Читать книгу Semiconductor Basics - George Domingo - Страница 28

Now consider the situation where the valence band is not full of electrons (case 2), as I show in Figure 2.6, or where the bands expand so much that the conduction band encroaches into the valence band (case 5), as I show in Figure 2.7. These two cases are very similar.

Even at absolute zero (as I show on the left), there is lots of space for the electrons to move. At room temperature, which is quite an increase in energy from absolute zero, the electrons have more than enough energy to move all over the place. It is like having a half‐full garage with two doors at each end connected directly to a freeway, and it is warm enough that people want to get moving. If we apply a voltage (the motivating force) across this material, the electrons have no problem going where they want to go, i.e. to the positive terminal. Also note that the cars that moved up to the freeway left empty spaces in the garage, so some cars in the garage can also move from one location to another inside the parking structure. As you may expect, this is a conductor. Tons of electrons are free to move as soon as a voltage is applied.

Figure 2.6 If the valence band is not full of electrons, there is a lot of space even at the lowest temperatures for electrons to move easily in the valence band.

Figure 2.7 Even if the valence band is full, if the conduction band encroaches on the valence band, there is plenty of space for the electrons to move freely when a force (a voltage) is applied.