Semiconductor Theory (Part 3) - Current Flow in a Semiconductor
We're continuing in section 8.2 and we're looking at current flow in a semiconductor. When a doped semiconductor has a voltage applied to it current will flow from negative to positive regardless of whether it is p or N-type material. Here we have a battery and whether this is N material or whether this is P material current will flow through it. However, the manner of the flow of current is radically different for the two types of material.
Current Flow Through N-type Material
N-type material has many conduction band electrons. If a voltage is connected across N-type crystal free electrons will move towards the positive terminal. If we have a battery and we have it connected N-type material those free electrons, remember current flow is negative to positive, those free electrons are going to be attracted to the positive terminal. As electrons are moved from one atom towards the positive terminal a hole is left behind allowing more electrons to shift toward the source voltage. Some of the valence electrons will also be moving. There will be holes created, electrons will be coming through here, some of them will move as free electrons, others will move through holes. There are more electrons than holes, as in the case in of N material, so current movement is electron movement.
Current Flow Through P-type Material
Current flow in P-type material causes the shift of holes towards the negative terminal because of the shifting of the covalent electrons. We're going to look at this. We have an image on the next screen that's going to depict this. Hole flow moves from positive to negative in a P-type semiconductor material. Actual current flow is still electron current flow from negative to positive.
Electron Versus Hole Flow
Electron flow in P-type material occurs in the valence band. In P-type material, we do have electron flow but it happens in the valence band. Electron movement in N-type material occurs in the conduction band. This is primarily. Electrons are the majority carriers in N-type material. They are holes in P-type material, so the majority carriers in P-type material are going to be the holes.
We have this image here. The arrows represent holes. This is representative of a hole that we could draw an atom here and we'll draw in one, two, three, four, five, six, seven. We'll draw seven electrons in this atom which means that there's a big hole right here. Electron current flow is moving this way. What we're going to find is that hole flow is actually going this way and here's he was this thought works. This is going to be another atom and we'll say that it has eight electrons in its valence shell and electron flow is moving in this direction, so what will happen is one of these valence electrons from this will move to this one. Once it is moved to this one, now this had the hole, this one, but now it's shifted to there. We're just going to move down to this next line. Here, now we have this that has a hole in it and there's electron number four it wants to move here, so it moves here. Number four now becomes the one with seven, and so there's a hole here. Five moves and you get the idea that in P-type material the electrons are moving through the holes. What's happening here is that the holes are tending to move in the opposite direction. Electrons are moving in the way we would expect them, but in P-type material, there is also the movement of holes going in the opposite direction.
Majority and Minority Carriers
The mechanisms for current flow. Conduction band electrons for N-type material. Those are the majority carriers. In N-type material conduction band electrons make up the majority carriers. Valence band holes make up the majority carriers for P-type material. These are the majority carriers. They're also minority carriers. Valence band electrons are minority carriers in N material. Most of them are going as conduction band, but also there are valence band electrons so there are going to be some electrons moving through holes, but the majority are going to be conduction band. Then, finally, conduction band electrons are minority carriers in P material. In P material, most of the movement is through holes, but some of them will also be in the conduction band. What we're talking about here is majority device the minority.
A device that depends on only one type material is called a unipolar device. A device that depends on both is called a bipolar device. Minority current carriers are for the most part a function of heat. This contributes to the heat sensitivity of semiconductor. We will find that semiconductors are very sensitive heat and it's a consideration that has to be looked at when we're considering current flow in semiconductors that heat can be a factor.
This concludes 8.2C. We've looked at minority and majority carriers, we looked at the concept of hole flow, and we looked at current flow through P material as well as N material.