1.9. Semiconductor Hole Flow
Earlier we described good conductive elements as those having a single electron in their valence shell or band. Insulators had their valence bands nearly filled. There are certain elements, however, whose valence bands are exactly half filled with electrons. Silicon and germanium, for example, each have four valence electrons in a band that can contain eight electrons. These elements belong to a class we call semiconductors.
In general, semiconductors are poor conductors of current. But suppose, for example, we add a small amount of impurity to an otherwise pure silicon structure. Antimony, arsenic, and phosphorous are three examples of elements that have five electrons in their valence band. If we allow an atom of phosphorous, for example, to take the place of a silicon atom in an otherwise pure crystalline silicon structure, an “extra electron” results, as shown in Figure 1-6. It is not truly extra, but it is not as tightly held as all the other nearby electrons. When we add elements that contribute extra electrons to the structure, we call it “n-doping” (“n” for negative) and say we are creating an N-type semiconductor.
Figure 1-6. Impurities added to an otherwise pure silicon crystalline lattice can create extra electrons or electron holes.
Similarly, boron, aluminum, and gallium each have only three electrons in their valence bands. If we allow an atom of aluminum, for example, to take the place of a silicon atom in an otherwise pure crystalline silicon structure, an “electron hole” results, also shown in Figure 1-6. Again, this is not really a shortage of an electron, but a nearby electron might be “captured” and held more tightly at this location than if the silicon structure were totally pure. We call this process “p-doping,” creating a P-type semiconductor.8
Now imagine that the electron immediately to the left of the hole in Figure 1-6 moves to the right and fills the hole there. That then creates a hole where this electron used to be. This might happen if there were an external force (voltage) applied to the structure. Here is the philosophical question: Did the electron move to the right (electron flow), or did the hole move to the left (hole flow)? In one sense, this is just a philosophical question,9 but certain phenomena are more easily explained from the standpoint of hole flow than from electron flow. Thus, the term hole flow is routinely used in semiconductor physics.
Sometimes people try to resolve the anomaly of current flowing from positive to negative in ordinary conductors, when the electrons are in fact flowing from negative to positive, by thinking in terms of hole flow instead. This can be appropriate in semiconductors, but there are no real holes in a copper structure. Electrons move because they are being pushed or pulled by an external force, not because there are holes to move into. Therefore, trying to extend the idea of hole flow to copper can just create more confusion. Personally, I prefer to live with the recognition that when it comes to conductors, it really doesn’t matter much how we view it.