An NPN transistor is connected to two direct current sources, as shown in the diagram. The two n-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?
The diagram shows an NPN transistor with two identical N-type regions, labeled N one and N two, sandwiching a P-type region labeled P. We are asked to identify the collector and emitter regions of this transistor. Now, this transistor is a bipolar junction transistor because it is made of a sandwich of doped semiconducting material. All bipolar junction transistors, or BJTs for short, have three regions: the collector, the base, and the emitter. The base region of a BJT is always the center of the sandwich.
Looking back at our diagram, this means that P must be the base because it is in the middle and N two and N one must be the collector and emitter regions. We just need to figure out which is which. To do this, we’ll need to understand the direction of conventional current in the transistor. From the orientation of the lower power supply, we know that conventional current will leave the positive terminal and then enter the base. In fact, we know the base current has this direction for the entire lower branch of the circuit.
Turning to the upper branch and again paying attention to the orientation of the power supply, we see that the conventional current enters the transistor at the region labeled N one. Following the current through this branch, we see that the direction of conventional current in the upper branch must be in the opposite direction to the conventional current in the lower branch. Since the conventional current in both the upper and lower branches points away from this node, there must be a conventional current pointing into this node, specifically the conventional current from the transistor region N two.
With all this, we can clearly see that conventional current enters the transistor at N one and exits the transistor at N two. We now recall that for an NPN transistor, conventional current enters the transistor at the collector and exits the transistor at the emitter. We’re therefore ready to answer both parts of this question. The collector region is the region N one because this is an NPN transistor and conventional current is entering the transistor at N one. The emitter region is N two because conventional current leaves this NPN transistor at N two.
Now to arrive at this answer, we had to rely on the fact that our transistor was an NPN transistor, and therefore the current enters at the collector. But the opposite is true for a PNP transistor. The current enters a PNP transistor at the emitter and leaves a PNP transistor at the collector. So, we have two different rules, one for PNP transistors and one for NPN transistors. But we can actually come up with a single uniform rule that will allow us to identify the collector and emitter in any BJT.
Looking back at our diagram, the direction of the conventional current is from left to right through the transistor. This current passes through two P-N junctions, one between N one on the left and P on the right and one between P on the left and N two on the right. Looking at the orientation of the current relative to the orientation of the junctions, we see that the N one-P junction is reverse biased, while the P-N two junction is forward biased. We see then that the collector is forming a reverse bias junction with the base and the emitter is forming a forward bias junction with the base. And this gives us a rule that is the same for both PNP and NPN transistors.
The collector of a BJT forms a reverse biased junction with the base, and the emitter forms a forward biased junction with the base. This also explains why the current rule is different for NPN and PNP transistors. If we changed our NPN transistor to a PNP transistor and reversed the direction of the base current, the transistor would still conduct. However, by reversing the N and P regions, the left junction becomes forward biased and the right junction will be reversed biased, but the current will still be moving in the same direction. So, in a PNP transistor, conventional current enters towards the forward biased junction, so it enters at the emitter, and it leaves from the reverse biased junction, so it leaves from the collector.