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Current time:0:00Total duration:12:45

Video transcript

in a previous video we've seen that if you reverse bias a PN Junction and put a very high voltage across it then it undergoes a breakdown a very heavy current starts flowing and that happens due to Avalanche effect where free electrons when they're moving through they start colliding with these bonded electrons knock them off creating more charge carriers but in this video we're gonna see another mechanism in which a diode a very heavily doped diode undergoes breakdown such heavily doped diodes are called Zener diode because it was in it was discovered the effect was discovered by this man called Zener and this breakdown phenomena is named as no surprise Zener breakdown so let's get to the bottom of this hole Zener thing if we go back to our conventional diode in normal PN Junction that we've been speaking about then even without applying any voltage across it you may recall that right at this Junction there is this depletion region over here so there is a depletion region a region which is depleted of charge carriers and there's a there's an electric field that exists over here this way and to see why that electric field exists we'll have to zoom in a little bit so let's zoom in over there and see what we get so if you take a small section of that and if we zoom in then we'll find in the depletion region over here we have these positive P ions and they have these negative of boron ions and it's because of this you can see there they're charged particles and the charged particles end up creating an electric field so that's really causing that electric field over here alright and this now is the current width of the depletion region now if you have say a very heavily doped diode so let's bring in a very heavily doped diode so we can imagine this is pretty much the same as this it also is a silicon and you put the same impurity but the only difference now is that you have more doping which means there are more impurity atoms more impurity ions per centimeter cube then what will find the difference over here would be its depletion region if you write down as depletion region that would be extremely narrow the depletion region would be very very narrow and the reason for that is because the field over here turns out to be extremely strong so the electric field again we're not applied any voltage but the inbuilt electric field over here turns out to be super super strong and I get to see why let's zoom in over here if we zoom in if we zoom in will find this now what we will see is because there are so many dopant there are so many impurities you'll find more number of charge carriers charge is not charge carriers more number of these ions per centimeter cube and now if there are more ions per centimeter cube if you compare over here I'm pretty sure you can agree with me that there will be a stronger electric field over there right so the electric field over here is very strong very strong much stronger than what we have over here and since the electric field is very strong oops since the electric field is very strong a small depletion region is formed because a very small region is enough to stop the charges from flowing unlike over here if the depletion region was small over here then maybe charges would be further diffuse because the field is not strong enough so you require a larger width so you see the more you dope the more impurities you get the smaller is the width of that so the key takeaway of heavy doping is that it causes very thin depletion region and that results from a very strong electric field over here all right now let's reverse bias this thing well when you reverse bias you attach a positive to the n-type and you put the negative to the p-type notice as a result you are increasing that electric field can you see that because this side was already positive over here ions so you made it more positive this was already negative you're making it more negative so as a result what happens is this electric this electric field over here increases and the depletion region widens that causes a minority carriers to go from here to here but if you apply the same reverse bias over here you apply that exact same reverse voltage over here the electric field gets way stronger the electric field strength becomes incredibly strong in this region the reason for that is you think of it like this you see if you you think about the voltage as the difference in height between two points like you have a low height over here and you have a higher height over here if the difference in the height is over a large span like this as you can see then notice that the slope that connects them is not much it's not much a steep slope on the other hand if you were to take this if you take this same voltage difference same height difference but if you were to make sure that it's not on a large space or a very tiny span then you can see as you make it tinier and tinier and you connect them notice that the slope increases can you see that it is now more sloping that before electric field is that slope all right so the tinier is the depletion width and even for the same voltage the tiny of the depletion width more is the electric field in this region so as a result even for a very modest reverse voltage that you provide like I don't know maybe two or three words the electric field over here skyrockets electric follow if you low here is still pretty much modest all right so as a result minority charge carriers start flowing in the opposite direction the holes over here start getting swept across electrons over here start getting swept across like this that causes a small current a tiny current from P to N and pretty much the same current flows over here as well we've seen that before it's in different of the voltage same current flows over here all right now let's see what happens when you turn up the voltage we've seen this before we're doing it one more time if we turn up that voltage let's say we go all the way to something like I don't ten words or maybe even 15 volts what happens well you have to zoom in a little bit again to see what happens remember right at this depletion region we don't have any free electrons but we have all these covalently Mauryan electrons there there we don't show them and now due to this strong electric field that's generated when you go to I don't know 15 words or something these free electrons maybe this is one free electron this glowing thing when this free electron gets a huge acceleration it can hit one of these covalently bonded electrons and it can knock it loose so it can knock it loose and due to this impact you can see more charge carriers are being created we call this usually as the impact ionization because due to impact you're freeing that whatever this due to this impact you get more and more charge carriers and these electrons can further go and impact more and that's where the whole avalanche effect takes place we've talked about this before but what happens over here in a heavily doped diode well over here if we zoom in the structure is pretty much the same but the chances of a free electron knocking any of these electrons loose is very tiny and the reason it's so tiny is because the depletion region is very tiny and so you know the probability is just very small however something totally different happens because the electric field over here is so incredibly strong that electric field itself is able to pull these electrons out of the covalent bond so it's the electric field that removes the electrons from their coal and bonds even at low voltage as you see because a strong electric field and we'll talk a little bit about why this happens later on but that increases the minority charge carriers over here and now the whole thing can undergo breakdown so you see the breakdown that happens over here a large current that starts flowing or here is not because of impact you see hole here it was due to the impact on the Avalanche effect this is not impact this is happening because of the strong electric field is the electric field that's pulling out all these electrons and as a result a huge current starts flowing this mechanism is called the Zener mechanism and this phenomena or this breakdown is called the Zener breakdown so if you look at the VI characteristics of under reverse bias for a conventional diode then we've seen that pretty much a very constant current flows very tiny and here is where this Avalanche effect is you know coming into picture and a huge current is flowing we call this as the breakdown voltage the reverse breakdown voltage but if we're to plot the same curve for a heavily doped dier zener diode then we would expect this same breakdown to happen at a much lower voltage due to the Zener mechanism right so this is what we would expect to happen for a heavily doped ire so the difference is that this happens at a much lower voltage so let me write that down this happens at a much yeah so this happens at a much lower voltage we call this as VZ the Zener breakdown voltage and because there is no impact happening over here there's hardly any heating taking place you see that's the major problem with the Avalanche breakdown it can heat up the entire diode and as a result that could melt but over here that won't happen because there are no impacts happening the electrons are being not they're not being knocked off they're just being pulled by the field and so this current does not produce a lot of heat alright that's pretty much it we could stop over here but let's go a little bit deeper and understand the mechanism or here it's a cool there's a very cool phenomena let's try to understand that let me get rid of this graph for a while here's a question why can't the electric field over here you know pull out electrons from the coal and bond why can't they do that why only that happens over here to answer that question we need to really understand what these electrons feel so we can think of these covalently bonded electrons as you know you can think of a mountain over here imagine you have some kind of a weird mountain like this is that down here and you can imagine that this is ball this ball represents the electron is sitting right over here okay so this covalent bond electron is like this ball you see the electric field is actually trying to pull these electrons they want to exert the electrons towards the right but they can't just like how gravity is trying to pull it down you see there's a slope over here gravity's trying to pull it down but they can't because of this barrier so if this this ball wants to fall down it first has to you know go all the way up here it has to overcome that barrier and then fall down and that's the reason why these electrons are not able to accelerate and that's why you need some impact so these free electrons that come an impact it's pretty much like you know some some energy is given to this ball so if some another ball comes and impacts is that energy is enough to take it over that hill and then they can accelerate down right so that's why impact ionization causes free charge carriers and acceleration and everything what happens over here well the difference over here is the depletion width is very tiny so the whole mountain they still have this maybe the same potential difference but the whole mountain becomes a sinner a sinner becomes narrower so the mountain becomes something like this all right now imagine you have a mountain like this and you have the same ball over here you might expect okay how does that help well now what's going to happen is that the ball will just go straight through and the next descent exhale it down you get that this is a quantum effect this cannot be explained by using Newton's laws or whatever because classically you might think that it has to go uphill and then come down but quantum mechanical it turns out it can just go straight through this effect is called tunneling alright it's a quantum effect it's called tunneling where the electron is able to overcome the barrier and just go straight through even though it doesn't have the energy to do that and that's why these electrons are literally tunneling through this barrier and becoming free and that's how they're you know accelerating so the reason will not talk about why this happens it turns out it's got to do something with the dual nature of electrons and it's probabilities or whatever but we won't talk too much about that but this phenomena why doesn't it happen over here well it turns out that this tunneling really depends on two things it depends on one how sloping this is more the slope more the chances of tunneling and more importantly how thin this is the thinner it is more the chance of tunneling so over here this diode is getting everything right for tunneling and that's why you know the Zener breakdown the tunneling effect is kicking in over there but over here the problem is that this mountain is very wide and once it becomes wide enough the tunneling probability almost goes to zero and that's why you don't see any tunneling effect over here but you see the tunneling effect over there so the Zener mechanism is truly due to the quantum mechanical tunneling