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Current time:0:00Total duration:6:50

Video transcript

in a previous video we've seen that a pn-junction at equilibrium has two currents one is called the diffusion current this is due to the majority charge carriers the holes and electrons are diffusing into each other causing a current from P to N and this is an indicator of how much the diffusion current is it's pretty low right now and the reason it is so low is because a barrier exists right at the junction you've seen that due to the recombination effect almost all the holes and electrons have destroyed each other an electric field exists and that pushes the holes and the electrons in the opposite direction and and we've seen that we can think of it as a potential barrier and the barrier is about point 7 volt for silicon at room temperature but there's a second kind of current and that current is due to this this barrier acts like a downhill for the minority charge carriers the minority charge carriers as a result end up moving in the opposite direction which causes the second kind of current we call this drift current and that current is from n to P in the opposite direction because holes are going in the opposite direction and again this is an indicator and it's very low because minority charge carriers have very low to begin with and at equilibrium the two currents are exactly equal to each other and opposite and so the total current is zero and we've seen this in previous videos so if you need more clarity it would be a great idea to go back watch those videos and then come back over here but in this video we're going to attach some metallic let's put some wires and attach and put a battery over here and see what's going to happen so let's do that and we can connect our cell in two is one we connect the positive to the p-type or we can even shift this and connect the positive to the n-type either way is fine so in this video let's see what happens when you put the positive to the p-type and this is no ordinary cell this cell comes with a voltage control device so we can change the voltage how much ever we want yay so we can play with this and right now the voltage is zero as you can see by the indicator and let's also attach an ammeter to our wire so that we can keep track of current notice right now the current is zero because you not applied any voltage to this all right let's apply some voltage let's ink the world edge a little bit let's increase it something let's say 0.2 or 0.3 world what do you think is going to happen well notice because the positive is coming to the p-type it starts pushing the holes this way and similarly because the negative is coming to the n-type it starts the battery starts pushing the electrons this way and as a result I hope you can see the holes and electrons are now the majority charge carriers are now being encouraged they are being pushed to defuse more and as a result the diffusion current starts increasing and so if you look over here the diffusion current will go up a little bit a little bit because you're putting a little bit of voltage and notice as a result diffusion is no longer equal to drift and because of that there will be a net current flowing now since diffusion is from P to N and that has increased there will be a total current now going from P to N and here's a simple way to think of what's going to happen to the entire circuit we know that you may have learnt already in circuits and everything that the electric current in the entire circuit like this must be the same so if the current over here is flowing from P to n the current should flow in the same direction everywhere so there will be a current through this wire as well and as a result our ammeter will now show some current and the current will be in this direction it will be from P to and inside the semiconductor this way now it's very interesting to think about how these holes eventually manage to come here and what really happens how how does the electrons start flowing and everything but we'll talk about it a little bit later as of now let's just concentrate on how the world it affects the diffusion current and there's another way to think about this instead of thinking that the battery is pushing the holes and the electrons and increasing their energy and that's why diffusion is increased we could think that the battery is just lowering the energy of the barrier same thing right increasing the energy of the charges or you lower the energy of the barrier the effect is the same and that's why most people I think like to think of it that way we'll do it that way as well we'll just imagine that the battery is not doing anything to the holes and the electrons but just end up decreasing so let's assume that the barrier the height of the barrier is decreasing this is easier because I'll tell you why because if you put point two volt over here we could just say that the barrier has reduced by point two world and because the barrier has reduced the charges can now start diffusing more it becomes easier to defuse and as a result I hope you can see even the width of the depletion region is decreasing and as a result we might expect the drift current to decrease a little bit because the drift was caused due to this potential difference in the first place right but the effect is extremely tiny on the drift and so we can pretty much you know assume drift current is a constant because it's a very tiny value anyways it's all about diffusion let's only focus on the diffusion okay let's let's increase this voltage what do you think will happen now well the effect continues let's say we double this voltage right now it's point two let's make it let's say let's double it and let's say they'll go to point four or something well the height of the barrier will further reduce and as a result the electrons and holes can diffuse more the diffusion current will increase and here's a thing you might think that if you double the voltage over here the diffusion current might double right no it the effect is nonlinear because diffusion is a bit more complicated it turns out the diffusion current rules become more than double it might be triple or something like that alright and the depletion region becomes even narrower and as a result the current will increase much more than double though the current will increase a lot and eventually if you get this voltage all the way to point seven world then all hell breaks loose now the barrier height is almost gone it's zero the depletion region almost radishes the electrons and the holes are now completely free to diffuse into each other and as a result the diffusion current skyrockets it becomes extremely high so the current becomes very high in the circuit and so as a result whoo I hope you can see when you connect a PN Junction this way p-type to the positive terminal n-type to the negative the diode starts conducting it allows the flow of charges and this connection is called forward biasing the word biasing just means connecting a battery all right so in the forward bias mode the PN Junction conducts quite heavily