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Extrinsic semiconductors N-type

What happens if you add impurities to a pure semiconductor? Impure semiconductors are usually called extrinsic semiconductors. In this video, we are going to explore the N-type semiconductors. Created by Mahesh Shenoy.

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Video transcript

in a previous video we saw that if you take an intrinsic or a pure semiconductor then at any temperature the number of electrons is exactly equal to the number of holes in this video we're going to explore what happens if we take an extrinsic semiconductor a semiconductor with some specific impurities in it what happens to their number of electrons and holes let's find out so to make our semiconductors impure we need to add impurities and this process of adding impurities is what we call doping so doping is the process of adding impurities so we add impurities and when we say impurity what we mean is anything other than silicon that's what we mean by impurity in general and why do we add that well it enhances its properties it makes our semiconductor better it makes it better and we're gonna see how it makes it better in this video makes it better now you may have heard of this word doping connected with athletes doping themselves illegally and issues like that and guess what even there the idea is exactly the same at least would dope themselves meaning they would add some foreign substance in their body they would be adding impurities to enhance their own properties that's the same idea over here so the big question that would be what impurities to add and by the way when you add these impurities where you get an impure semiconductor we're gonna call that as extrinsic semiconductor so in pure semiconductors the technical name is extrinsic that's all it's just a name it's basically impure but another big question is what impurities can we add can we add anything we want turns out not to be so all right so this is one of those rare moments in physics where a periodic table is going to get popped up and if you look at the periodic table you can see silicon is sitting right over there under group 14 and it turns out to really enhance its properties to make a difference we need to add either elements from group 13 that's one less or from group 15 that's one more either these elements are these ones and if you add these you get one kind of impure semiconductor extrinsic semiconductor and if you add these you get another kind of extrinsic semiconductor impure semiconductor so in this video we're gonna focus on group fifteen elements and another video will focus on group thirteen and we can choose any one candidate nitrogen phosphorus arsenic will choose any one let's choose phosphorus let's just phosphorus and by the way you may be curious as to why we have to choose only these elements and we'll talk a little bit about why only these elements work towards the end of the video but anyways if you look at phosphorus notice that it has 15 electrons one more than silicon that means since silicon has four valence electrons phosphorus will end up having five valence electrons so here is our phosphorus atoms five valence electrons and the way we add this phosphorus impurity into the semiconductor there are very various complicated process in that one of them is like diffusion they pass phosphorous gas at some temperature over this silicon and then the phosphorous will well the atoms will enter the silicon and sometimes it will replace some of the silicon atoms sometimes there will be there will be some space over here they'll be a silicon atom missing there'll be some defects and so this whole thing can go and fill up that missing gap so many things are possible let's assume that the phosphorus is going to replace one of the silicon atom it's gone maybe it was not there Crystal defect and now our phosphorus is going to enter over there and it's going to fit nicely over here and when it does notice that four of its electrons are going to nicely form covalent bonds with the neighboring silicon and so these really kind of pretty happy because they really don't care that it's silicon or phosphorus all they care about is getting there eight electrons and they've got it but the key thing to note over here is this fifth electron that's going to be our focus so what's going to happen to the fifth electron is it free to move through the crystal lattices because it's not bonded or we have to be careful we have to look at the energy level of that electron so let's you know let's get back our band diagram we've been using band diagram for quite a while now we've seen that at very low temperatures our pure semiconductor this is the band diagram for pure semiconductor it would have completely filled valence so what is the energy level of this electron does it lie in the valency band is it lying the conduction band there is it but one can do the math to figure out exactly where it will be and if you do it it turns out that its energy level lies somewhere over here right below very close to the bottom of the conduction band all right so all these v electrons of all the phosphorus phosphorus atoms would be at that level so let me just show it that way now at first you might be like wait wait how is that possible isn't this the forbidden energy gap ironed electrons forbidden from being here yes but those are the electrons of silicon but phosphorus electrons will have its own energy levels right so it just turns out that the energy level of this will be over here and so we did not figure out what's going to happen well at of course at a very low temperature again this is going to be acting like an insulator because there is again notice that these electrons are not free there because these are in the forbidden region there are no energy levels available but what happens when you come to room temperature we've already seen that at room temperature in a pure semiconductor some of the electrons from the valence band can absorb thermal energy and jump into the conduction band and become free that this is called as thermal generation but now think of this way if these electrons can get thermal if the thermal energy can excite these electrons don't you think terminology can readily more easily excite these electrons this electron these electrons because the energy gap is so tiny I mean this energy gap the forbidden gap is about one electron volt for silicon this is what it turns out for phosphorus about about point zero five electron volt it is so minuscule that these electrons can be easily jumped into the conduction band and as a result it turns out that at room temperature almost all the phosphorous atoms almost all the phosphorous atoms end up donating their v electrons so all of them will end up jumping into the conduction band and notice as a result the addition of group 15 elements increases the number of electrons let's write that down and this by the way is the summary of the whole thing so long story short adding group 15 elements is going to give you a lot more got more electrons than holes lot more electrons that's it that's the idea so let's just now end this with some names that we give and some of the curious questions we might have since we have ended up with a lot more electrons the majority charge carriers are negative type electrons this semiconductor is called n-type semiconductor n stands for negatives a negative type so whenever you hear this word n type what must come to your mind that n is telling that in this semiconductor it's extrinsic impure it has a lot more electrons than holes so n E is way higher than M Hetch and since this phosphorous has donated an electron to the silicon for conduction this this impurity is also called as donor impurity so all group 15 impurities are also called donors donor impurities okay some some names that we usually use and this level at which the electron was before it got jumped into the conduction band that we'll call that as the donor level donor level a couple of more interesting things before we end this one is a big misconception that I had I should think that since now we have a lot more electrons in holes this must be negatively charged semiconductor right anyone this n-type is - you convince me I make it - that's telling us well actually no think about it we started with a neutral pure semiconductor all silicon where they're uncharged and we added in neutral phosphorus so how can the whole thing be charged it has to be neutral what is important to understand over here is although we have now all these extra electrons but for every extra electron donated our phosphorus has lost an electron and so it has now become a positive ion so our phosphorus has become positive so at every donor site there is a positive charge that cancels the negative charge of these extra electrons keeping the whole thing neutral so it is still neutral and one last thing might be why are we using group 15 I mean if you bring back our periodic table maybe you could have added groups Steen sulfur it has six six valence electrons that means I would get two free electrons right more electrons isn't that better well answer is no the key is to look at the donor level the reason it works out with the group 15 elements so nicely is because it turns out for all these elements the donor level is very close to the bottom of the conduction band and as a result the electrons the extra electrons provided by the donors can be easily promoted to the conduction band and that's there for almost all of them will end up promoting but if you look at any group 16 or group 17 you if you do the math it turns out that the donor level goes lower and lower and lower now imagine what's the point of having a donor level so low promoting those electrons even if you have twice the number or thrice the number promoting those electrons will be very difficult right and so it would be totally besides the point and so the whole reason for using group 15 is that its donor level is very close to the conduction band bottom of the conduction band so with all this hard work have we achieved what we wanted is this now a one-way conductor well let's find out so this is how we usually depict here it is this is how we usually depict the n-type semiconductor we ignore all the silicon atoms we only show all the donor ions remember it's positive because they have donated electrons so here are all the donated electrons the whole thing is still neutral and of course due to thermal generation there are few electron hole pairs also extra created and now electrons are majority the holes are minority and these are immobile you want though they have charges don't think they can move but does this act like a one-way conductor well all we have done is increase the number of electrons I don't think that's going to change anything right it doesn't act like a one-way conductor whether you put electric field to the right or hard to the left it's still going to conduct either ways it's much better conductor than a pure semiconductor so we're still not getting what we wanted so an n-type semiconductor all by itself is still pretty useless but we learn how we can make something useful out of this in the future videos