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Reverse biasing a PN junction

What happens if you connect the negative terminal to the P-type? In this video, we will explore what happens to a PN junction when it's P side is connected to negative terminal and N side to Positive terminal of a battery. This connection is called the reverse bias. Created by Mahesh Shenoy.

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Video transcript

we've seen in previous videos what happens when you take a PN Junction and attach a battery in such a way that the p-type is connected to the positive and the n-type is going to negative our PN Junction conducts very heavily very nicely it allows the flow of charges from p-side to n-side and we call this as forward biasing in this video we're going to figure out what's going to happen if you reverse the connections what will happen if you connect the P site to the negative and n site to the positive so let's do that so let's reverse the connections reverse the connections excellent so now that now we have a negative side connect to the P and the positive side connected to the N let's see what's going to happen well as of now nothing will happen because the voltage is pretty much zero we are still at equilibrium the diffusion is happening drift is happening in the opposite direction the two currents are equal the total current is zero you've spoken all about this in previous videos so if you need more clarity it would be great idea to watch them and then come back over here we've also seen there is a potential barrier of 0.7 volt in built barrier we even without any external batteries it's already there okay so what do you think is going to happen if we turn up the voltage knob if we increase give some voltage so let's do that let's give some voltage to it so let's zoom in to the voltage knob and let's turn the voltage let's make it I don't know maybe one would let's turn it to one would what do you think is going to happen just pause the video for a while and think about it well there are two ways to think about it one is we could say notice now that we have a negative terminal attached to the P it starts pulling on the holes because there are a lot of holes over here remember holes are like positive charges and so the negative term and starts pulling on the holes the positive terminal starts pulling on the electrons and as a result notice that the majority charge carriers are moving away from the junction exposing the charge is exposing the ions in other words they are increasing the depletion region can you see that so the the main thing that's going to happen is that the depletion width the depletion width is going to start increasing so the depletion width the barrier starts increasing it becomes why all right and another way to think about this is we could say that because the batteries it has this way the potential barrier has increased can you see that now it takes more energy for for a charge to move from here to here for a positive charge let's say to go from here to it it takes more energy before it took less energy now this height of the barrier is increased so the barrier has increased and you have to excuse my drawing the barrier is entering into the semiconductor but it is okay the barrier has increased and guess what if you add 1 volt the barrier increases by 1 world all right either ways what do you think happens to the diffusion current think about this well now notice the diffusion has become harder it makes holes and electrons will they want to diffuse into each other but because of this huge potential barrier now it becomes even harder for them to diffuse and guess what if the potential barrier is large enough diffusion might even stop all right so the diffusion current over here diffusion current might decrease and it might even go and become zero you know it might even go and become zero so diffusion has stopped for all practical purposes what do you think happens to the drift current think about that I remember a drift current is due to the minority charge carriers there are some holes over here and the hole enters into this region whoa it's gonna accelerate this way and similarly even the electrons enter into this region the X or it that way what do you think happened to the drift current well you might think now that the barrier has increased the electric field has extended the drift current might increase right well turns out the drift current doesn't change much it pretty much remains a constant we'll explore why in another video but the drift current does not depend on this voltage or any ways as a result not as the equilibrium is broken and there is a net current flowing from n to P the drift current is winning diffusion is almost gone so there is a net current from n to P and so the PN Junction starts conducting in the opposite direction starts conducting in the opposite direction this way n to P or opposite direction means what happened in the previous case in the forward bias alright what do you think the ammeter will do well the ammeter won't show anything maybe like why because chances are that this current is extremely tiny you know why it's extremely tiny well you can actually see it over here it's a very tiny current because it's due to the minority charge carriers the word minority is telling us that you know minority that are very small in number so this current is in fact in fact if you do the calculation turns out it's about a thousand times smaller than what we get in the forward current so this current is so tiny we can actually assume that this current is approximately zero so this current we can pretty much approximate and say it's zero and so if you think of it this way and naught is we could pretty much say that the PN Junction is now acting like an insulator when you attach the battery this is when you try to push the charges this way it acts like an insulator because it hardly conducts and so guess what if you forward bias it the PN Junction conducts now if you attach the battery this way we call this as the reverse biasing because exactly the reverse of what we did so this is called reverse biasing so if you reverse bias it the PN Junction doesn't conduct and guess what that's exactly what we wanted we have now built something that only allows the flow of charges in one direction but it doesn't allow almost almost doesn't allow any flow of charges in the opposite direction