Semiconductor Theory (Part 1) - Intrinsic Semiconductors
We're in section 8.2 and we're looking at semiconductor theory. We're starting out with a discussion of intrinsic semiconductors. Silicon, germanium, and gallium arsenide are primary materials used in semiconductor devices. Silicon and germanium are elements and are intrinsic semiconductors. In pure form, silicon and germanium do not exhibit the characteristics needed for practical solid state devices.
Isolated Semiconductor Atoms
Silicon and germanium are electrically neutral, that is each has the same number of orbiting electrons as protons. They are neither negative or positive. They are neutral. Both silicon and germanium have four valence band electrons and so are referred as tetravalent atoms. This is an important characteristic of semiconductor atoms. You may recall from chapter one we talked basically about this concept. Remember, we had a conductor and we talked about things like gold, silver, copper, and their characteristic that we noted was that they had only one valence electron so that it could be easily dislodged and this would support current flow. Then, we looked at other materials. We called it an insulator. This material, the characteristic of an insulator was that it had eight electrons in the outer valence shell and this made it very stable so that it wasn't subject to chemical reactions, materials like glass, ceramic. These are materials that have eight electrons in their outer valence shell.
We're on the subject of semiconductors now and semiconductors have the unique property of having four electrons in their outer valence shell. This will make them suitable for some very unique processes. In their base form, they won't be suitable, but we're going to be able to modify them so we can do a lot of things with them.
Tetravalent atoms such as silicon, gallium arsenide, and germanium bond together to form a crystal or crystal lattice. Because of the crystalline structure of semiconductor materials, valence electrons are shared between atoms. This sharing of valence electrons is called covalent bonding. Covalent bonding makes it more difficult for materials to move their electrons into the conduction band. Here we see this lattice structure that is referred to in reference to tetravalent atoms. Tetravalent atoms bond to form a crystal lattice. The picture shows a one-dimensional view. Each atom has four valence electrons. In the outer ring, they share four atoms with other atoms for a total of eight valence electrons. This creates a very stable tightly bound structure. The shared electrons are referred to as covalent bonds. Here this is one of the silicon atoms, and it has four electrons in its valence shell. What's happening here is you're seeing that are eight in its outer shell. The reason there are eight is because it is sharing an atom with one of the other atoms that are right to next to it. Since there is four right next to it, then it has an additional four atoms, and then this makes for rather than four it has eight electrons in its outer valence shell. Four of them are what we call covalent bonds because they are a shared electron. Then, as we go on we can others out here and what we find is that all of them would have the eight electrons except those that are on the very outer edge, and then they wouldn't have the advantage of having the extra electrons.
Let's consider the distribution of electrons at two temperatures. You have a picture in your textbook that you might want to refer to this. I did not have access to that picture, so I couldn't put it into my presentation. We're going to consider how these electrons are distributed at absolute zero and at room temperature. At absolute zero, electrons are at their lowest energy level. The material acts as an insulator because no energy and no electron flow or movement and there are no electrons in the conduction band. You would figure that this would be the way it would be with a semiconductor because remember on a previous screen we looked at the structure and we saw that there were eight electrons in that valence shell. This is a rather tightly bond structure, but at absolute zero there's no energy, and so the possibility of one of these electrons becoming free and becoming what we would call a conduction band electron is just virtually impossible because at absolute zero there's no extra energy, and it's going to stay in this tightly bound state.
At room temperature, valence electrons have absorbed enough energy to move into the conduction band. At room temperature, a valence electron can receive enough energy to break away and move to the conduction band. At room temperature, there is energy that's being picked up by these electrons and one of these electrons can actually move out of this valence band and become a free electron.
Atoms with broken covalent bonds, missing an electron. This one right here, this broken, it's missing an electron right here. They have a hole present where the electron was. This is a concept we have not talked before, but here we have this piece of silicon, silicon atom. It had an electron right here. That electron moved out to become a free electron and what is in its place is we call that a hole. There's nothing there. A hole just simply means that it is a vacant spot where an electron was. For every electron in the conduction band, there is a hole in the valence band and they are called electron-hole pairs. One concept that we will here is that in pure silicon you'll have an equal number of free electrons as you have holes. That just makes sense.
Electron distribution. As more energy is applied to a semiconductor, more electrons will move into the conduction band and current will flow more easily through the material. Therefore, the resistance of an intrinsic semiconductor material decreases with increasing temperature. That's an interesting thing. The resistance of an intrinsic semiconductor material decreases with increasing temperature. The reason is the more heat, the more electrons that are free. This is referred to as a negative temperature coefficient. As the temperature goes up the resistance goes down.
This concludes our introduction to semiconductors and specifically to intrinsic semiconductor devices.