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Question Video: Understanding PN Junctions Physics • Third Year of Secondary School

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In a p–n junction, both free electrons and vacancies can diffuse through the junction, as shown in the diagram. Toward which side of the junction is the direction of the net diffusion current? In which of the following regions is the concentration of free electrons greatest? [A] The p-side [B] The n-side [C] At the middle of the junction. In which of the following regions is the concentration of both free electrons and vacancies smallest? [A] The p-side [B] The n-side [C] At the middle of the junction

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Video Transcript

In a p–n junction, both free electrons and vacancies can diffuse through the junction, as shown in the diagram. Toward which side of the junction is the direction of the net diffusion current?

In our diagram, we see a p–n junction, and we know that the blue dots represent free electrons and the red dots, vacancies. Both free electrons, which have a negative charge, and vacancies, which have an effective positive charge, are mobile charge carriers. This means that they’re capable of moving throughout the junction. The n-region of our junction is so named because the mobile charge carriers are negative; that is, they’re free electrons. Indeed, most of the dots on this side of the junction are blue representing negative free electrons. In the p-, or positive, region on the other hand, most of the mobile charge carriers are positively charged vacancies.

We can think of our p–n junction as a volume enclosed by this rectangle. In the n-region of that volume, we see that there is a relatively high concentration of free electrons, while in the p-region, there’s a relatively high concentration of vacancies. Both of these types of mobile charge carriers will tend to move from areas of high concentration to areas of low concentration. That process is called diffusion. And because the objects that are diffusing, in this case, do carry charge, we have what is called a diffusion current in our junction. Let’s start with the free electrons, the blue dots, and consider how those might diffuse throughout the junction.

The concentration of free electrons is high in the n-region and low in the p-region. Overall then, free electrons will tend to diffuse to the right from high to low concentration. If we consider this movement as a current, that is, a transfer of electrical charge, we can recall that conventionally electric current is thought to be the motion of positive charge. That means the conventional current of our diffusing free electrons, the effective flow of positive charge, is to the left towards the n-region. That’s the direction of the diffusion current due to the free electrons.

Note though that our question asks about the direction of the net diffusion current. In other words, we’ll need to consider the diffusion current due to the vacancies as well. The positively charged vacancies have a high concentration to the right in the p-region and a low concentration in the n-region. They will tend to diffuse then to the left. And since these charge carriers are positive, that indicates a diffusion current to the left. The diffusion current due to both types of mobile charge carrier is to the left. This means that rather than canceling out, these currents will add so that the net or overall diffusion current will be to the left as well. For our answer then, we say that the direction of the net diffusion current in the junction is toward the n-side.

Let’s look now at part two of our question.

In which of the following regions is the concentration of free electrons greatest? (A) The p-side, (B) the n-side, (C) at the middle of the junction.

Recalling that free electrons are represented by blue dots, we know that there’s a high concentration of blue dots on the n-side of our junction and relatively lower concentrations at both the middle and the p-side of the junction. Since the density of blue dots is greatest towards the n-side, we choose answer option (B). The concentration of free electrons is greatest in the n-side of our junction.

The next part of our question asks, in which of the following regions is the concentration of vacancies greatest? (A) The p-side, (B) the n-side, or (C) at the middle of the junction.

From our diagram, we know that vacancies are represented by red dots. We see that these dots are concentrated on the p-side of our junction. There are far fewer red dots on the n-side or at the middle of the junction. Therefore, we choose answer option (A). It’s at the p-side of our junction where the concentration of vacancies, positive charges, is greatest.

Let’s now consider the last part of our question.

In which of the following regions is the concentration of both free electrons and vacancies smallest? (A) The p-side, (B) the n-side, (C) at the middle of the junction.

In earlier parts of our question, we’ve seen that on the n-side of our region, the concentration of free electrons is high, while the concentration of vacancies is low and that this situation is reversed on the p-side of the region, with lots of vacancies but few free electrons. In this part of our question, we want to identify where the density of both free electrons and vacancies is small. From our diagram, we see that this occurs at the middle of our p–n junction. It’s in this region where the concentrations of both of our charge carriers, free electrons and vacancies, are both small. We choose answer option (C) at the middle of the junction.

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