Video: Understanding What Is Meant by “P-Type Semiconductor”

Which of the following describes a p-type semiconductor? [A] A p-type semiconductor is a semiconductor material that carries a net negative charge. [B] A p-type semiconductor is a semiconductor material that carries a net positive charge. [C] A p-type semiconductor is a semiconductor made from an element in period 3 of the periodic table. [D] A p-type semiconductor is a semiconductor material that contains an impurity such that there are more free electrons in the material than there would be in the pure semiconductor. [E] A p-type semiconductor is a semiconductor material that contains an impurity such that there are fewer free electrons in the material than there would be in the pure semiconductor.

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Video Transcript

Which of the following describes a p-type semiconductor? A) a p-type semiconductor is a semiconductor material that carries a net negative charge. B) a p-type semiconductor is a semiconductor material that carries a net positive charge. C) a p-type semiconductor is a semiconductor made from an element in period three of the periodic table. D) a p-type semiconductor is a semiconductor material that contains an impurity such that there are more free electrons in the material than there would be in the pure semiconductor. E) a p-type semiconductor is a semiconductor material that contains an impurity such that there are fewer free electrons in the material than there would be in the pure semiconductor.

Okay, so these are all long and worded options that we have to decide between, but before we choose and before we go through the options, let’s actually remind ourselves a bit about semiconductors. Now semiconductors are most commonly made from an element called silicon. Let’s say this is an atom of silicon. Now in the outer or valence shell of silicon, there are four electrons. Remember though that we’re just talking about the valence shell because there’re core electrons beneath this valence shell, but they don’t play any important part in semiconductors. So let’s talk in more detail about a semiconductor made from silicon.

In the bulk of a semiconductor, lots of silicon atoms are bonded together in this way such that for a silicon atom that’s surrounded by as many other silicon atoms as possible, there are now eight electrons surrounding it: one, two, three, four, five, six, seven, and eight. In each bond of course, one silicon atom provides one electron and the other provides the other electron. But this is simply what the bulk looks like for a pure or undoped semiconductor. When we do end up doping the semiconductor, it can turn into either a p-type, or positive type, or an n-type, negative type semiconductor. So what does this mean?

Well let’s consider doping our semiconductor with an atom of boron, which by the way only has three electrons in its outer valence shell. Now when we do dope a semiconductor with a boron atom, essentially the boron atom replaces one of the silicon atoms. So let’s say we replace this central silicon atom with a boron. Now what we have is silicon atoms once again, but this time we’ve got a boron atom in the center. And because boron only had three electrons in its valence shell, unlike four for silicon, this means that surrounding the boron atom once other bonds have been made, there are only one, two, three, four, five, six, seven electrons, not eight anymore, because of course each silicon provides one electron per bond. So that’s four silicon atoms, so four bonds. And then the boron provides the three from its valence shell. So that’s all together seven electrons.

This means that originally where there would have been an electron, there is no longer an electron. And this lack of an electron is what makes all the difference in a p-type semiconductor. Now it’s called a p-type semiconductor because it’s a positive-type semiconductor. And the reason is called this is because there would have been an electron there if we had the pure or undoped semiconductor. But when we dope it with boron, there no longer is an electron there. Therefore, a negative charge that would’ve been there normally is no longer there. So a lack of a negative charge is what is said to turn it into a positive-type semiconductor.

Now we need to be careful here. We’re not saying that the bulk is overall positively charged because there isn’t an electron there anymore. No the charge on this whole substance is still neutral because, remember, even though there isn’t an electron there when they would’ve been in the undoped semiconductor, this is compensated for by the number of protons in the nucleus of boron because a boron atom has five protons in its nucleus and five electrons orbiting it. Now they arrange themselves in this electronic formation: two electrons on the inner shell and three electrons as we’ve already seen in the outer or valence shell. So the overall or net charge on the boron is actually neutral. However, the only reason it’s called a positive-type semiconductor in this situation is because there’s not an electron so there’s a lack of a negative charge when there would’ve been one in the undoped or pure situation.

So bearing all of that in mind, let’s go through the options A to E and see which one makes more sense. Starting with option A, a p-type semiconductor is a semiconductor material that carries a net negative charge. Now we’ve just said that there is no net charge on any of the semiconductors whether they’re undoped or doped. So option A doesn’t make any sense. Option B then, a p-type semiconductor is a semiconductor material that carries a net positive charge. Once again, this is not true; there is no net charge on the semiconductor. So let’s look at option C: a p-type semiconductor is a semiconductor made from an element in period three of the periodic table. Now if we look at period three on the periodic table, we can see that it consists of sodium, magnesium, aluminum, silicon, phosphorus, sulphur, chlorine, and argon. So we can see that silicon is in period three of the periodic table. and that is what makes up the majority of our doped semiconductor.

However, this is not a very good description because boron, which is commonly used to dope these semiconductors to make them a p-type semiconductor, is not in period three. Plus, none of these other materials are used to make a p-type semiconductor. So why we are describing it in terms of the third period of the periodic table? No, this is not the answer we’re looking for either. Option D then, a p-type semiconductor is a semiconductor material that contains an impurity such that there are more free electrons in the material than they would be in the pure semiconductor. Now this is more like it; we’re talking about impurities now. As we’ve discussed, boron is an impurity here. However, the problem with this definition is that it says there are more free electrons than there would be in the pure semiconductor. So option D is actually the definition of an n-type, negative-type semiconductor.

This type of semiconductor is made when we dope our pure semiconductor with something like antimony, which has five electrons in its valence shell. And therefore, it results in having more electrons surrounding the central antimony atom. So although option D is quite close to what we’re looking for, it’s actually the definition of an n-type semiconductor, not a p-type. Option E, a p-type semiconductor is a semiconductor material that contains an impurity such that there are fewer free electrons in the material then there would be in the pure semiconductor. Now this is the exact definition we’re looking for. Firstly, it talks about an impurity, boron in this case that we’ve discussed already. And secondly, it says that the impurity results in fewer free electrons in the material than there would’ve been in the pure semiconductor, just as we saw earlier. And so our final answer is option E: a p-type semiconductor is a semiconductor material that contains an impurity such that there are fewer free electrons in the material than there would be in the pure semiconductor.

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