 try to understand the characteristic graph for p n junction itself right now. This is p n junction diode. Now in order to draw characteristic graph, we have to measure potential difference and current across the diode and see the different values measure it and then plot the graph simple. So the overall circuit to do that it's like this. You have a p n junction by the way this is how you represent a p n junction in a circuit like this. This flat side this is p side and this is n side. So you have a p n junction like that and in order to measure the potential difference across a p n junction you connect a voltmeter across a p n and I also have to measure the current through the diode fine. So this is milli emitter and then it is connected like this. Tell me whether it is forward bias or reverse bias? Forward. It is forward because p side is connected to the positive terminal of the battery. Now this potential difference which is the supply voltage I can change. So that is why there is an arrow like that fine. So I can change this voltage vary it and accordingly I will get the different values of V and I. So this is forward bias. Now for a reverse bias also I can have a very similar looking circuit. So we will have a circuit. Now in a reverse bias negligible amount of current flows. So rather than milli emitter I will connect micro emitter so that the slightest of the current that is there should be measured. This is the reverse bias now and I am connecting a voltmeter across a diode like this. So reverse bias there should be no current only no. So reverse bias there is a small amount of current that happens. Let me show you what we have discussed earlier. So this is the reverse bias. This one is reverse bias. So if few electrons from this side jump to p side then there will be a current. Now the electron that could jump from n side to p side that is because of what? Because of diffusion are you getting it? So higher concentration is on the n side. p side has lower concentration. When equilibrium is attained then this width is constant. So there will be a small amount of diffusion current because of the flow from higher concentration to lower concentration. But then that is negligible so that is why we say that it is like insulator. But then if you supply tremendous amount of voltage suppose you supply huge voltage then breakdown may happen. What do I mean by breakdown is that all these valence electrons which are there in this junction barrier all the bonds will get broken and then you have lot of free charge carriers carry the electricity. So in a reverse bias also if I have huge amount of voltage then at a certain point the entire junction diode will break and a surge of current you will observe. So if I plot current versus voltage using these circuit trading reverse bias as negative voltage and forward bias as positive voltage and this is what I will get. This is current in the milli here this is voltage. Now if I say that positive voltage is forward bias and a negative voltage is reverse bias then since current also changes its direction I can say that the current in the reverse bias is negative and current in the forward bias is positive. So when it comes to forward bias you will have a plot in the first quadrant where voltage and current both are positive and if it is reverse bias then since voltage and current both are negative it will come in the third quadrant. Now tell me that if I supply small amount of voltage in the forward bias like very very less let's say 0.01 volt will there be any current will there be any current oh amog is watching from youtube I didn't know that amog you can also answer amog you went to picnic you're watching this from your picnic spot are you at home you are at home tell me if I have a very small voltage applied let's say 0.01 volt will there be any current yes or no see when you apply a very small amount of current very small amount of voltage let's say 0.01 volt that voltage may not be sufficient for electron to jump over that barrier which was created earlier fine the electron has to jump from n side to p side for electricity to get conducted and in between there is a potential barrier so external voltage should be more than that junction barrier then only current will flow fine so typically the junction barrier in forward bias is around 0.6 to 0.7 volt fine so if you are applying battery or voltage less than 0.6 0.7 then the diode will not get activated the current will be close to zero only but if you apply let's say one volt or three volt then electrons will gain sufficient kind of energy to overcome the barrier and there will be a good amount of current okay and the entire diode will behave like a conductor with a potential difference of whatever is a junction barrier okay let's first draw the diagram then I'll speak more about it this is how it will be it goes like this and then it grows this is around 0.8 volts where it starts jumping this is the new voltage fine this is a forward bias characteristic all right so let's say I have a diode so this is p and this is n and the resistance are this is connected like that let's say this is 5 volt can you guess what will be the current how much is the current this p and junction will act like a reverse voltage of 0.7 volt so the current will be equal to 5 minus 0.7 which is forward bias junction barrier divided by r fine that's how it will repeat that see what I was saying that in forward bias p and junction just creates a junction barrier okay of let's say around 0.7 volt and after that junction barrier if you overcome that it behaves like a metal okay so if you supply external voltage of 5 volt 0.7 volt it requires to overcome the barrier and rest of the voltage you can treat as if it is connected uh add to a metal piece right so 5 minus 0.7 volt is the effective voltage across the resistance r so in a forward bias it behaves like this fine now comes the reverse bias in reverse bias you have negative potential connected to the p side fine so in a in a forward bias I'm slow increasing the voltage and current increases very fast in reverse bias I can increase the voltage faster I can say that this 20 volt this is 40 60 this is around 80 fine so if I do that and then measure the uh current and voltage then this is what I will get it goes on like this and then there is sudden surge of current this is the breakdown that happens this is breakdown voltage at this uh voltage the entire uh entire diode breaks down what do you mean by breaking down is that at the junction barrier the bonds get broken off and a lot of free electrons get generated that can start conducting electricity fine so there's a surge of free electrons and then the y axis measures the current and in the reverse bias I'm not measuring current in milli ampere but I'm measuring in micro ampere okay because it's a very small amount of current up till 80 volts till the breakdown happens okay now one thing which is noticeable in in this graph is that you know this particular portion this portion is flat why it is flat because after 80 volt one breakdown happens then it becomes exactly like a metal piece the there is no junction barrier also and sudden surge of uh electron comes in and also there is a large amount of external potential that is around 80 volt is already there so suddenly current will rise and this line could be very flat okay you may think that in a forward bias also this line is flat but then if you notice on the x axis the potential is increasing very slowly 0.1 0.2 up to 0.8 okay so a flat looking line here and a flat looking line this side where the potential is changing rapidly there's a much there is a lot of difference okay a flat line over here is way flatter or more steeper than this line okay because here this entire axis is shrink and this axis is stretched so even after stretching the axis this line remains flat fine so I can say that after breakdown the voltage is sort of constant any doubt till now no sir no sir fine so this is with respect to the characteristic of the PN junction diode now there is no point defining you know uh static resistance for this particular what is static resistance a ratio between voltage and current so there is no point talking about static resistance because at every point it changes and usually what happens we are we are going to use PN junction at the varying voltage scenario or varying current scenario what we do we measure the dynamic resistance that is change in voltage divided by change in current okay this is called dynamic resistance at any point in time okay because the slope of V and I is not a constant so this is not a straight line okay even that may not be exactly straight line so there is a curved portion also here so what we do we measure delta V by delta I and say that that is the dynamic resistance fine so