Explain how the depletion region is formed inside a p-n junction.
As we start out, let’s remind ourselves of what a p-n junction is. Say that we have two types of semiconductor materials. One is a p type, which means that, in this semiconductor, there’re an excess of positively charged holes, which serve as charge carriers. And we’ll show these holes like this, just an empty loop that is the absence of an electron.
Different from this p-type semiconductor material is the n type. In this case, rather than positively charged holes being the charge carriers, we have negatively charged electrons that serve that function. So then, we have our two separate semiconductor material types. But what happens if we form a junction out of them? That is, what happens if we bring them together? When we do that, we now have materials with opposite type charge carriers bumped up against one another. When these two different types come in electrical contact, there’s an immediate reaction on both parts.
Notice that the concentration of positively charged holes is much greater on the left side of our junction. And the concentration of negatively charged electrons is much higher on the right. Now that our p and n types are joined together, they’ll try to address that imbalance by sending positively charged holes over from the p to the n side and in the opposite direction negatively charged electrons from the n to the p side. The physical reason this happens is because there’s a tendency to even out the concentration of these two charge carriers across the whole junction. The movement of these positive holes in negative electrons from one side to the other actually forms what’s called a current, a diffusion current.
We can say then as a first part of our explanation for how a depletion region is formed in a p-n junction that, first, due to different concentrations of charge carriers, positive charge carriers in the p type and negative charge carriers in the n type, diffusion current occurs. As this current continues to flow, something interesting happens. As the diffusion current continues, more and more charge carriers, both positive holes and negative electrons, separate from the atoms they were initially associated with. This creates what is called uncovered atoms.
We could think of it this way. Say that, within the lattice structure of our p-type semiconductor, we have a boron atom. A boron atom is trivalent. So that means when it’s in the crystal structure of the semiconductor, they will have seven valence electrons in its shell. And then, importantly, it will have one absence of an electron, a positively charged hole which can serve as a charge carrier. Now, this hole, as we’ve said, it can migrate as part of the diffusion current over to the n-type side of the junction. The way it could be allowed to do that would be if an electron came from elsewhere in the junction and filled this hole. But notice if that happens, the boron atom now has one more electron than it did before when it was neutrally charged. So with the addition of this electron, this boron atom now has an overall negative charge. That is, it’s become a negative ion.
And in fact, something similar happens over on the n-type side of the junction. The n-type lattice would be doped with an impurity that has five valence electrons, such as phosphorus, which would mean that one of these impurity atoms within the crystal structure would have a full valence shell of eight electrons plus one extra. That’s the negative charge carrier it can contribute. At this point, the charge of this phosphorus atom overall is neutral. But what if it loses an electron due to the diffusion current. In that case, it will have lost one negative charge. And, therefore overall, it will be a positively charged ion.
What we’ve seen then is that this diffusion current, this exchange of negative and positively charged charge carriers from one side of the junction to the other, creates positive and negative ions on either side of the junction. In particular, these positive and negative ions tend to accumulate at the junction between the p and n types. This is step two in the formation of a depletion region. And we can explain it this way. We can write that uncovered positive ions appear in the n region. And uncovered negative ions appear in the p region. The appearance of these ions is due to the diffusion current. And it’s the movement of the charge carriers, positive and negative, which we say uncovers these ions.
After all that, notice what has happened in this region with high concentration of positive and negative ions. Considering the negative ions, we see that after it loses its positive charge carrier, it has no charge carrier. And likewise for the positive ions, once they give up an electron, they have no charge carrier to contribute. That means that, in this entire region, where we have a high concentration of negative and positive ions, we’re lacking charge carriers. It’s very difficult for charge to be exchanged across this junction. Because this region has been stripped of charge carriers, we say it’s depleted, hence the name depletion region.
You may notice that the existence of the depletion region does a very good job at preventing charge flow across this junction. And that’s true. If we had a p-n junction in an electrical circuit, then we would need to apply a forward voltage bias in order to overcome this region. As a last step in our explanation, we can say then that the region containing these ions, the positive and negative ions, is free from charge carriers. And it’s called the depletion region. This then explains the formation of the depletion region inside a p-n junction.