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Current time:0:00Total duration:7:08

Video transcript

we've seen in previous videos that when you forward by as a pn-junction it conducts and when you reverse bias it it doesn't conduct in this video we'll see how to summarize this entire behavior of a PN Junction in a single picture this picture should tell us how the current through the PN Junction changes as we change the voltage across it so the picture we're gonna draw is going to be a graph of voltage versus current so the world edge across the device will be plotted along the horizontal and the current that falls through it will be along the vertical alright let's begin let's start with the forward biasing when forward bias we've already seen that the positive is connected to the p-type and the negative is connect to the n-type and a current flows it conducts a current flows from P to n the direction of the current is from P to n so what does the graph look like well initially when the voltage is very tiny the current is not very significant you may recall that there is a depletion region that exists over here and as a result there is only a small amount of charges are about to flow through it acts like a barrier but then as the voltage increases that depletion region becomes smaller and smaller and as a result the current is growing I hope you can see that and eventually after a particular point the depletion region vanishes and the current skyrockets so this is how the how the PN Junction behaves in the forward bias and this point after which the current increases very quickly we usually call that as the cut in voltage let me just draw that over here so this point approximately this voltage over here we call that as the cutting voltage this voltage is called the cut in voltage it's also called as the knee voltage or whatever but you usually can call it as the cutting voltage and that for silicon diode is approximately 0.7 volt this is about 0.7 volt and so you can see before points are in world the current is slowly increasing but once you hit 0.7 then even for tiny changes in the voltage the current skyrockets the current is very dramatically so--that's forward by us now let's look at what happens in a reverse bias you may recall that reverse bias is when we connect the positive to the n-type and the negative to the p-type we'll plot this voltage along the negative axis and in this case is a very very tiny current that flows in the opposite direction from n to P a very tiny current so we'll plot that current along the negative current axis so what does this graph look like well in this graph we've seen that that current is very tiny so pretty much it remains near zero itself so current is remaining near zero itself but more importantly the current is almost a constant it's almost a constant we've seen that in the previous videos and so the graph that you get is something like this and if you're an electronic engineer you would absolutely love this graph because without getting into the details of how and what and why it pretty nicely summarizes what this PN Junction does under various circumstances it tells us that when you have forward bias it the current is very low until points are north then very quickly skyrockets and it becomes extremely sensitive to voltage after this point under reverse bias it hardly conducts it acts like an insulator but a very tiny current flows through it pretty much a constant it's also telling you that this is not a straight line and as a result this is a non ohmic device you might recall that if you have a conductor then the VI graph for that would be a straight line and so it's an ohm flaw works for that but over here Ohm's law doesn't work so it's also telling us that the PN Junction it's a non ohmic device Ohm's law doesn't work ok that's pretty much it but now let's look at some technical names of this an engineer would call this as a characteristics it's called it as the VI this graph is called as VI characteristics characteristics of a PN Junction all right now even though this is a non ohmic device so Ohm's law doesn't really work over here we like to think in terms of resistances and so if you go in the forward bias this is the forward bias over here no is because it conducts very nicely we could say it is having low resistance so we usually say that in ford by us pn-junction has low resistance low resistance and similarly if you go to now reverse bias because it hardly conducts its acting like an insulator we would say it has a very high resistance to current so we would say high resistance high resistance in reverse bias and one last thing is if you were to you know draw a circuit with the PN Junction in it it would be tedious to keep drawing this diagram over and over again so we have to come up with a circuit symbol right so here's a circuit symbol of a PN Junction the symbol looks like this so whenever you are seeing a PN Junction used in a circuit you probably see something like this so this this side represents the P side and this side represents the N side now notice what the symbol is telling us the circuit symbol is telling us in what direction our PN Junction conducts the arrow mark is telling us that the PN Junction conducts along P - n from P to n so if you want this thing to conduct you better put a positive here and negative here followed by acid on the other hand if you try to make it conduct from n to P uh-huh it won't work see there's a barrier over here it doesn't work so that's the motivation behind this symbol and one last thing is that people usually call this PN Junction as p-n-p-n diode or semiconductor diode or a junction diode and the word diode has two things in it it has the word die di die die means - - and that's referring to the two terminals that we have one side is P and one side is n the two terminals and the word or well I really don't know what this means because there are a couple of sources which one source tells us that it is a short form of the word electrode because we use electrodes here another phone is telling us that it spot whichever it is I don't know what is really is it could be a short form electoral part but what is trying to tell us the word dire is telling us that this is a device in which you can create a part from one electrode or one terminal to the other terminal for charges to flow you can have a path from P to n under circumcircle circumstances like the forward bias but if you reverse bias it it doesn't allow that that path is not there so usually when you hear the word diode what what usually comes to my mind is it's a device which conducts only in one direction method