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Extrinsic semiconductors P-type

In this video, we will explore the P-type semiconductors.  Created by Mahesh Shenoy.

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Video transcript

in a previous video we saw that if you take a pure semiconductor and AD group 15 elements like nitrogen phosphorus Lee took phosphorous as an example then because of five valence electron one extra electron compared to silicon at room temperature these phosphorus atoms end up donating those extra electrons and our entire semiconductor now has a lot more electrons a lot more negative charge carriers compared to holes and as a result we ended up with what we call n-type semiconductor and in this video we're gonna find out what's gonna happen if we add group 13 elements we'll take will take boron as an example and remember usually we only add group 13 and group 15 elements for making our semiconductors impure and we're gonna speak a little bit about again why that is true towards the end of the video and the process and the stuff that's going to happen when we add boron or any other element over here is going to be identical very similar to what happens when we add group 15 elements so it'd be much easier to follow and it will be much easier you can even predict what's going to happen beforehand if you have followed those previous videos so if you have not seen them or if you if you need a refresher then it'll be a great idea to go back and watch those videos first and then come back over here so anyways let's add boron to the party now if you write the electronic configuration of boron you will see that it has three valence electrons we can quickly write and check that confirm that so since if boron has five electrons it's electronic configuration would be 1s2 2s2 for done one more would be to p1 so notice that it's valence electrons are just three hooves and so if you add boron to the semiconductor a pure intrinsic semiconductor and the way of adding that is again you're complicated we won't get into that then some of the electrons some of the silicon atoms could be replaced by boron atoms or at some places there won't be silicon atoms we call them as crystal defect no crystal is perfect so at some places there will be vacant sites so one can just go and swoop in and fit over there so we just assume that one of the cross borders just going replace one of our silicone so let's remove one silicon atom and fit our boron into the mix over here as the boron fits over here notice because it has three valence electrons it can nicely form covalent bonds with three neighboring silicon atoms so these atoms are pretty happy they don't care who it is because they have filled up their octet structure but you can see that this silicon is not happy because it's not fill its octet structure and the reason is there isn't one electron over here it Robin great to have an electron so you know what this silicon desperately needs one electron over here so there is a vacant site for an electron which is not occupied by anyone and it's desperate for an electron the question is what's going to happen can any electron from here swoop in over here can it do that well again we have to be careful we need to look now at the energy level of this vacant site produced by boron so let's get rid of this video right table and let's bring back our band diagram we've been using that a lot this would be the band diagram at very low temperature of an intrinsic semiconductor but now the moment we add boron we were interested in what is the energy level of this site well it turns out again if you do the math then that energy level is found to be very close to the top of the valency band can you see how this is beneficial for us so what will happen at room temperature if we were to get this all the way to room temperature then one effect we have already seen in pure semiconductor that's gonna happen here as well there will be thermal generation some of the electrons will gain thermal energy and jump into the conduction band but notice since there's another energy available over here some of the electrons can even jump from here to here does that make sense because this energy gap is incredibly small it's much smaller than this energy gap it's much easier for electrons to jump over here in fact what's going to happen is that almost all these vacant sites will now be completely filled by some or the other electrons and as a result all those will be filled up by some of the other and when they jump they would leave behind a lot of holes so over here if you were to see it we could imagine say this electron has jumped from here to here any electron can go like that anyone in the neighborhood can do that and as a result notice it has left behind a hole and this hole now is in the valence band it is free to move so notice by adding our group 13 element like boron we have now produced lots and lots of holes again let's write that down so this is the summary of the entire thing groove 13 elements if you add group 13 elements then we end up with lot more holes lot more holes than electron and so because now the majority charge carriers the majority of conduction is done by holes which act like positive charges remember that they're not positive charge they're not even particles really but they act like positive charge in the in the sense that if you apply an electric field holes appear to move in the same direction so since you have a lot of positive type charge carriers we call this as P type semiconductor P for positive so we call this as P type and so whenever someone says Peter what comes to my mind but P is telling me there are a lot more positive type positive type means holes so a lot more holes in hatch number of holes is way way larger than number of electrons number of electrons and that's pretty much it we just have a couple of names technical names that we use when it comes to adding impurities the process of adding impurity is called doping and since this boron it accepted an electron and that's how we got holes we call this impurity as an acceptor impurity we also call this as Group thirteen or call as acceptors acceptor impurities are just acceptors and the level the energy level that they had did or at which the electrons were able to jump into that energy level that boron introduced is called the acceptor level so it's called acceptor acceptor level and now we could ask a big question why are we only adding group thirteen elements I mean if we bring back our periodic table if we bring back our periodic table I mean group thirteen elements is fine but why not add group to valium and say like I don't know zinc maybe because if you work out its electronic configuration you will see it has just two valence electrons you can just check that and so if we added zinc maybe then it has only two valence electrons and it'll have to waken sites yay more electrons can occupy right no it doesn't work that way what's important is to look at that acceptor level it turns out that if you add Group two all elements or any other group elements the acceptor level will just go higher and higher and higher what's the point of having multiple vacant sites a lot of vacant sites if you're accepted level is so high because then it'll be extremely difficult for these electrons to jump from here to here we take a lot more energy and so most of those will not be occupied by the electrons at room temperature and so hope you think will be pointless and that's the real reason why we only add group thirteen elements because in group thirteen elements it turns out that the accepted levels of all these elements almost almost all these elements the accepted levels are very close to the top of the valency band and that that's really the key to to understanding why that's the key to why we add 13 group 14 elements so lastly one big misconception that we might have because I always had that is that now that we have a lot of holes lot more holes compared to electrons isn't this like a positively charged semiconductor is because also it says p-type right I always used to think p-type positive type I always had in mind p-type means positive semiconductor well we have to be careful I can't even think about it we start with a neutral pure semiconductor all silicon atoms with neutral and we added neutral boron so how can the whole thing be charged the whole thing is still neutral but then what we need to understand is that although we have had these extra holes that you might think that might contribute to positive charge remember when the boron accepted an electron the boron ended up becoming negative so all those extra electron holes that we have gotten their charge can be balanced if you compare if you realize that the boy all the acceptor impurities now have a negative charge so that's become an ion and as a result the whole thing is still pretty neutral all right but now the big question is is this what we wanted remember our we want something that only conducts in one direction can it do that well let's find out so if you go around I have the whole thing ready over here so this is how we usually depict a p-type semiconductor we're gonna show we're gonna ignore all the silicon atoms only show these boron ions notice since it's an acceptor in accepted electrons and it became negative but as a result of accepting those we now have a lot of holes that can freely move majority are holes and we also have some electron and hole pair form due to thermal generation so this is in a nutshell what a p-type semiconductor we would like to show it this way and now does it conduct only in one direction the answer is no because if you apply electric field to the right almost all the holes will just go to the right if you apply electric field to the left they also just move to the left so all we have done is improve the conducting property of our pure semiconductor but it still doesn't have any directional property so if you locate that way it's still pretty useless so a p-type semiconductor just all by itself is still pretty useless we have to do something more to make it useful and we'll do that in future videos