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Current time:0:00Total duration:11:50

Forward and reverse current mechanism

Video transcript

in a previous video we saw that if you forward by us a pn-junction which means you connect the positive terminal to the p-type then our PN Junction conducts current from P to M but if you reverse the battery if you reverse bias it that is you connect the negative terminal to the p-type then the diode doesn't conduct of course a very tiny current flows through from n to P due to minority charge carriers in conduct well in this video we're going to go a little bit deeper and explore the subtleties involved in the mechanism of the charge flow both in forward bias and in the reverse bias this will be extremely important later on to truly understand the mechanism of a transistor so let's begin let's start with forward bias we've seen that if the forward voltage is roughly around points on world if you reach point seven volt a heavy current flows over here the holes migrate into the end region and the electrons end up migrating into the P region but here's the big question what's really causing this current why are the holes moving there and why are the electrons moving over here now since we have two kinds we have holes and electrons moving it would be a little bit difficult to keep track of everything so let's pause this animation for a while and let's only concentrate on one of them let's concentrate on the holes the electrons will then be similar to each other so here's the question what's causing these holes to migrate to the N region what's causing that well I should think of it this way let me just concert on one hole and show you what I should think so if there's a hole over here I would I would think like you know there's a positive charge over here and that pushes the hole there's a negative charge over here that pulls the hole negative terminal that pulls the hole and as a result the hole starts moving from P to n very nicely and that's what's causing the current simple right guess what that's not true and that's not how it works this is how conduction works in conductors maybe but not in a forward biased semiconductors and here's the reason why remember that even before we attach the battery there was a depletion region formed and in that depletion region there was an electric field a strong electric field over here and it was this electric field that was preventing that hole from crossing the right because it is acting like a barrier it's pushing it backwards well guess what the battery is only supplying the hole enough energy to overcome that barrier at point seven volt it barely gives it enough energy to overcome the barrier it doesn't have any energy left after it all comes it doesn't have any energy left to go all the way to this side that's not possible so what's really causing that current is the big question the answer is diffusion you see because there's a lot of holes on this side there's a high concentration of holes on this side but there's very low concentration of holes on this side this difference in concentration makes the holes go from here to here this process is called diffusion so it's the diffusion that's driving the holes from here to here not because they're being pushed by the battery same story for the electrons because you have high number of electrons here less number of electrons here that's making the electrons diffuse from n to P so diffusion is the reason that's happening however there's another question now we can ask you see diffusion only diffusion only happens as long as there's a difference in the concentration and that's happening right now makes sense but as time passes by as more and more holes diffuse from P to n don't you think that the number of holes the concentration of the holes who here will increase and the concentration of the holes here would decrease wouldn't eventually the concentration of holes equalize and if that were to happen diffusion would stop because they would no longer be a difference in the concentration right I mean that's what would have happened if you had a gas in a room initially the gas would diffuse all the way until it has filled up the entire room and then there would be no more motion on at least on a macroscopic scale so why doesn't that happen if that was to happen then the diffusion would have stopped and the current would have stopped in the in the circuit but that doesn't happen the current doesn't stop so what's happening what's going on well the answer or the key to understanding this is recombination recombination the recombination is what makes sure that the diffusion never stops and here's how it works you see when the holes diffuse into the n-type don't think so again let me think of this hole again so when this hole diffuses we can get rid of this electric field let's forget about this field now it's not there okay once this holes dis whole diffuses into the n-type don't think that the hole will keep moving due to the diffusion it can't do that because there are a lot of electrons over here and as a result even after travelling a very tiny distance there's a very good chance it'll recombine with the hole and it'll be destroyed another way to think about this is that the chance of a hole reaching all the way to here is extremely tiny because it has to all it has to cross so many electrons is a very good chance they'll just recombine somewhere over here so if we look at the concentration of the holes on this side this end the hole concentration will always be minimum there will be a minimum number of holes over here simply because due to a lot of recombination holes have a very tiny chance of reaching over here but we can say the same thing about electrons right electrons also have a very tiny chance of reaching over here and therefore there will be minimum number of electrons on this side which means there will always be a maximum number of holes on this side that make sense maximum number of holes and guess what as we move from the here to here we will see that the concentration of the holes gradually decreases gradually decreases and this concentration gets maintained it stays that way it doesn't change with time it doesn't change because of recombination the number of holes over here can never increase because the chance of holes reaching over here is very tiny number of holes over here cannot decrease because chances of electrons reaching over here is very tiny and the same thing you know works out throughout the entire region and because this difference from here to here gets maintained with time diffusion can never stop so what's really causing this forward current in this forward bias is the recombination and that's why in some textbooks or in some articles you would see that they actually call this as the recombination current because if the recom was not there if electrons and holes were not to recombine with each other then the concentration would have equalized and eventually the the current would have stopped so the forward current is due to the recombination all right one last thing could be what happens to these holes when they eventually reach I mean yeah some of the holes eventually do reach at this end right so what happens to these holes which make it all the way over here well remember there are electrons in this metallic contact and as a result the electrons gets sucked in over here and the hole gets destroyed but because this part missed this metallic contact lost an electron that pulls an electron from here and that ends up pulling an electron from here and that's how all the electrons get pulled one by one and so that's how the current even flows in the external wire and eventually an electron gets pulled from here that pulls one electron from the coal and bond over here which makes a hole at this point so one hole gets generated over there which means for every hole that is lost over here one hole gets generated over here and that's how the story continues so long story short in forward bias is the diffusion that's causing the flow and the diffusion never stops because the concentration never equalizes because of the recombination effect all right now let's talk about the reverse bias you see because the negative terminal is connected to the P side and the positive connect to the N side what happens is the depletion region widens and as a result the strong electric field is present in this depletion region which means diffusion almost stops however the minority charge carriers for example the holes in the n-type again let's concentrate on Leon one hole the minority charge carriers if they ever reach into this depletion region the note is due to the electric field they get accelerated and that causes a current in the external circuit so there is a very tiny current flowing from n to P due to the minority rgeous but here's the thing you see this current is pretty much independent of the voltage I mean even if you increase the voltage the current doesn't change pretty much it remains a constant and that what I'm going to talk about over here why is that happening whoa over here notice that since the hole is inside this end region which is a lot of electrons over here as the hole is moving there's a very good chance that it recombines immediately and as a result what's happening over here is this hole is recombining and then somewhere else due to thermal generation that hole is coming back again it might recombine and again due to thermal generation it comes over here so don't think that the hole just gets swept across very nicely no no oh it spends a lot of time over here in order to get swept across we need the hole to get thermally generated in the depletion region so suppose that the hole recombines and then maybe maybe by chance it gets thermally generated over here once it gets tunnel generated then it gets swept across and so in order to have a current we need thermal generation happening right at the depletion region now I imagine due to the thermal generation we have five holes being created for a second as an example five holes being created per second then we'll see that all these five holes will get swept across and as a result we'll have a current due to these five holes but if we were to increase that voltage if you were to increase the strength of this electric field maybe then the holes would accelerate more true but if there are only five holes being generated per second don't you see that it will still cause the same current because current by definition is the number of charges flowing per second the number of shells flowing over here the number of holes is still the same five so you see even if the holes are getting swept faster and faster if you increase the voltage that current pretty much stays the same because the current is the number of holes that are being swept across for a second so if you want to increase that current then what you need to do is you have to increase the generation happening over here and that can either be done by increase in the temperature because if you increase the temperature there are just more chances of generation and as a result more chances they're more carriers will be generated over here or maybe you could shine light we'll talk about it in other videos but you can shine light and again you can increase the charge carriers over here but whatever it is the long story short the the current over here is not driven by the voltage it's only driven by the generation effect and it's for that reason this reverse current is often called as the generation current it is often called as the generation current so you see the forward current so this is in forward bias forward bias the forward current is usually called the recombination current this this allows the diffusion to happen and the reverse current is caused due to the generation the generation is what causes that in the reverse bias and this is pretty much independent of the voltage