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Current time:0:00Total duration:8:54

Video transcript

in a previous video we spoke about the VI characteristics of a PN Junction diode we saw that if you forward bias it then after a particular voltage call as the knee voltage the current just skyrockets the diode conducts very nicely on the other hand if you reverse bias it we saw that the current is pretty much zero a very tiny current flows it's almost zero but more interestingly that current was pretty much a constant it was independent of the voltage that was applied now if you need more clarity on this VI characteristics would be a great idea to go back and watch that video first and then come back over here but in this video we're going to explore what happens if you increase the reverse bias voltage to a pretty high value we'll see that something very interesting happens over here alright let's do that so imagine here is our PN Junction diode and let's apply a reverse bias to it remember reverse bias is we are connecting the negative terminal to the P side so we put a negative over here and the positive terminal to the N side positive over here the dired ideally shouldn't conduct but there's a small current that starts flowing in this direction from n to P and remember why that current flows this is color wise it couldn't even flowing well if you zoom in right at the junction then you'll find there's something called the depletion region is the region where charge carriers are almost gone and they have exposed their ions and as a result there's a very strong electric field that exists in this region that electric field exists this way we've spoken about this in previous videos and as a result what's happening is that the minority charge carriers for example the electrons in the P region once they come into this depletion region they're getting accelerate towards the right as you can see and similarly the holes from the end the minority they're being excited towards the left and that's causing the current from n to P all right now on a normal voltages like when the voltage is about two wars or three wars I put it that much nothing but nothing interesting is going to happen you have pretty much a constant current but as the voltage increases the depletion region widens that means the electric field becomes longer but not just that it turns out the electric field ends up becoming stronger as well and as a result now these electrons and they end up getting more acceleration then of getting more kinetic energy what happens because of that well to understand what happens you need to zoom in even further so if you take a small patch over here and zoom in even further so let's do that zoom in even further or remember that there are actually silicon atoms in between remember that you've not shown that and and all these atoms are covalently bonded these electrons which have not shown what you usually don't show they're there they're all currently bonded they're immobile but they're still there and I've shown one free electron over here that's the electron that's being accelerated I put a small glow here so that we can differentiate between the free electron the conducting electron and these non conducting electrons over here so let's see what happens so imagine we have this free electron because of the electric field the same electric field that exists here this electron is going to excite towards the right now as it exudes towards the right if it has enough kinetic energy it can knock off it can hit this electron maybe and knock that electron into the conduction band that's possible you see usually the electron can go jump into the conduction band only by thermal generation we straight about that right due to thermal energy but this is another way in which electron get knocked off from that covalent bond so as a result maybe this electron gets knocked off and now you end up getting two free electrons you have two free electrons and that is because of this you've got an extra free electron and we got an extra hole remember that part has now become a hole so this is a hole so we have gained two extra charge carriers and now both the holes and electrons will end up accelerating electrons will accelerate towards the right that's what he was doing and the whole wall surrender backs are in towards the left the electrons will start you know taking places like this and now as a result what can further happen is this free electron I don't know maybe after collision after collision it bounces over here maybe it goes and hits and knocks this one off as a result this one now becomes free and you see how you get one more electron hole pair so we just draw that over here you get one more electron hole pair over here and now these three electrons can further go so you see what's happening one electron became two that can even become four this electron can go further and cause more electro on hold pairs and as a result what will happen in a fraction of a second you will see that one electron will multiply into I don't know maybe billions of free electrons and holes so there's a tremendous increase in the charge carriers over here the minority charge carriers a shin just increases incredibly very high increase you'll find and as a result the value of the current becomes incredibly high so a strong current starts flowing this effect is what we call as the Avalanche effect all right we call this as let me write that down here the avalanche avalanche effect and you may have heard of this thing called as an avalanche we usually use that in you know snowy snow covered mountains the process is pretty much the same you see initially if a small tiny piece of snow starts falling down that can hit another piece of snow and other two pieces of snow fall down and that can hit further to put more pieces and so before you know it a huge chunk of snow is falling down this process is called as avalanche so the same thing is happening over here the electrons are producing an avalanche effect and as a result the current starts increasing tremendously and so let me just get rid of this drawing for a while all right and so what do you think will happen as we go further and further towards the left well let's try to finish that complete that craft now so well we would just expect the graph to go forward right well it doesn't in fact because of the avalanche effect after some point so I don't know let me show you some point over here the graph will go like this and this is where that Avalanche is happening and as a result the charge carriers are exploding in number and as a result the current just you know just increases it shoots up alright and this part of that graph this is this voltage this voltage at which this Avalanche becomes significant so this voltage over here oops for that over here this voltage at which the Avalanche becomes significant we give a name to that we call it as the breakdown voltage we break down and the word breakdown I mean it sort of makes you think that the our diode is breaking or something no it's not breaking the word breakdown sort of means that all the covalently bond all the coal and bonds are broke and because a lot of : ones are breaking a heavy current is flowing that's the meaning of the word breakdown all right so long story short in the reverse bias if you increase the voltage too high then we'll find that a heavy current starts flowing okay why should we come I should be be concerned about this oh because you see ideally we would want to make sure the diet doesn't conduct in the reverse bias right that's the whole idea behind using a diode we don't want it to conduct but notice that if the voltage is high enough then it starts conducting and our purpose is lost and as a result when you're using a diode in any circuit if you want to make sure that it works properly we have to make sure that in the reverse bias the voltage that comes across it doesn't exceed the breakdown voltage and every diode who always have a rating for that they will be giving what the breakdown voltage is so for example if if you find out the breakdown voltage for a diode is about I don't maybe seven volt then you're gonna make sure that in the reverse bias the voltage will not come close to seven volt because then you start conducting very heavily another problem that can happen because of this breakdown is that so you see so you might ask you what happens with a diode is a diode like said you know I'm braking or something I really know you see so if you exceed the breakdown certainly the current shoots up and if you now come below the breakdown voltage ideally the diode must revert back all this extra you know charge carriers created must recombine and destroy each other all that should happen and the diet should revert back to its normal conditions but when you are in that breakdown region because of this very high flow of current and because of a lot of collisions happening there is a chance that a lot of heat heating takes place over here and that could potentially melt the diode or that could you know but it might destroy the diode so it's the heat that could destroy that diode the current really doesn't do anything to it but the heat does and so if you're ever conducting an experiment in your lab to you know figure out the VI characteristics and suppose you are near the breakdown region you'll actually see that once you hit the breakdown the current just increases like anything and it's always advised to make sure that you don't run the circuit for a long time in the breakdown region I mean I have done that many times and what you would see is that the diode will slowly start getting smokes from the side and the smokes are coming because of the huge amount of collision which is causing the heat you