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Current time:0:00Total duration:10:06

Video transcript

we've seen in previous videos that if you take an intrinsic semiconductor and add a group 13 element like say boron which has only three valence electrons then it accepts electrons it acts like an acceptor impurity and as a result ends up giving us a lot of holes and so the boron itself becomes negatively charged but it's in mobile it can't move it's a bulky thing stuck in the crystal but the holes are now the majority charge carriers and as a result we can imagine that we now have a bed of negatively charged semiconductor on top of which we have this gas of holes nice way to visualize this which acts like a blanket and as a result the whole thing is still neutral since the majority is positive type holes we call them as p-type semiconductor and similarly if you dope it with or if you add a group 15 element like phosphorous which has five electrons one extra electron ends up donating that as it is the ends up getting a positive charge but still these phosphorous ions cannot move they're in mobile they're pretty much stuck and again we can assume that it's a bed with with a a positive charge a stuck positive charge and you have a gas of electrons on top of that which are randomly moving around and as a result again the whole thing is neutral and since we have the majority as the negative charges we call this as n-type semiconductor and we've talked a lot about this in previous video so if you need more clarity on this it'll be a great idea to go back and watch those videos first and then come back over here but in this video we're gonna do the most tempting thing ever you're gonna take a p-type and n-type and join them together like I've seen over here and ask the question what's going to happen so let's do that so if you take these two separate crystals and sort of glue them together then nothing will happen because if you glue them there will still a lot of microscopic gaps in between and as a result it will act like two different containers having gases and nothing can happen if you really want something to happen you have to build a single crystal one single crystal with P side or p-type on one side and n-type on the other and building that in reality could be a little bit complicated but let's just assume that we end up doing that and let's just stop this animation for a while it's giving me a headache there we have it a single crystal with a lot of holes on one side and who are of electrons on the other just pause this video and think what will happen and we might think that electrons and holes attract each other because one is negative another one is positive but this is wrong this is a huge misconception don't think of it this way holes can't attract electrons because in reality holes are not really particles remember they are absence of electrons their vacant sites in : bonds they cannot attract electrons so don't think of it that way a better way to think of them is as gases imagine that you have one kind of gas on one side of a container and another kind of gas on the other side of a container and the partition between them is removed what will happen well we know what happens to gases they end up mixing with each other and we call that as diffusion it's a random process in which particles end up moving from a high concentration region to a low concentration region since we have a lot of holes over here these holes will end up diffusing into the n-type and similarly these electrons over here will end up diffusing into the p-type but notice that diffusion can only happen over here you wouldn't expect diffusion to happen over here you wouldn't expect these holes to start moving like this because remember there might be a lot of holes but there is no difference in the concentration and similarly no diffusion over here so diffusion is only happening very close to the junction and notice because of the diffusion electron holes are becoming very close to each other and then they come very close to each other we know that they end up destroying each other recombination and so right at the junction those charge carriers are completely destroyed what happens next well next we might expect the diffusion to kick in over here and here we might expect the holes these holes now to diffuse and these electrons to diffuse and they might recombine and eventually as time progresses we might expect all the holes slowly and steadily all the holes and electrons would have diffused into each other mixed with each other just like how gases would do and they would have recombined and destroyed each other and remove all the excess holes and electrons that would be extremely bad because we worked so hard to get to this point right well fortunately that happen because there's one key difference between gas molecules and holes and electrons we are dealing over here with charged particles and as a result all the holes and electrons warm mixed with each other this mixing will stop immediately can you think why that would happen this is important I want you to pause the video and think but just look around and just see can you come up with the reason as to why all the holes and electrons won't mix together well the secret lies over here naught is due to the earlier destruction of charges they have exposed the negative and the positive ions and now as a result when more holes try to diffuse over here notice that these positive ions start repelling these holes and as a result when they try to go there between repulsion they can't make it and they have to turn back then returning back and similarly when the electrons when the electrons try to diffuse I mean forget about this hole that's a minority don't worry about the as of now when the electrons try to diffuse over here notice this boron negative ions they start repelling the negative electrons again they go try to diffuse up no they can't do that and so notice because they have exposed this charge carrier these immobile ions the impurity ions that acts like a barrier for diffusion but diffusion may not give up there might be some molecule some holes in some electrons which are energetic enough maybe there might be these holes might be energetic enough that even after repulsion they might still diffuse and similarly there might be some electrons over here on the other side which are energetic enough and do to do even even off to the repulsion they might still diffuse but notice they might go over here and they might get you know color recombine somewhere similarly these might end up recombining somewhere because there's so many places to recombine but notice that the more they diffuse more ions they expose and they make it even harder for diffusion because the repulsion starts increasing so I think we can see where this is going as the diffusion continues we see that more and more Chinese will get exposed ions will get exposed and as a result diffusion might slow down and might eventually stop right well not quiet you see it's right that the diffusion keeps slowing down because the barrier starts increasing the repulsion starts increasing but there are always electrons and holes energetic enough to overcome that and still keep defusing we always have them so what will happen eventually well we've been ignoring the minority charge carriers far too long remember in the P side we have these electrons over here and here some few electrons and on the N side we also have some holes well we didn't talk about them so far but what happens next they play a key role in it so let's focus on one electron let's say we take one minority charge carriers over here notice that nobody cares about that everybody is about diffusion so it's just minding its own business it's just doing you know it's wandering around it's pretty lonely and suppose it enters into this region over here near the junction notice because of this positive charge it starts attracting the electron and zip it goes into it into the n-type it gets sucked into the n-type and similarly if you have like say a minority charge carriers a a hole again nobody cares about this it can't diffuse it's all about diffusion everybody's just focused on diffusion it's lonely the poor poor folk is just going over here and finally he enters over here and now notice the negative charge will attract the hole and Zoop the hole will get sucked in can you see that there's a second kind of motion that's happening completely in the opposite direction I mean think about it for diffusion the holes are moving from P to n and this charges are acting like a barrier but notice for minority charge carriers like a hole in this region over here there was a hole over here let's see the holes are here for the minority charge carrier the the the motion is in the opposite direction and this charge is actually aiding the motion it's it's benefiting that motion can you see that so there's a second kind of motion that's coming it's in the opposite direction of the diffusion and so as time passes by diffusion starts decreasing but the second kind of motion starts increasing and since the two are in the opposite direction eventually we will reach a point for every hole that diffuses into n-type one hole will get sucked back due to these charges and similarly for every electron for every energetic electron that still manages to diffuse to the other side one electron will get sucked back and when this happens notice that on an average the total number over here and the total number over here on an average remain the same and that is when we would have reached our equilibrium so that's the story of PN Junction very interesting very subtle concepts but yeah it was very intense so let's quickly summarize when the PN Junction just gets formed majority charge carriers start diffusing the whole start diffusing into the end the electrons start diffusing into the P and as a result they start recombining and destroying each other and they expose the charge on the impurity ions this starts acting like a barrier which slows down diffusion but diffusion still continues but now the minority charge carriers start getting sucked in due to this due to these charges in the opposite direction eventually equilibrium is reached when for every hole that diffuses one hole gets sucked back and for every electron that diffuses one electron sucks but gets sucked back and now equilibrium has reached and this device is super useful because this will act like a one-way conductor