Transcription
In this video, we're going to talk about semiconductors. When you hear the word semiconductor, what do you think about? Semiconductors have properties that are in between an insulator and a conductor.
Insulators do not conduct electricity at all. They have a very high resistance to the flow of electricity, and insulators consist of non-metals such as sulfur, iodine. Those do not conduct electricity at all.
Conductors, on the other hand, such as metals, they conduct electricity. For example, zinc, copper, aluminum. They're excellent conductors of electricity. Semiconductors are basically metalloids, and they don't conduct electricity as well as conductors. However, they do conduct a small amount of electricity. Some good examples of naturally occurring semiconductors are silicon and germanium. Now, germanium conducts electricity better than silicon, but both of these are semiconductors. They conduct a small amount of electricity.
Now, how can we change the conductivity of a semiconductor? How can we increase it? One way is to change the temperature. If you increase the temperature of, let's say, silicon, the electrical conductivity of that semiconductor will increase. Now, the opposite is true for metals. Let's see, if you have copper metal, if you increase the temperature of a metal, the electrical conductivity decreases. So, what this means is that metals conduct electricity better at colder temperatures. Semiconductors, they conduct electricity better at higher temperatures.
Now, the other way in which we can increase the connectivity of a semiconductor like silicon is to add impurities to it. We can dope it with other atoms. Now, before we get into that, let's briefly review the structure of the silicon crystal. Silicon has four valence electrons. Silicon has an atomic number of 14. So, every silicon atom has 14 electrons, four of which are valence and 10 are core electrons. But the valence electrons are those that participate in chemical reactions. So, I'm going to put other silicon atoms next to the one at the center, and each of those silicon atoms also contain four valence electrons.
Now, it takes two electrons to form a covalent bond. So, these two electrons will form a bond, and those two electrons will form a bond, and so forth. So, in the silicon crystal, silicon is bonded to four other silicon atoms, and the same is true for those four silicon atoms. They're bonded to another four silicon atoms. And so, the structure of silicon is very similar to the structure of diamond. However, the properties are very different.
So, what happens if we dope this silicon crystal with an element with five valence electrons, let's say like phosphorus? What's going to happen? So, if we replace this silicon atom with a phosphorus atom, let's see. Each silicon atom used four valence electrons to create four bonds. Now, phosphorus is going to do the same thing. It's going to use four of its five valence electrons to create four bonds with the neighboring silicon atoms. However, it still has one valence electron left over, and that extra electron, it's free to move in the conduction band, and so that electron increases the conductivity of the silicon crystal. So, you really only need to add a small amount of phosphorus to a large silicon crystal in order to increase its connectivity significantly.
Now, this is known as an n-type semiconductor. The charge carriers of this type of semiconductor are electrons. And so, that's one way in which you can increase the conductivity of silicon is by adding atoms with five valence electrons, and you can create an n-type semiconductor.
Now, what about adding an atom that has less than four electrons? Let's say like aluminum, which has three valence electrons. What type of semiconductor will we get? So, let's try aluminum. And here are the three valence electrons of aluminum. Let's see what's going to happen if we place it in the crystal silicon. Each contains four valence electrons. Now, as we recall, it takes two electrons to form a covalent bond. So, these two will form a covalent bond, same with those two, and those two. But notice that we have a missing electron here. So, therefore, a hole will be created. So, we're going to get a structure that looks like this. So, this silicon still has the valence electron, and aluminum has a hole. And so, what we have, if you add an atom with three valence electrons to a pure silicon crystal, is you're going to get a p-type semiconductor.
Now, a p-type semiconductor conducts electricity better than pure silicon. So, the connectivity goes up. But the charge carriers are not electrons, as in the case of the n-type semiconductor. But for a p-type semiconductor, the charge carriers are holes. So, it's the movement of holes, or electron vacancies, that is responsible for the connectivity of a p-type semiconductor. In the case of an n-type semiconductor, where we had phosphorus, the charge carriers were electrons. So, electrons were free to move in the n-type semiconductor.
Now, here's the question for you: What happens if we take, let's say, a p-type material and combine it with an n-type material? If we put these two together, what's going to happen? Well, you're going to create something known as a p-n junction. The contact between the p-type and the n-type semiconductor creates the p-n junction. And what happens is, these electrons will want to drift to the p-type crystal and fill in the holes. As the electrons move to the p-type crystal, you're going to have a build-up of negative charge on the left side of the p-n junction, and the right side is going to be electron deficient. So, there's going to be a positive charge on the right side.
So, this is the basis for constructing devices like diodes, transistors, and even solar cells. But let's talk about why the charges are the way they are. Now, let's go back into something called formal charge. As we learned, a simple way to calculate the formal charge, it's equal to the valence electrons minus the number of bonds and dots. So, before phosphorus loses this electron, it's neutral. Phosphorus naturally has five valence electrons. In that structure, it has four single bonds and it has one dot. So, five minus five is zero. And in this structure, aluminum is also neutral. It has three valence electrons, and in this structure, it has three bonds, no dots. That's a hole. This dot is for the silicon, it doesn't come from aluminum. So, therefore, in that representation, aluminum is neutral.
But now, when phosphorus gives up one of its valence electrons, let's see what the formal charges will now be. So, now the hole that was here is now filled with an electron. And if you have two electrons, that equates to a single bond. So, we have something that looks like this after the transfer of one electron from phosphorus to aluminum. So, now the formal charge on phosphorus is going to be the five valence electrons minus the four bonds, and it no longer has any dots. So, now phosphorus has a positive formal charge. Aluminum still has three valence electrons naturally, but now it has four bonds. So, now aluminum has a negative one formal charge.
And so, whenever you have a p-type semiconductor mixed with an n-type semiconductor, the p-type will initially have holes. Let me draw that in a different color. And the n-type will have excess electrons. Now, these electrons will naturally drift to fill in the holes. So, as the electrons move to the left, over time, you're going to have a buildup of negative charge across the p-n junction. So, this is the p-n junction, that line I just drew right there. So, I'm going to highlight it in green. So, as electrons move from right to left, you can have some negative charge build up. And because the right side is now electron deficient, this is going to have a positive charge. And so, an electric field will be generated across the p-n junction. And so, new electrons, it's going to be difficult for them to cross that p-n junction. Those electrons on the right, they still want to fill these holes. So, they still feel a force that wants to accelerate them towards the left. However, they're repelled by the negative charge. And so, when these two forces reach a state of equilibrium, then electrons no longer cross the p-n junction. And so, this is going to lead to a constant injection potential.
This is the basis of the formation of a diode. A diode is an electronic device that conducts electricity in one direction and does not conduct electricity in both directions. So, if you were to connect this across the battery, if you don't connect it the right way, there's got to be no current. If you connect it the right way, electricity will flow.
So, that's it for my tutorial on semiconductors. I want to give you a basic introduction into semiconductors, the relationship between temperature and electrical conductivity, and the difference between an n-type and a p-type semiconductor. So, thanks for watching.