Semiconductor Theory (Part 2) - Semiconductor Doping
We're continuing in 8.2 with semiconductor doping. Impurities are added to intrinsic semiconductor materials to improve the electrical properties of the material. The process is referred to as doping. The resulting material is called an extrinsic semiconductor. That's [inaudible 0:00:25] the intrinsic semiconductor we talked about previously. There are two major classifications of doping materials. First of all, there is trivalent. Materials that come under this category are aluminum, gallium, boron. These are materials that have three valence electrons, and I have some pictures to come in future slides here. Then, there are pentavalent materials things like antimony, arsenic, and phosphorus. These are materials that have five valence electrons.
Pentavalent and Trivalent Materials
These materials have essentially the same structure as silicon except that they have one or more or one less valence electron. They either have one more or one fewer valence electrons. These materials when mixed, and this is where we get the term dope, with the semiconductors, appear to create a defect in the lattice structure. This is a pentavalent atom here. You'll notice that there are five electrons in the outer valence shell. This is a trivalent atom here and you'll notice it has three atoms in the outer valence shell. You'll recall from the previous lesson we saw that pure silicon when it was in its lattice structure like this it had eight electrons in its outer shell. If we mix this pentavalent material in there there's going to be an extra one. If we mix this material in there, there would be a lack, there would be seven. Some examples here: Pentavalent, antimony, arsenic, phosphorus, and trivalent, boron, aluminum, and gallium.
Silicon is the most widely used semiconductor material. By adding a trivalent material to the crystal structure holes are introduced and provide a mechanism for conduction. Because trivalent materials can accept an additional electron they are called acceptor atoms. A silicon crystal doped with the trivalent material is called, and here is a term, p-type material. P-type material and this a result of trivalent doping. Here's the process. We start out with intrinsic silicon, then we add trivalent atoms and what do we get? P-type silicon. When doped with trivalent material holes are created in the semiconductor valence band. The doping impurity, in this case, is aluminum. Here we see the lattice structure that we see in silicon, except that here we find an aluminum atom. Aluminum has three atoms in its outer shell and so what we get? Instead of eight here now we have a hole right here. Here's another hole right here, and then here is an another hole. We got our lattice structure. This appears to create an impurity in this one because we did have this nice pure, this nice balanced system, eight everywhere, but now every place that we see this aluminum atom there are going to be seven and not eight.
Doping silicon with pentavalent material results in extra electrons being available, improving the conduction characteristics. Pentavalent materials donate electrons and therefore are called donor atoms. Once the silicon crystal has been doped with the pentavalent material it is called, and here is another term, n-type material, n-type semiconductor material. Here is the process, same thing. Start with intrinsic silicon, we add pentavalent atoms and what do we get? N-type silicon.
Pentavalent Atoms are Added to a Silicon Crystal
Impurity doping material phosphorus is added to the silicon. Note that valence shell is full with eight electrons and the extra electron is now available. It's not called n-type material. Here we have the nice silicon lattice structure until we get right here to this phosphorus guy. Since he has five electrons in his outer shell, the outer shell is full with eight, and so now here we have that extra electron. Then, here's another extra electron, and here's another extra electron. These are all associated with that phosphorus atom that was mixed in and making our nice silicon impure. It's got this phosphorus in it now.
Energy Levels in P Material
We want to take a look at energy level, both in P material and in N material. We have this image up here. In fact, this is available for this particular part. I didn't have it when we talked about this previously. Here we have p-type semiconductor and we're looking at absolute zero. Up here is what we'll call the conduction band. This represents free electrons, and then in this lower level, we look at the valence band and what is available in the valence bond. We're looking at P material first.
At absolute zero, all electrons are at their lowest energy level. There are no conduction band electrons. There is no energy. No energy available, so there's nothing going on in the conduction band. Note that there are holes in the valence band since there are not enough electrons to fill all the holes. You remember that when silicon is doped with the trivalent material it actually reduces the number of electrons in the valence shell, so at absolute zero there's going to be holes present without any energy whatsoever because that's just the nature of the material.
At room temperature, this is room temperature over here and this is p-type material, the electron receives thermal energy and some of them break their covalent bonds and move to the conduction band. This creates a hole in the valence band. In fact, remember that many of them still have eight, but it doesn't make any difference. At room temperature, there's going to be energy available and some of these electrons will move to the conduction band which would be in this band here. This creates a hole in the valence band, so this will create additional holes in the valence bands of our P material. Because of the presence of the trivalent material, there will always be more holes in the valence band than electrons in the conduction band. Remember, when we looked at pure silicon we had an even number of electrons as holes. With P material there will always be more holes in the valence band than there are free electrons in the conduction band.
Energy Level in N Materials
Here we're just looking at the same thing again only we're considering N material at this point. At absolute zero, no energy, there are no holes in the valence band. Notice there's no holes in the valence band and free electrons are available in the conduction band. The covalent bond supports a max of eight electrons. The impurity material will add one extra electron and is forced to a higher conduction level. Remember that with the N material the lattice structure was completely full. In fact, there was an extra electron there. At absolute zero you the eight electrons are bound into the valence shell but there is an extra one, so even at absolute zero, there are extra electrons available for conduction band.
At room temperature, some of the electrons will break away from their valence shell and move to the conduction band. This will create holes. At room temperature, remember we still have our eight. What we're going find is that at room temperature some of those will become free electrons, and so we will have some holes created as a result of that. As result of pentavalent impurity, there will always, and note this, more free electrons in the conduction band than holes in the valence band.
Let's review what we've said here. We started out talking about semiconductors. Remember, a pure semiconductor, they'll have equal numbers of free electrons and holes. Remember we had the nice balanced atom that had eight electrons in it and for every free electron there was going to be one hole created and they had equal numbers of each. With p-type material, this is doped with trivalent material, will always have more, notice more holes than electrons in the conduction band because the nature of the material is to have holes. We'll still have his phenomenon where an electron will leave and create a hole, but because we started out with holes there'll always be more holes than electrons in the conduction band. That's the nature of P material.
N material is just the opposite. It is doped with pentavalent material. It will always have more free electrons in the conduction band than holes. Again, even in n-type material, you'll still have the electrons leading, you'll still have the creation of holes, but because the nature of the material is to extra electrons then you will have more free electrons than holes.
This concludes our discussion of doped silicon. We consider pure semiconductors, we looked at P material, and we looked at N material.