Video: Eq17S1-Physics-Q43B

Draw a labelled diagram for a circuit that uses an NPN transistor as a switch in an OFF state.


Video Transcript

Draw a labelled diagram for a circuit that uses an NPN transistor as a switch in an OFF state.

Starting out, we know that one element will include in this diagram is an NPN transistor. But what is this type of transistor? This type of transistor is essentially a sandwich of three semiconductor materials. The materials on the outside are called n because they have negatively charged charge carriers, electrons, while the semiconductor material in the centre is called p type because it has positively charged charge carriers or holes where electrons could be.

Based on these charge carriers signs, positive and negative, the interfacing regions in these different types of semiconductor materials tends to allow electron movement in one direction across the barrier but not another. For example, moving from a p region into an n region, the allowed type of charge flow is for positive charge to move in that direction or in other words for negative charge to move in the opposite direction. The same thing happens at the other p-n boundary. Positive charge is allowed to move from p to n and negative charge is allowed to move in the opposite way.

Now, let’s imagine we were to connect up this pnp [npn] transistor to a circuit. We can start to see what will happen if we pass conventional current in a clockwise direction toward our transistor. We can see right away that this current will run into an issue. Our first boundary region between adjacent semiconductors will only allow positive charge to move from right to left. Positive charge motion in the opposite direction is prevented. When two semiconductor materials with oppositely charged charge carriers oppose the flow of conventional current, they’re set to be reversed biased.

That’s what’s going on here. The positive charges in the conventional current created by our cell want to move clockwise through our transistor but are prevented from doing that. Notice though that if we can get past this hurdle, if we can get over this barrier between our n and our p semiconductor types, then we’ll be in the clear. That’s because the bias direction of the other side on the second joining of semiconductor materials is in the opposite direction. Relative to the direction of our conventional current, we would say that this second bias is in the forward direction. It allows current to flow through. So what can be done in order to overcome this barrier here posed by our n and p junction?

One method that’s been discovered is empowering. That is, providing potential difference to the p part of this transistor. With enough voltage supplied by this source, the conventional current is able to overcome any electrical barriers to flow across the transistor. When arranged in this npn sandwich, it turns out that each of these semiconductors has a special name. The n-type semiconductor on the right is called the emitter. That’s because it emits electrons which are gathered up by the other n-type semiconductor, called the collector.

Since we just talked about charge moving left to right through this transistor, the idea that the emitter emits charge towards the collector may seem backwards. But importantly, the emitter emits electrons, negatively charged particles, towards the collector. So just like we’re used to saying that conventional current is the flow of positive charge when really we know that it’s electrons that do the moving in electric circuits, so we can understand that while the flow of negative charges through this transistor is right to left, the flow of positive charge — conventional current — is in the opposite direction as we’ve shown it. So the emitter emits electrons which move to the collector. And they crossover this middle p-type region which is known as the base.

In the very early days of transistors, the collector as well as the emitter were jammed into a common base material. While the design of transistors has evolved over time, that name has remained. The base is what connects the emitter with the collector. As well, the base is also where we supply potential difference if we want current to flow through our transistor. And that has to do with whether our transistor will function as a switch in an ON or as we want in this case an OFF state. Let’s start to create this diagram then that uses an npn transistor as a switch in this state.

We will start our sketch with an npn transistor. But we’ll draw it differently than we did before. In this case, we’ll use the symbolic representation of this transistor that often shows up in circuit diagrams. We said that an npn transistor has a base, a collector, and an emitter. And this is how we represent that. On top is our collector. On bottom is our emitter. And if you look, it looks like both collector and emitter are stuck into another platform. That’s our base. The way we did it earlier. We drew this transistor as part of a circuit where conventional current flowed into the collector. We’ll set up a similar circuit to that. But this time, we’ll have a resistor designated for the collector.

So what we have here is a circuit identical to the one we drew before, except we’ve added in a resistive component. But the overall operation of our npn transistor in this circuit won’t be affected much by that resistor. According to the polarity of our battery, conventional current will flow anticlockwise through this circuit or at least it will try to. As we said, there’s a reverse bias that occurs between the base and the collector in an npn transistor. Without overcoming that bias, this current isn’t able to make it through the transistor and complete the loop. This transistor then is effectively blocking the flow of current in this circuit.

However, that doesn’t quite satisfy the requirement because we want it to work as a switch. In other words, something that could be turned on or could be turned off. In this case, our transistor functions simply as a stop for any flow of current. It can’t be turned on or opened up. That is unless we change our circuit. Remember we said that the way to overcome the reverse bias opposing the flow of current from the collector to the base was to supply voltage to the base. We saw that by providing a cell which gave enough voltage to the base of our transistor, the reverse bias could be overcome and positive charge could flow from collector to emitter. Or as we mentioned, negative charge actually could flow from emitter to collector. With this arrangement, we’ve allowed current to make it through our transistor and cycle through the circuit.

This means that if we were to create a voltage output somewhere in our circuit loop to the right. Let’s put that output right above our transistor. And we’ll call the potential difference out, 𝑉 sub out. Then, with our circuit the way it’s drawn and our transistor setup as shown, we would be able to provide voltage to this output. That is, whatever device is there — whether it’s a light or stereo or other electrical component — it would be powered on. So we now have an npn transistor which does function as a switch. But it’s in the ON position, while we want it to be in the OFF state. So what could we do to the circuit which allows our transistor to continue to function as a switch? That is, it could potentially be ON or OFF. But in the arrangement we draw it, it’s in the OFF state.

An effective way to do this is to take our cell which is supplying voltage to the base of the transistor and invert it, flip it around. If we do that, then notice we no longer have a complete circuit in the left loop of our diagram. This conventional current positively charged is opposed by the bias between the emitter and the base. Moreover, it’s not going into the base to help it overcome the reverse bias between collector and base, which means that the current which used to flow through the right loop in our diagram no longer is able to. And as a result of that, this circuit is no longer providing output power.

We could say that whether our npn transistor functions in the ON or OFF state depends on the direction of this cell. And when the cell is oriented this way, the reverse bias in the transistor prevents current from flowing through. And the transistor works as a switch in the OFF state.

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