 Hi, I'm Zor. Welcome to Nezor Education. Today I would like to talk about using semiconductors in electronics primarily to make diodes. Now diodes, as we know from previous lectures, are such electronic devices which allow the current to go only in one direction. So if you have some kind of a diode that has basically two ends and if you put electricity plus and minus in one way, the current will actually go through. If you change the polarity, the current will not go through. Now before we were talking about implementation of this particular device using electronic vacuum tubes and obviously you're welcome to go to the previous lectures where it's all explained. Now it's a completely different concept based on semiconductor activity which will implement the same functionality. The current will go only in one direction and if you change the polarity, it will not. Now this lecture is part of the course called Physics for Teens implemented on Unizor.com. Well Unizor.com is an educational website where you can find another prerequisite course, mass for teens. So I strongly recommend you to familiarize yourself with all the mathematical concepts used in physics and mass for teens course provides it. Well and obviously it stands by itself at the course of mathematics. What else? Well the site is completely free, there are no ads, no strings attached, you don't even have to sign in if you don't want to. Okay now this lecture actually is preceded by another where I basically explained what is P type semiconductors and type semiconductors. I will briefly repeat this type of thing in this lecture but again I strongly recommend you to to take the course basically where where all the topics are linearly related and logically related to each other. Okay so we're talking about diodes implemented as a P-end junction. Okay so let me start again from very brief review of what semiconductors actually are. Now consider the atom of silicon. Now the atom of silicon has a certain number of electrons but we are concerned only in electrons which are on the outer orbit which is called valence electrons and there are four of them. One, two, three, four. Now also there is another very important thing that atoms are trying, well as if they're some kind of integers with this mind, they're trying to have eight electrons on their outer orbit, valence electrons and well obviously it's not always possible because right now we have certain number of protons inside the nucleus and certain number of electrons and the outer orbit contains only four but if you have another atom of silicon nearby and each of those atoms also have four electrons then what's very interesting these pairs of electrons serve both atoms this one and this one. Well you can say that they are maybe rotating around some kind of a midpoint or I don't know really how it is actually arranged inside but now this one can count four of its own and four of neighboring electrons and this one also counts four of its own and four of the neighboring electrons. These are related to each other with some kind of covalent bonds so this is a concept which I have explained in the previous lecture so there is basically it's like a force if you wish which forces these together and basically count as valence electrons for both this one and this one and this pair is for this and this so these covalent bonds are holding much stronger the whole structure basically enabling crystalline structure of the silicon as a material so that's all about silicon. Now let's talk about semiconductors in case of semiconductors we are introducing certain impurities into silicon so sometimes we are replacing certain atoms of silicon with atoms of let's say phosphorus. What's important is for element which we are adding is to have five electrons on the outer orbit five valence electrons so what happens well these four will be related through covalent bonds with neighboring atoms and this one will not have any covalent bond but obviously there is a bond between this electron and the nucleus nucleus has also corresponding number of protons same as number of electrons so there is a positive to negative attraction obviously that's what keeps this particular electron near this atom but not as strongly as all other valence electrons because all other are also connected through a covalent bond to a different atoms of silicon in this case this is also silicon and this one has no covalent bond so it doesn't participate in the crystalline structure that's very important now another kind of impurity which we can introduce is when instead of four electrons we have three boron element boron has three electrons in the outer orbit three valence electrons one two and three these are connected but this electron of the silicon has no covalent bond connecting to this one so again this one is held only by its attraction to the nucleus and not by any covalent bond so basically you can say that this is some kind of a hole if you wish now the crystalline structure and covalent bonds actually are very strong and sometimes what might happen is let's say this particular electron since it's still you know on the outer orbit if excited by something like heating or or applying some voltage for instance it can actually start acting very very actively and it can jump and fill up this particular hole but that would make this hole now so we will restore the covalent bonds in this particular case but this hole will jump to this particular thing so we're talking about holes as basically absence of valent electron but in this case again the hole might actually migrate it might move from one place to another that's all about how semiconductors are arranged now if no excitement introduced into this particular picture no heat no external voltage no sunlight or whatever else can excite electrons well the whole thing is neutral even with impurity because impurity has less protons in this case and less electrons in case of a phosphorus for instance we have an extra proton and extra electrons so the whole thing is electrically neutral however existence of extra electrons or extra hole in the whole structure actually makes certain differences when some some some activity actually is observed if some excitement of these electrons can be observed based on as I was saying heating or sunlight or electric voltage etc so those semiconductors with impurity that introduce the extra electron is called n-type and those like boron for instance where there is a deficiency of electrons in the structure in the crystalline structure they're called p-type n stands for negative p-type stands for positive because hole can be considered as a positive charge well the electron is negative charge the same thing as in any other electrical kind of thing what is positive positive means it's it's lacking electrons deficiency of electrons so the whole is like a positive charge so we have p-type and n-type now what happens let's say let's talk about b boron introduced so it's a p-type semiconductor what happens if I will apply plus and minus to the whole body of semiconductor well let's just think about it we have basically two important forces which are acting in this case but in any case one force is the attraction between positive and negative charge so protons for instance in the nucleus are attracting electrons around at the same time we have covalent bonds which are also holding electrons together and makes the crystalline structure at the same time we have plus and minus here which are also kind of attracting plus attracting electrons minus repels electrons so what happens in this particular case if the voltage becomes stronger and stronger well if it's very weak nothing happens but as soon as we are trying to increase the voltage a little bit what happens well electrons can actually be attracted so what happens in this case let's say this electron is attracted here now there is a hole in between right instead of this electron there is a hole in crystalline structure now the covalent bonds are strong and some other electron can jump and replace this so this becomes whole this covalent bond becomes whole now we have we do have already some whole so the whole actually will move and this whole will also be filled up by some because there is a general movement of electrons towards the positive at the same time minus provides certain supply of new electrons so as soon as certain number of electrons migrated here this becomes the whole thing becomes positively charged and attracts electrons from minus and they will replace certain places here and to make certain things whole but plus will still continue doing what it's doing so again the holes the electrons will move towards plus and the holes basically will move to an opposite direction the stronger the potential difference between plus and minus is well the faster the more intense this process will occur so there will be an electrical current so that's why they're called semiconductors in it's in a weak difference of potentials there is no current but so it's not really it's it's insulator but if there is some kind of a excitement pressure it becomes conductive okay now what if it's n-type but it's n-type so we have a phosphorus and extra electron here well same thing electrons there are extra electrons that will go here and now this becomes positive because the proton inside extra proton is positive it lost this electrons so it will attract electrons from here so again we will have the electrons are moving with proper voltage this actually occurs that's why it's called semiconductors sometimes if it's if it's a small voltage it doesn't really conduct electricity but with a substantial voltage and substantial I don't mean hundreds of volts we're talking about three five this type of voltage the and obviously it also depends on how much impurities we have introduced how many extra electrons or extra holes in case of p-type or n-type we have the more holes or extra electrons we have well the better conductivity will be okay and now we will consider this junction think that's the purpose of this lecture what happens if we will connect together so let's say we have this p-type and this n-type of semiconductor which means here we have extra electrons extra electrons which are not participating in construction of the crystalline structure not bounded by covalent bonds so if you have something like a phosphorus added to silicon we have these extra extra electrons which are not engaged into any kind of a covalent bond so they're well they're not free because they're still attracted to its own atom but with a small push small activation they might actually jump and even in a normal condition these balance electrons which are not really bonded together by covalent bond into a crystalline structure they're still fluctuating not a lot but but still now on the p-type we have holes which I will put as pluses because again plus means lack of negativity lack of electrons so there are certain holes here throughout the whole body if it's a boron with silicon boron has only three electrons on the outer orbit so we have whole basically so you see this covalent bonds and electrostatic forces between plus and minus both are participating in this game so what happens now if nothing if no voltage is applied anywhere well there is certain migration as I was saying just random walking of these extra electrons because they are not covalently bonded with other electrons only the electrostatic forces and they're not as strong so if there is certain degree of activity some electrons can actually jump here now what happens well they are disappearing from here and they are converting into neutral from the covalent bond viewpoint atoms in the border line near the border they fill the holes now strong forces covalent bonds are holding these electrons as soon as this electron jump here well the covalent bond actually is formed and it's well relatively stable it doesn't move any further this particular electron because it maintains the crystalline structure of the semiconductor on the top of the p-type okay but what happens with electrostatic forces since electrons move from here to here and are held here by covalent bonds this thing used to be neutral right but now it becomes negative because the electrons are moved here and this becomes positive so the whole thing now becomes electrically charged okay what next well this negative charge prevents new electrons from moving further so there is certain layer of negative charge formed by those electrons which jumped and were captured by covalent bonds and there is certain positive charge in the area near the border near this junction on the end type and the process actually ends doesn't really move any further there are a certain number of extra electrons here which are filling up the holes and there is a certain lack of electrons here but we don't really need them to maintain the crystalline structure because they did not participate in the crystalline structure it was an n-type it was an extra electron on each atom of phosphorous so the process ends now it happens well let's say we put plus here and minus here attached some kind of contact well now we have introduced yet another electrostatic force this is plus this is negatively charged so the electrons actually now might move there well if the voltage is sufficient enough because again covalent bonds are still acting but if this attraction is stronger than these extra electrons will move towards the positive and go to whatever the batteries now what happens well now this becomes again positive not positive sorry holes doesn't become it becomes neutral electrically neutral but but the crystalline structure remains with holes because these electrons which were used to basically put the structure together will will migrate towards the plus which means that there is no more barrier of electrons here and extra electrons here can continue this migration and fill up these holes now this becomes again well it used to be positive now it's even more positive but we do have a source of electrons because this is a negative so this is positive this is negative so electrons will go here and will be extra electrons which do not fit into crystalline structure so all these electrons which don't fit crystalline structure are potentially migrants to fill up the holes here but as soon as they do this plus again will attract them and they will go again to the positive electron and this process will continue this will supply new electrons to the n-type these will be migrating into the p-type to fill up the holes and this positive charge will attract them even further again leaving the holes bare again which will be substituted with new electrons so the electrons will move all the time from negative to the positive and the current in the opposite direction is what if we will do a completely opposite sync but this is minus and this is plus well if this is minus these will not attract will not be attracted so they will remain in the covalent bond structure so they might actually be kind of pushed back since it's the negative right then this is negative there is a force which pushes back but again it it's not strong enough to basically push it across the border because there is no covalent bonds here ready for them to be kept you see when electrons moved there there are additional forces both to fill up the covalent bonds and attraction of the plus but if electrons move move here there is no covalent bonds which are ready to grab these electrons and the force of repelling is not sufficient to move electrons here so that's why it's semiconductor it's all depends on the level of voltage level of density of impurities in both sides etc but definitely under certain circumstances we can think about it but this force is not enough to move actually a lot of electrons back here because there is no covalent bond available for them which means that these negative electrons which are in the borderline after their migration so it's neutral but it's the covalent bonds are filled but electrons are still here there are no holes here well they will just stay there and they will prevent any other electrons to go this way or that way and the covalent bonds here are strong enough to resist the attraction of this plus so that's exactly where we have to really have some kind of a very finely calculated voltage against the covalent bonds obviously if I will put a very strong minus and very strong plus I will just rip off all the electrons all the valence electrons from everywhere everywhere so we're not talking about this that's not interesting what's interesting is to have certain density of impurities in both cases and certain voltage and it's all calculated in such a way that this particular device which is called a p-n junction diode will only conduct electricity when electrons are moving this way and not that way that's basically how our diode is arranged this is the p-n junction diodes that make basic principle so again what's important is that it works only if there is certain finely calculated balance between intensity of the applied voltage in both sides and certain density of impurities which we introduce into n-type and p-type if it's all done correctly and obviously by now we have a very good experience how to do it correctly we will have this diode working for certain particular voltage so obviously it's low voltage all our computers are basically low voltage computers and semiconductors basically help in this particular case because it's exactly this range of voltages where they work now we are not talking about technology of how to introduce these impurities how to make this junction etc it's all technological thing which not subject of this course at all we are talking about principle and the principle is the covalent bonds are strong the electrostatic attraction and repelling are strong and they are supposed to work in conjunction which with with each other and that's how the whole thing is made I suggest you to read the notes for this lecture on junisor.com you go to physics 14 course electromagnetism and then there is a in a semiconductor part there is a lecture about electronics related to semiconductors well that's it for today thank you very much and good luck