Video Transcript
A PNP transistor is connected to a direct current source, as shown in the diagram. Which of the two identical p-regions of the transistor is reverse biased? P one, P two, both regions are reverse biased.
The diagram shows a PNP transistor with the two p-type semiconductor regions labeled P one and P two sandwiching the n-type semiconductor region labeled N. From the question, we know that the two p-type regions are identical, which means we don’t have to worry about material variations when answering this question. Note the question itself is asking us to identify which of these two p-type regions is reverse biased.
Reverse biased and its counterpart, forward biased, are the two descriptors we typically apply to a PN junction. And note that our transistor consists of two PN junctions, one with P one on the left and N on the right and the other with N on the left and P two on the right. So we need to understand what it means for a PN junction to be forward or reverse biased.
Forward and reverse biasing of a PN junction are determined by the junction’s orientation relative to an external power supply. When the p-type semiconductor is connected to the positive terminal of the power supply and the n-type semiconductor is connected to the negative terminal of the power supply, we say that the junction is forward biased because charge can flow freely across the junction; that is, it is conductive. On the other hand, when the p-type semiconductor is connected to the negative terminal of the power supply and the n-type semiconductor is connected to the positive terminal of the power supply, the junction is insulating. No charge can flow across the junction, and therefore we call it reverse bias because this behavior is opposite to forward bias.
Looking back at our diagram and noting that the positive terminal of the power supply is on the left and the negative terminal is on the right, we see that P one is connected to the positive terminal of the power supply, so it must be forward biased. P two, on the other hand, is connected to the negative terminal of the power supply. So looking back at our definitions, we see that P two is reverse biased. So of P one, which is forward biased, and P two, which is reverse biased, it is only P two that is the reverse-biased p-type region of our transistor.
