Video: Physics Past Exam • 2017/2018 • Pack 1 • Question 38A

Physics Past Exam • 2017/2018 • Pack 1 • Question 38A


Video Transcript

Compare P-type semiconductor, N-type semiconductor. Point of comparison: valence impurity.

Since the two objects we’re comparing are both semiconductors, the difference we see will come down to whether they’re P-type or positive-type or N- or negative-type. And in particular, we suspect that the difference between these two types of semiconductors will have something to do with the valence structure of their impurities.

Now what do we mean though when we talk about an impurity of the semiconductor? Well, say that we have a big chunk of pure semiconductor material. One of the most common materials for this is silicon.

Now a silicon atom has the property that it has four electrons in its valence or outermost electron shell. That means that if we string a whole bunch of silicon atoms together so that they’re arranged in a lattice formation, then thanks to electron sharing between adjacent silicon atoms, the interior atoms of silicon to the bulk will have a full valence shell. They’ll have eight electrons in their outermost electron shell.

Now that’s a great thing for the stability of these atoms. It means they’re very unlikely to donate or accept any more electrons. But it’s not good for the conductivity of these atoms. If we want to increase the conductivity of our semiconductor overall, we do that through a process called doping using impurities. This involves adding an element of a different type to this bulk material, but not just any element. We’ll pick the elements specifically to create either a positive-type or a negative-type semiconductor. Here’s how we would do that.

Let’s say we wanted to create a P-type semiconductor. In that case, we would find an element that has three electrons in its valence shell compared to silicon’s four. An example of this type of atom is boron. Then if we doped our silicon with this impurity, let’s say that would mean that a couple of our interior silicon atoms are then replaced by boron.

Now when we do this, something interesting happens. Remember that boron has one fewer valence electron than silicon. So now the boron atoms that are in this bulk lattice no longer have a full valence shell. But rather they’re down one. These absences of electrons which would complete an outermost shell are called holes. And since they’re the absence of a negatively charged particle, these are considered positive charge carriers.

So if we had an electron that wanted to traverse our semiconductor, then now it would have a place to go. It would be attracted to the hole in the first boron atom and then could be transferred to the hole in the second one, and so on and so forth, until it moves across the material.

We’ve now created, thanks to this doping of boron, a P-type or positive-type semiconductor. Incoming electrons can use these positively charged holes almost as stepping stones to make it across the semiconductor. All this was made possible because we chose for our impurity an atom that had three electrons in its valence shell. There’s a name for that. It’s called a trivalent atom. And we’ll write that in as our description of the valence impurity for a P- or positive-type semiconductor. They’re characterized by having three valence electrons.

But then what about an N- or negative-type semiconductor? To see how that’s created, let’s go back to our all-silicon bulk material. We’ve removed the boron atoms completely. And now we have a lattice purely of silicon. If doping our silicon with an impurity that has three valence electrons created a P-type semiconductor, you might guess what kind of element we will need to use to create an N-type.

In this case, we use an atom that has five valence electrons. An example of this is phosphorus. If we now replace some of our interior silicon atoms with phosphorus, then not only do those phosphorous atoms have a full valence shell of eight electrons, but they also contribute an extra electron each. These electrons are extra, in the sense that they aren’t needed to fill the valence shell of any adjacent atoms. As such, they’re free to roam. And this means that they’re effective charge carriers for increasing the conductivity of the semiconductor.

And the reason the semiconductor is called an N- or negative-type is because these charge carriers, electrons, themselves have a negative charge. The key to creating this surplus of negative charge carriers was doping our semiconductor with an impurity that has five electrons in its valence shell. The name for an atom like that is pentavalent. In this name, “penta” indicates five and “valent” indicates that we’re talking about valence electrons.

This then is the main point of comparison for P-type and N-type semiconductors. P-type semiconductors are doped using trivalent impurities, while N-type semiconductors are doped using pentavalent impurities.

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