If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:8:38

Video transcript

let's talk about the names that we give to the different parts of a transistor and also talk about some design related details about it we will take MPN transistor as our example but the same will hold true even for a PNP so we'll just take one example and work with it now the names given to these three regions are really based on their functions what they do when you take your transistor and make it work like an amplifier in a previous videos seen how this transistor can behave like an amplifier and so if you need some refresher it would be great idea to go back and watch that video and then come back over here but we saw in previous video what we did we made some connections we connected this side to the ground we had connected this side to you know I think about a positive of 5 volts any voltage is fine here and over this side we had connected about positive 0.7 volt and you quickly recall what happens you see because of the forward bias over here the P side is going to the positive the electrons are emitted from this Junction all the way into this Junction and before they have any time to recombine because of very small amount of holes over here most of them just get swept across and collected over here and that's because of the voltage that positive voltage we have supplied over here so based on that we can actually see the names behind this since this is emitting electrons we call this region as the emitter so this is called emitter and since this is the region that collects the electrons we call this as the collector and this thin region in between is called the base so the three parts of the transistors are emitter base and collector and notice in order for amplification to work if you want to make this work as an amplifier then the necessary condition that the two Junction must must follow is that the emitter base has to be forward biased this has to be forward biased and the collector base look at what's happening to collector base it is reverse biased because notice the n-type is connected to more positive than the p-type so this Junction has to be reverse biased this is the necessary condition and we'll see what happens if you don't followed by us this if you've reverse bias the emitter base Junction then majority charge carriers can't diffuse through and as a result these electrons will not get emitted and nothing will happen and similarly the collector base Junction has to be reverse biased because these injected electrons have to get swept across they can only get swept across remember these injected electrons in this region act like the minority charge carriers in the p-type electrons are minority and for them to get swept across we need a reverse bias we've spoken about this in previous videos all right and similarly if you had forward biased it what would have happened well if your forward bias this Junction first of all it would be hard for these electrons to get swept across but more importantly the electrons from the collector would start diffusing into the base and as a result the net flow of electrons from across upwards from base to collector oh that would be decrease right because there's be some flow this way and there will be some fall this way as well and so just mess up with our amplifier working and therefore we don't want this region to be forward biased alright now we may have one question in your head is in the collector and the emitter identical if you were to flip the transistor upside down it won't look any different so what's the difference between them well from the operation point of view there is no difference however if you look from the design point of view there are certain differences and here's some of them first of all since the emitters job is to you know inject electrons into the base we would like to dope the emitter very heavily so emitters are always always heavily doped heavily doped regions and the idea is the more you dope them the more electrons get injected when you forward bias it and so more electrons could get collected and as a result you would have more amplification on the other hand look at the base region the base region we have already seen for proper working of a transistor it has to be thin very thin oops very thin and lightly doped very lightly doped okay okay now what about the collector well if you think about it the collector doping doesn't really matter for us right I mean look at in our amplification what matters is how many electrons get injected and that depends upon the doping of the and how many electrons recombine that depends on this right the electrons that don't recombine will eventually get collected regardless of how many electrons are there in the collector so strictly speaking you can whether whether you dope collector very heavy or you dope it very lightly it's not going to affect our our mechanism all that much now having said that there is one thing we need to take care of you see this PN Junction is reverse biased and we've seen before that when a PN Junction gets reverse biased if the voltage exceeds a particular value it undergoes breakdown that means a very high current starts flowing from n to P we don't want that to happen if that happens that'll mess up with our with our amplification action we don't want this Junction to ever undergo breakdown when our transistor is working as an amplifier so what we have to do is we have to make sure that this Junction has a very large breakdown voltage so that it can it can withstand a lot of voltage before undergoing breakdown let's just quickly write that down we need the collector base Junction we need the C B Junction to have large large reverse breakdown voltage we usually call it as V R so how do we achieve that now here's a simple way to think about it without getting into too much details the lesser you dope this region because this region is already very lightly doped so the lesser you dope this region the larger the breakdown voltage I like to remember our Zener diode remember Zener diode is very heavily doped and under such heavy doping it has a small breakdown voltage that's the speciality well lighter doping means larger brittle voltage so if you need more clarity on how that happens and how it's affected how the depletion region gets affected it would be great I do go back and watch that video on Zener diode again but anyways so to make sure that this reverse breakdown voltage is high enough the collector is usually lightly doped you get that is nothing to either operation of amplification but just to make sure doesn't undergo breakdown so this is lightly doped now you may ask well how lightly doped is it and this is not really all that rigorous it depends on different transistors it turns out in some transistors the doping concentration of collector is moderate between that of the emitter and out of the base well in some other cases it's seen that the collector doping is even kept smaller the of the base so there is no hard and fast rule but what is important is that the emitter must be heavily doped and the base has to be thin and very lightly doped the collector doping is only the moly to ensure all these other design related issues all right so to make it visually more accurate let me show a little bit less electrons in the collector compared to the emitter one more difference in emitter and collector is in their size it turns out that the collector is very big in size all right so to be more visually accurate it would be better to draw a transistor a little bit big and that's what you will see in most drawings you will see that the collector is usually drawn wrong bigger than the rest of the regions of a transistor and again this has nothing to do with the amplification action it's again from the design point of view the idea is roughly this you see we want the transistors to be big enough so that when the currents are flowing through it it doesn't heat up very quickly if it's very small it's temperature will shoot up very quickly and again we can have problems emitter or we can't make that region big because you know we want to make it heavily doped so it's easier to dope something very heavily if it's small base is anyways very thin it has to be thin for our upper for our amplification it's the collector that really doesn't matter how much you dope it and since we are moderately or lightly doping it we like to make the collector big another reason for collector being big is that's the design so this this is not what a real transistor a practical transistor would look like in reality a translation would look someone like this you ready here is a real transistor looks somewhat like this this is how the design is for a real transistor so you again have the emitter notice how small it is you have a thin base region and you have this huge collector region and the terminals are over here so this will be encapsulated and you will have three wires coming out from here so we can do our attachments and so collector is usually moderately doped and it's also one of the biggest regions of our transistor