silicon with boron you create what's called a p-type semiconductor because the charge carriers are positively charged so to make a diode all you have to do is take an n-type semiconductor and a p-type semiconductor and push them together in the n-type you've got these electrons moving and in the p-type you've got these holes moving around and you can think of them a bit like a gas and as you know gases diffuse so when you put these two bits of semiconductor together this cloud if you like of electrons in the n-type semiconductor will diffuse a little bit into the p-type semiconductor in other words the electrons will drift over and fill in those holes and what you end up with is a kind of equilibrium because actually as electrons diffuse over into the p-type semiconductor you get a buildup of negative charge there and similarly where the electrons have abandoned the n-type semiconductor you get a buildup of positive charge and that creates an electric field that is pulling the electrons back across so what you end up with is an equilibrium between electrons diffusing across the junction and this buildup of an electric field that pushes them back across but the point is there are no freely moving charged particles in the depletion zone so what happens when you connect a battery to a diode well it depends which way you connect it with the positive terminal of the battery connected to the n-type semiconductor those freely moving negatively charged particles those electrons are attracted to the positive terminal of the battery because opposite charges attract and a similar thing is happening on the other side the positive holes in the p-type semiconductor are pulled towards the negative terminal of the battery because opposite charges attract so in this scenario the electrons are being drawn out from the right and the holes are being drawn out from the left and the depletion zone increases in size there's this larger region of non conductive material and at some point you reach a new equilibrium where this extra buildup of charge in the depletion zone is able to resist the pull of the battery and no current can flow I think it's useful at this point to just reestablish the fact that we're modeling the holes as if they're particles and link it back to what's really happening what's really happening is the negative terminal of the battery is repelling electrons in the p-type semiconductor because like charges repel but most of the electrons the p-type semiconductor can't move because they're locked up in four shells the only ones that can move are the ones that have holes next to them that they can move into so think about an electron that's here just to the right of a hole the negative terminal of the battery repels that electron into the hole so that hole vanishes and a new hole appears where the electron was but if you step back from that picture it looks as though the hole has moved to the left and that's why we are able to model those holes as moving particles because they move around in that way so what if we switch the battery around now the negative terminal of the battery is pushing electrons into the n-type semiconductor and the positive terminal is pushing holes into the p-type semiconductor and it's pushing those electrons and holes towards the junction it's shrinking the depletion zone and with a large enough voltage from the battery electrons are actually able to jump across this reduced depletion zone they hop into the holes in the p-type semiconductor and suddenly you can have a flow of electrons through the diode so with the Batchelor entered this way electricity is able to flow and the diode has fulfilled its purpose but there's so much more to diodes than that because when an electron jumps across the depletion zone and falls into a hole it goes from a high energy state to a low energy State and you might know that when an electron does that it emits a photon for a brief explanation of why that happens you can watch this video but the important thing is when you push electricity through a diode in the right direction it emits light and that's why we can make LEDs light emitting diodes but let's remove the battery for a second and shine light onto the diode let's kind of do the reverse so if the photons are energetic enough they can kick an electron out of a shell leaving a hole behind in other words creating an electron hole pair and because of the electric field that's present there in the depletion zone the electron is pushed into the n-type semiconductor and the hole is pushed into the P type semiconductor in other words you can generate a voltage across a diode by shining a light on it and that is why solar cells are also made of diodes a quick note about materials we've been talking about silicon based semiconductors but silicon based LEDs emit infrared light like that you can't see that's what the LED inside your TV remote control is made of but if you want visible light LEDs then you need to use an alloy of gallium arsenide and gallium phosphide you mix them in different proportions to get red orange and yellow LEDs and if you want blue LEDs then you use gallium nitride and it's all to do with when the electron drops into a hole across the depletion zone basically it's a drop in energy so the electron is losing that energy to a photon so the the further the electron has to drop the the bigger the bandgap the more energetic the photon that is released and you might know that there's a link between the energy of a photon and its color so the more energy of photon has the further up the light spectrum you go so with silicon you start off with infrared but then with gallium arsenide phosphide you go through red orange and yellow and it's gallium nitride that has the really big drop in energy the really big bandgap that gives you your blue photons because solar panels and LEDs are both made of diodes you can actually use them in Reverse so if you want to see a video of solar panels acting like LEDs and LEDs acting like solar panels then check out part 1 this video is made possible by my patrons on patreon and brilliant org if you don't know what brilliant org is yet it's a website full of maths science and engineering problem solving courses but it's so much more than that it's designed and curated to make you smarter as you work through the problems one of my favorite features is the daily problems it's a great way to just keep your mind ticking over on a daily basis my routine is if I haven't solved the problem by the end of my commute then I get my wife involved in we work through it together we solve it together all the problems are presented with the context and framework that you need to be able to get the answer and if you really like a particular problem you can use it as a launchpad into a full-on course that explores the same ideas in more detail and if you ever get stuck there's a community of thousands of learners just like you discussing the problems and coming up with the solutions together to start your journey from curiosity to mastery one day at a time visit brilliant dot org forward slash Steve mold the link is also in the description there the first 200 people to use that link will get 20% off annual membership which gives you access to the full daily problems archive and to every single course I hope you enjoyed this video if you did don't forget to hit subscribe and I'll see you next time [Music]

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