Let’s step back to Part 1 for a moment. In Part 1, we said some sort of excitation method, such as Thermal or Optical Generation, was necessary to bump electrons up into the Conduction Band. What we did not consider at that point was the ‘hole’ left behind when an electron was smashed from a bond.
In fact, these holes
are charge carriers too. Holes can move through the material just as electrons do, and they contribute to the conductivity of the material as well. This is a difficult concept to grasp; the idea of a hole is an abstraction, since the hole is not actually an object, like an electron, but rather the absence of an object. To help understand, consider that the hole exists in the Valence Band, not the conduction band (since it was created by an electron getting pulled from the Valence Band to the Conduction Band). Thus, electrons in other bonds can easily slide into that hole without requiring any extra energy. In that fashion, an electron from one bond can move into a hole left in another bond. Then, yet another electron from yet another bond can move into the hole left by the previous electron, and so on. The diagram below should help to visualize the process.
http://media.hardwareanalysis.com/articles/small/10649.gif" alt="Semiconductor Physics">Fig. 3 - Hole movement through the material. The 'hole' is an abstraction; it has no substance and does not actually move itself, but movement of electrons in the opposite direction is perceived as the hole moving.
As you can see from the diagram, the movement of a hole is not, in fact, the hole itself moving, but rather all of the other electrons moving in the opposite direction. As all of the electrons slide to the right, the hole is perceived as moving to the left. Now consider again our small silicon wire example. Suppose this wire had no free electrons in the Conduction Band, but had free holes in the Valence Band. If we tried to push an electron (or a hole) into one end, the electrons (or holes) inside the silicon could shuffle around, an electron (or hole) could emerge from the other end, and the net result would be the exact same number of electrons and holes as there were to begin with, thereby obeying space charge neutrality. If you’re having trouble grasping that, have a look at the graphic below.
http://media.hardwareanalysis.com/articles/small/10650.gif" alt="Semiconductor Physics">Fig. 4 - The hole as a charge carrier. Again, the hole is an abstraction, and cannot conduct charge itself, per se. However, by providing a means for electrons to move, it effectively serves as a conductive charge carrier. Remember that holes and electrons move in opposite directions. A hole 'entering' one end of a material is not actually a hole entering, in fact, but an electron leaving, in the opposite direction, thus leaving a hole in its absence.
We can now take a moment to discuss some of the properties of the two types of charge carriers; electrons and holes. First of all, as a hole is the absence
of an electron, it is considered to have a positive charge. Secondly, when we use Carrier Generation
(such as Optical or Thermal Generation) to excite electrons, we also create holes in the same number. In other words, electrons and holes are created in pairs
. Third, we can now define a property of carriers called mobility
. Mobility can be thought of as the ease with which the carrier moves. Since electrons move by themselves, where as hole movement is in fact the movement of every other electron, it is not difficult to see that electrons can move much more easily than holes, and thus have a higher mobility.
Fourth, and most important, when we pass current through a material, holes and electrons move in the opposite direction
. Remembering that electrons move opposite the direction of current, holes, then, must move in the same direction as current. The graphic below should help to clarify.
http://media.hardwareanalysis.com/articles/small/10651.gif" alt="Semiconductor Physics">Fig. 5 - Holes and electrons flow in opposite directions. Because of the error in the definition of current, holes are perceived as flowing in the same direction as current, while electrons are seen to flow in the opposite.
The whole idea of the hole may seem vague and silly at first, but its usefulness will become evident later.