# Video: Transistors

In this lesson, we will learn how to describe how transistors can be used as electrical switches in circuits.

13:33

### Video Transcript

In this video, our topic is transistors. Transistors are part of many modern electronic circuits. And this includes microprocessors that we find in computing devices. There are many different types of transistors, but in this lesson we’ll focus on two and see how they work.

Both of these transistor types we’ll talk about are bipolar junction transistors. To better understand this term, let’s begin by considering this word “junction.” From our knowledge of electrical diodes, we can recall that a junction is formed whenever we take a semiconductor material of one type, say, a p-type, here and join it up with the opposite type of semiconductor material, n-type. A diode could either be formed by p and then n second, or we could flip it around and have n and then p. Either way, at these interfaces where the two different semiconductor material types meet, we have a junction. With these two types of diodes, p-n and then n-p, we could see that they’re different just by their polarity.

If we hook up our p-n diode to an electrical circuit with our voltage source oriented this way, we know that with the positive terminal of our source pointing to the left, it will tend to send current clockwise through the circuit. And with this particular diode arrangement, a p-type semiconductor then joined to an n-type, charge can flow across the junction between them. As a result of this, current can exist all throughout the circuit. So as far as the polarity of our diode goes, we can say that it’s forward biased. It’s arranged in such a way that charge can successfully flow all the way through it.

But now let’s consider attaching our circuit to the n-p diode instead. This is identical to p-n, but it’s been switched around. Its polarity is reversed. And that means when effective positive charge begins to flow clockwise through the circuit, in this case it’s unable to cross through this junction from n-type to p-type. So the diode blocks the flow of charge. And this means no current can exist anywhere in the circuit. When a diode is arranged this way relative to a voltage supply, we say that it’s reversed biased.

Now we bring all this up because when we talk about transistors, we can think of them as being two diodes joined together. Say, for example, that we take our n-p diode and we attach it to the end of our p-n diode. We would get then this stack of semiconductor materials, n then p then n-type. When we do this, we formed a bipolar junction transistor. And specifically, this is called an NPN transistor. Up to a point, we can think of this transistor as two diodes joined together. We’ll talk in a moment about when that doesn’t apply, but in many ways, thinking of a transistor this way helps us understand how it works.

To see that, let’s look at how this transistor works when we connect it up to a circuit. With our cell arranged this way, we would expect conventional current to point in a clockwise direction. But we see that when that current encounters our transistor, it runs into what is essentially a reverse-biased diode. This means that charge can’t cross the junction between these n- and p-types. Since charge doesn’t flow then through this transistor, current can’t exist anywhere in the circuit. So right now, our transistor really isn’t working as it should. All it’s doing is shutting down the flow of charge.

But in general, a single transistor can be used as a switch in a circuit or to amplify a current. For that to happen, we need to modify our circuit a bit. We’re going to add a branch in our circuit right here. And along with that, we’ll include a second voltage supply. This supply will send conventional current up into the p-type part of our transistor. And recall that if we consider this current moving in this direction through our junction, then we can say that this diode we’re effectively working with is forward biased. So because of that, charge can flow through this part of our transistor. And therefore, current can exist in this particular part of our circuit.

With all that charge flowing in this smaller loop, if we then consider the current generated by what we called our primary voltage supply, now with charge already flowing this way through part of our transistor, the current coming from our primary supply actually is able to cross this junction here than it normally wouldn’t be able to. We mentioned earlier that we can think of transistors as two diodes joined together but that that’s only true up to a point. Well, this is that point. Because if we were just thinking of our n- and p-semiconductor types as a diode, then charge flowing across this junction wouldn’t be possible.

But since we’re working with a transistor and we already have charge flowing through this forward-biased portion of it, under those conditions, charge actually can cross this barrier between the n- and the p-type. And then once electrical charge has made it that far, there’s nothing to stop it from continuing on all the way through the transistor. And so then charge is able to flow all the way around this outer, larger loop in our circuit.

Now in a typical transistor circuit, the current here, that in effect turns the transistor on, is very small in magnitude compared to the current here coming from our primary source. By this smaller current effectively switching the transistor on, we can see then that we’re amplifying current through the transistor as well as allowing current to exist at all in this portion of our circuit.

Now that we’ve got our transistor circuit working, notice that there are three wires that connect to the different parts of the transistor. Each one of these wires or connection points has its own specific name. Recalling that conventional current travels clockwise through our circuit, this electric wire here is called the collector. This is how our transistor collects charge. Paired with this, on the other end of our transistor, we have the emitter. This is where conventional current or charge comes out. And then this last connecting wire here that goes into the middle of our transistor is called the base. The base is what we supply with this smaller amount of current from our secondary voltage source that then travels out through the emitter and then allows charge to flow overall from collector to emitter.

When we see a transistor like this drawn in a sketch or a diagram, sometimes the collector, base, and emitter will just be indicated by single letters. And note that we could also think of these names as applying to the different regions of the transistor. When we see this, it can help us remember what each letter stands for and why those names have been given.

So far, we’ve been looking at one particular type of bipolar junction transistor, the NPN type. The second closely related transistor we’ll consider is the PNP transistor. Like we might guess, that type of transistor is formed by a p-type then an n-type then a p-type semiconductor joined together. If we connect this kind of transistor up to a circuit and if we once again consider this connection point the collector, this one over here the emitter, and this one the base, then if we once again connect up primary and then secondary voltage sources, for this transistor circuit to work properly, we actually need to reverse the polarity of both of these sources.

Set up like this, if we only consider the effects of our primary voltage supply, we know that, oriented this way, that will send current in a counterclockwise direction. When it reaches the transistor, that charge will be able to cross over this p-to-n junction here. But then when it reaches this effectively reverse-biased junction between the n- and the p-types, the charge will be blocked. So then no charge will flow through the transistor and, therefore, no current will exist in the circuit.

Just like before, though we have this second voltage supply that sends current in a counterclockwise direction through this smaller loop. And charge can indeed flow through this forward-biased p-to-n portion of our transistor. When that happens, just like before, it enables the passage of this current traveling through the larger outer loop by letting the charge in that current pass over this junction here and continue on out the collector of the transistor. And so by turning on our transistor, we could say, by establishing current through the smaller loop, we allow charge to flow all the way through the transistor and, therefore, all the way through our larger circuit.

So these are the basic ideas behind how PNP and NPN bipolar junction transistors work. Though these two types share similarities, notice that the direction of current through them is opposite. In the NPN transistor, conventional current flows into the base and then out the emitter, which allows what we could call the main circuit current to enter through the collector and leave through the emitter. On a PNP transistor though, everything is reversed. The conventional current coming from the base branch of our circuit enters the transistor through the emitter portion. And this allows the main circuit current, we could call that the current in the larger outer loop, to travel from emitter to collector through the transistor.

So depending on whether we’re talking about an NPN or PNP transistor, the direction of conventional current flow through the transistor varies. Knowing all this, let’s get some practice with these ideas through an example.

A PNP transistor is connected to a direct current source, as shown in the diagram. The two p-regions are identical. Which of the regions of the transistor is the collector region? Which of the regions of the transistor is the emitter region?

Looking at our diagram, we see our PNP transistor connected up to an electrical circuit. The three regions of this transistor called P one, N, and P two describe a specific type of semiconductor material, either p-type or n-type. We’re told that the two p-regions in this transistor, P one and P two, are identical. And seeing how this transistor is arranged in this electrical circuit, we first wanna answer the question of which of these three regions in the transistor is the collector region.

Now, this term “collector” refers to a connection point into the transistor. And we see from our diagram there are three of those, this point right here, this one, and then this one. To help us figure out which of these connection points attaches to the collector region, we can recall that in general a PNP transistor has a collector region, a base region, and an emitter region. And note that the collector and the emitter both correspond to p-type semiconductor regions.

So considering the PNP transistor in our sketch, we know that the answer to this first question, which of the regions is the collector region, is either going to be P one or P two. That is, one end of this transistor is the collector and the other is the emitter. But which one is it? Is P one the collector and P two the emitter? Or is it the opposite?

To answer this question, we’ll need to look at the way that conventional current travels through the transistor. Because of the polarity of our voltage supply, we know that conventional current will point in a counterclockwise direction all through this circuit. And note that we’re assuming our transistor has been switched on so that current can indeed exist all through this loop. Now that we knew this, the real question, we could say, that this first part of our question is asking is, for a PNP transistor, does conventional current enter through the collector or does it enter through the emitter?

Based on the names of these regions, we might expect conventional current to enter through the collector and leave through the emitter. As it turns out, though, that’s only true for an NPN transistor. But here we have what we could call the opposite type, PNP. That fact means that the direction of conventional current travel through our transistor actually moves, as we’ve drawn it, from right to left, from the emitter to the collector. So then it’s the second p-type region that our current reaches which is the collector in a PNP transistor. And as we look at the way charge will flow through this transistor in our diagram, we see that that second region is P two. Since that is the last region of our transistor that charge passes through as it moves in this circuit, we know it’s the collector region of our transistor.

Now that we figured this out, finding the answer to the second part of our question is simpler. Here we want to identify which region of the transistor is the emitter region. We’ve seen that in a PNP transistor, charge moves from the emitter to the collector. And therefore, in our diagram, the first region of our transistor that conventional current encounters is the emitter region. That’s P one. And that’s our answer to “Which of the transistor regions is the emitter region?”

Let’s take a moment now to summarize what we’ve learned about transistors. In this lesson, we saw that transistors are electrical components that are made of n-type and p-type semiconductor materials joined together. Two common types of transistors, both called bipolar junction transistors, are NPN and PNP. When an NPN transistor is set up in a circuit, charge is first to set up to flow through the forward-biased p-n junction of the transistor here. This means that charge flows into what is called the base region of the transistor and out the emitter region. And this allows charge in the larger circuit loop to flow in to the collector region of the transistor and then out the emitter.

With a PNP transistor, on the other hand, when this transistor is in a circuit, all these current directions are reversed. First, charge flows through the forward-biased p-n junction here in a counterclockwise direction. And then this allows charge to flow this way from emitter to collector through the transistor overall. So then in an NPN transistor overall current travels from collector through emitter, while in a PNP transistor it travels from emitter to collector. This is a summary of transistors.