Video Transcript
Each of the following diagrams shows a p-n junction. The charged sides of the junction’s depletion region are shown in red and blue. The junction is shown under conditions of forward bias, reverse bias, and zero bias. Which bias shown correctly represents zero bias?
In answer option (A), the red and blue sides of our depletion region are what we can call medium sides. In answer option (B), these two sides are a good deal larger in size. And in answer option (C), they’ve disappeared entirely. There is no depletion region. We want to know which of these three answer options corresponds to zero bias across our p-n junction. Let’s start by thinking about what happens when we join together a p-type and an n-type semiconductor material. The line along which these materials meet is called their junction. And the region around this junction is called the depletion region. It’s called this because mobile charge carriers, whether holes or electrons, do not stay in this region.
Considering our answer options, let’s imagine that this red region or the depletion region corresponds to this negatively charged side of that region. Let’s likewise imagine that the blue portion of that region indicates the positively charged part of the depletion region. With our semiconductor diode as it is now, we say that it experiences a condition of zero bias. That is, we’re not applying any external voltage across the diode. So, in a condition of zero bias, we would expect our depletion region to have some size to it. We can change the size of this region by applying what are called forward and reverse biases.
To set up a forward bias, we can connect either end of our diode with a cell oriented this way. Conventionally, positive charge flows from the positive terminal of the cell, in this case clockwise, while negative charge will flow from the negative terminal, in this instance counterclockwise. The incoming positive charges in our current here repel the mobile positive charges on the p-type side of our diode. This pushes those positive charges to the right into the depletion region. The same thing happens with our incoming negative charges here on the right side of our circuit. They repel the free electrons on the n-type side of our junction and push the free electrons into the depletion region.
The result of all this is that the depletion region shrinks. In fact, if the voltage provided by the cell is strong enough, the depletion region can vanish entirely. This is what we see happening in answer option (C). As we’ve seen, this is an outcome of what is called forward bias.
Returning to our circuit, let’s say that we open up a switch so that the current in the circuit goes to zero. In that case, our semiconductor diode is once again zero biased and its depletion region returns. Let’s now imagine taking our cell and flipping it around so that now the positive terminal is on the right. If we close the switch in our circuit, we know that now positive charge will move counterclockwise while negative charge will move clockwise.
Under these conditions, the incoming positive charges on the right side of our circuit will draw toward them the free electrons in the same way the incoming negative charges on the left will attract the positively charged mobile holes. As the mobile charged carriers are pulled away from the junction between the p-type and n-type materials, the depletion region will expand. This is what we see happening in answer option (B).
Just as a side note, when a diode is strongly reverse biased, it’s almost impossible for mobile charges to move across this now very large depletion region. The current through the diode goes to zero. On the other hand, a forward biased diode, where there may be no depletion region, allows the movement of mobile charge through the diode much more easily.
Anyway, we’ve now seen what forward and reverse biasing look like in terms of the depletion region of a diode, and we know that a diode with zero bias has a depletion region with a size in between these two extremes. Therefore, we choose answer option (A) as the bias that correctly represents zero bias.
