Question Video: Determining the Forward Biased n-Type Region in a Transistor | Nagwa Question Video: Determining the Forward Biased n-Type Region in a Transistor | Nagwa

# Question Video: Determining the Forward Biased n-Type Region in a Transistor Physics • Third Year of Secondary School

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An NPN transistor is connected to a direct-current source, as shown in the diagram. Which of the two identical n-regions of the transistor is forward biased? [A] N₁ [B] N₂ [C] Both regions are forward biased.

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### Video Transcript

An NPN transistor is connected to a direct-current source, as shown in the diagram. Which of the two identical n-regions of the transistor is forward biased? N one, N two, both regions are forward biased.

The diagram shows an NPN transistor with two n-type regions sandwiching one p-type region. From the question, we know both of the n-type regions are identical, which means we don’t have to worry about variations inside of the transistor itself. What we are trying to determine is which of these two n-type regions is forward biased. Forward biasing and its counterpart reverse biasing are the two possibilities when a PN junction is connected to a direct current source. Here, we’ve drawn a picture of a forward-biased and a reverse-biased PN junction. Note that the only difference between these pictures is the orientation of the power supply, that is, which terminal is connected to the p-type semiconductor and which terminal is connected to the n-type semiconductor.

A forward-biased junction has the positive terminal of the power supply connected to the p-type semiconductor and the negative terminal of the power supply connected to the n-type semiconductor. We call this configuration forward biased because charge can flow freely across the PN junction. In other words, the PN stack conducts. When a junction is reverse biased, the positive terminal of the power supply is connected to the n-type semiconductor and the negative terminal of the power supply is connected to the p-type semiconductor. In this configuration, charge cannot flow freely across the PN junction. The stack is insulating, and since this behavior is opposite to that of a forward-biased junction, we call it reverse biased.

Looking back at our diagram, we see that the positive terminal of the power supply is on the left and the negative terminal is on the right. The transistor also consists of two PN junctions, one with N one on the left and P on the right and the other with P on the left and N two on the right. Starting with the PN junction on the left, we see that N one is connected to the positive terminal of the dc current source, which means using our definition of reverse and forward bias, this is a reverse-biased PN junction. Looking at the other PN junction, we see that N two is connected to the negative terminal of the dc current source, which means that this region is forward biased.

And there’s our answer. Of the regions in our transistor, N two alone is the forward-biased n-type region.

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