Читать книгу Semiconductor Basics - George Domingo - Страница 27

Let's consider first the situation in which the valence band is full of electrons (case 1) and there is a large gap separating the valence and conduction bands (case 4). I illustrate this in Figure 2.5. Imagine that the valence band is a parking lot full of cars (electrons), and above it is the conduction band, equivalent to an empty freeway. The cars in the parking lot cannot move because there is no space for them to go to. No cars are on the freeway, so there is no movement up there, either. If I apply a voltage to this material (right, in Figure 2.5), nothing moves. The gap between the parking lot and the freeway is too large for cars to jump, and thus no matter what voltage I apply, the electrons cannot move; there is no current flowing in this material at all. This is the case for insulators. No electrons can move under an applied voltage.

Figure 2.5 In an insulator, the valence band is full of electrons, the conduction band is empty, and the separation between the two bands is very large.

At this point, I would like to clarify the concept of bands, which can sometimes be confusing. The bands are not a physical location where electrons reside, just as earth's orbit is not a railroad track or a circular road on top of which the earth travels. A cannonball follows a parabolic path even though there is no “path,” pipe, road, or track that the ball rolls over. The concept of energy bands is similar. The electrons are anywhere in the material, but they have energies with values restricted to certain allowed ranges – energy ranges that we call bands. The electrons are not allowed, under any circumstances, to have energy between the highest energy of the valence band and the lowest energy of the conduction band.