Читать книгу Semiconductor Basics - George Domingo - Страница 25

We saw in the previous chapter that an atom's electrons have precise energy values (we represent them as orbits or levels). We also saw that electrons must have distinct quantum numbers (designations), which limits the number of electrons in an atom that can have a given energy. As atoms have more and more electrons, the electrons have to occupy higher and higher energy levels. An electron must absorb from somewhere the exact energy needed to jump from one level to a higher one. When it falls back to a lower level, it donates the same amount of energy. This is what happens in a gaseous state where the atoms are separated by large distances and do not interact with each other. This model beautifully explains the absorption and emission spectral lines of the elements, the sun, and the stars.

In this chapter, we are going to push the atoms closer and closer together until we form a solid. Now the atoms and electrons start interacting with each other and forming bonds, which is what keeps them together in a crystallographic structure. As we push them together, the energy levels have to separate because, in a system, according to Pauli's exclusion principle, no two electrons can have the same quantum number. The levels split into bands. Depending on how the bands spread, the material behaves like a conductor, an insulator, or a semiconductor.

Finally, we will analyze the specific case of semiconductors and how the electrons fit into the bands. We will also see how the lack of an electron, which we call a hole, is equivalent to a positive charge.