Video Transcript
In a PN junction, an electric field is produced in the depletion region due to the
different concentrations of donor ions and acceptor ions on either side of the
depletion region. The electric field produces a current called the drift current. Is the net current produced by the electric field in the depletion region in the same
direction as the diffusion current or in the opposite direction?
The question asks us to compare the directions of the diffusion current and the drift
current. Before starting, let us recall some important facts about the sides of a PN
junction. Firstly, the p-side of the depletion region contains a relatively higher
concentration of negatively charged ions. And the n-side of this region contains a higher concentration of positively charged
ions. These ions exist because, in the depletion region, mobile charge carriers tend to
combine with one another, creating these charged atoms. A second fact to remember is that the p-side overall contains a higher concentration
of vacancies and the n-side overall contains a higher concentration of free
electrons.
Next, let’s recall what diffusion current is. In our diagram, we see the PN junction, and we know that blue dots represent free
electrons and red dots represent vacancies. Both free electrons, which have a negative charge, and vacancies, which have an
effective positive charge, are carriers of mobile charge. Both types of mobile charge carriers will tend to move from areas where there is
higher concentration of charges of the same sign to areas of lower concentrations of
charges of the same sign. This motion is called diffusion. Because the objects that are diffusing in the PN junction are charge carriers, we
have what is called a diffusion current at our junction.
Let’s start with the free electrons, the blue dots, and consider how they might
diffuse along the junction. The concentration of free electrons is high in the n-region and low in the
p-region. In general, then, free electrons will tend to diffuse to the right from a higher
concentration to a lower one. If we consider this movement as a current, that is, a transfer of electric charge, we
can remember that conventionally electric current is thought to be the movement of
positive charge. This means that the conventional current of our free electrons in diffusion, the
positive charge flow, is to the left towards the n-region. That is the direction of the diffusion current due to the free electrons.
But positively charged vacancies have a higher concentration on the right in region p
and a lower concentration in region n. Then, they will tend to spread 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 the total diffusion current stream will also be to the left. So, for our answer, we say that the direction of the net diffusion current at the
junction is towards the n-side.
Opposite sign charge carriers that diffuse toward each other, where the p-side and
n-side join, tend to recombine, neutralizing each other’s charges. Recall that the p-side of the depletion region contains a higher concentration of
negatively charged ions and the n-side contains a higher concentration of positively
charged ions. With the charges of the free electrons and vacancies neutralized, the charges of the
positive and negative ions produce an electric field. The electric field produces a potential difference, which acts towards the
p-side. The potential difference produces a current called drift current in the direction of
this electric field. We can observe that the drift current direction is opposite to the diffusion current
direction.
For our answer then, we can say that the current produced by the electric field in
the depletion region is in the opposite direction as the diffusion current.
