Video Lectures created by Tim Feiegenbaum at North Seattle Community College.
We're in section 8.3, semiconductor junctions. When P-type material meets N-type material within a single silicon crystal a PN junction is formed.
Here we have P and N material and this area right here is the area we're referring to as the PN junction. The PN junction is formed in the process of creating the semiconductor device. We want to look at this junction in what we'll call an unbiased condition meaning that there's no voltage across it. Before carrier migration, there are equal numbers of holes and electrons on either side of the junction. This results in a net charge of zero. Because of random thermal energy, some electrons will pass across the PN junction mating with holes on the other side. This is referred to as recombination. Your text has a picture depicting recombination.
I'm just going to draw a picture here and we'll say this is P material and this is N material. Remember, that P material has an excess of holes and N material has the donor electron, so it has extra electrons. Right at the point of the junction, this won't be throughout all the material, this is just right at the point where these two materials are touching each other, there is going to be something interesting that occurs. Before they're put together, remember, that the net charge is zero volts. There is no charge on either of these materials. When we bring them together, something occurs. These are free electrons, remember, they see that available hole over there. The free electrons are going to move over to those holes. What that is going to do is since this material on the end side is losing an electron this will result in a net positive charge on the end material. The holes over here which were net charge of zero, suddenly they received an electron and now they have a net negative charge. We're going to have this region develop that has a net charge and we're going to have a name for it. The end result of recombination is the creation of the, here it is, the depletion region which means an area depleted of carriers.
What's that mean? The free electrons in the end material have filled the holes in the P material. That's what we were talking about right here that the free electrons moved over to the P material. Because the N material lost an electron is now has a net positive charge. Because the P material gained an electron it now has a net negative charge. In this region, it is now depleted of carriers. In the area of this junction, those electrons that were able to move did move and the holes which are able to serve and transport for electrons they're all full of electrons. We have a region that is depleted of carriers and it actually has a potential voltage across it.
After a time, the region will be depleted of charged carriers because of the migration of electrons and holes. This leaves an area known as the depletion region of the PN junction. Further electron migration will not take place until the barrier potential is overcome. That region, that barrier potential, there is actually a voltage there now. There's that difference of potential and to overcome that it will take 0.6-0.7 volts in silicon, and if it's germanium it'll take 0.2-0.3 volts. If you work with diodes you'll find that there will be no current flow in this until you have overcome this barrier potential. If you were to connect a power supply down here, you'd have to have at least 0.7 volts to cause this to conduct.
An external source can either oppose or aid the barrier potential. If the positive side of the voltage is connected to the P-type material and the negative side to the N-type material, then the junction is said to be forward biased, forward biased. What we'll have here, notice, remember we had the free electrons in the N material and we had the holes over there in the P material. Let's look at what's going to occur if we connect the battery like this. Remember, current flow is from negative to positive. Here we have this negative force connected here and we have these free electrons. These free electrons want nothing better than to move over to the positive terminal. When it's connected like this those negative electrons, those electrons that are negative, will be forced to move through the N material and most of them will move through holes through the P material and they'll come over here. The positive force here, the positive on this one, remember the holes want to move from positive, remember the holes are not moving anywhere, but this positive force will facilitate the whole process causing electrons to move through the P material. This will cause the depletion region to narrow drastically. This is a forward biased junction and current will flow easily through this junction.
Let's talk about it a little more. In a forward biased junction, the following conditions exist. Forward biased overcomes the barrier potential, the electrostatic field created by the barrier potential had stopped electron migration. That was initially, that electrostatic field created the barrier potential and stopped electron migration. A voltage of 0.7 volts, if this is silicon, will overcome that barrier. Forward biased narrows the depletion region. Forward biased forces the majority carriers in both materials towards each other, thus narrowing the depletion region. We just discussed that. There is maximum current flow with forward bias. The power supply provides an abundance of free electrons and overcomes the barrier potential. Electrons move as conduction and electrons in the N material and as valence band electrons in the P material. When forward biased, the PN junction resembles a short circuit.
Reverse bias occurs when the negative source is connected to the P-type material. Negative is connected to the P-type material and the positive source is connected to the N-type material. Reverse bias will have the effect of strengthening the barrier potential. Reverse bias widens the depletion region and current flow is minimum at best. What's going to happen here is remember in the N-type material we have these electrons and these free electrons that want to, let's see, what do they want to do under this condition? They've got a power supply connected here and the positive terminal is here. Remember, those electrons want to go to the positive side of the power supply. What we're going to end up doing is in this region here, in fact, all the way across this material we are depleting this of electrons. Electrons are going to be pulling away from this barrier towards this positive force. Again, the holes aren't going anywhere, but remember hole movement would be from positive to negative, and so the holes are going to tend to want to move in this direction. That will thus inhibit electron flow as well. With reverse bias, we are going to cause a depletion of electrons in this region and thus we will effectively shut this junction off.
More discussion about reverse bias. A reverse bias junction has zero current flow, and that is ideally. Reverse current is temperature dependent. The higher the temperature, the greater the probability for reverse current. If reverse bias is increased enough the reverse current increases dramatically causing the PN junction to break down. This is referred to as junction breakdown. The voltage required to reach this point is referred as the reverse breakdown voltage, RBV, and this is usually specified on data sheets. Actually, this is something to be avoided. As the breakdown occurs, an avalanche may occur and destroy the device if uncontrolled. Most PN junctions are designed to not operate in a reverse bias junction, a reverse bias condition. There are a few devices like zener diodes that actually do operate in reverse breakdown, but for most devices, if they go into junction breakdown you're looking at the destruction of the device.
In this section, we have looked at PN junctions, we looked at reverse biasing, we looked at forward biasing, we looked at the unbiased junction. That concludes 8.3.
Video Lectures created by Tim Fiegenbaum at North Seattle Community College.
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by Aaron Carman