Matter is made up of atoms, and objects made up of atoms repeatedly arranged in a certain period are called crystals. When atoms combine to form a crystal, because the distance between atoms is very close, the electron orbits (quantum states) between different atoms will overlap to different degrees. Of course, the orbits of the outermost electrons of two adjacent atoms in the crystal overlap the most. The overlap of these orbitals allows electrons to be transferred from one atom to another. As a result, the electrons originally belonging to a certain atom are no longer owned by this atom alone, but can move in the entire crystal and become common to the entire crystal. This phenomenon is called the commonality of electrons. The degree of overlap between the inner and outer electron orbits of the atoms in the crystal is very different. The more the outer layer of electrons, the greater the degree of overlap, and the less bound it is by the nucleus. Therefore, the shared characteristics of the outermost electrons are the most prominent.
Although electrons in a crystal can be transferred from one atom to another atom, it can only transfer between quantum states with the same energy. Therefore, there is a direct correspondence between the shared quantum state and the original energy level. Due to the commonality of electrons, the entire crystal becomes a unified whole. These energy levels are very close to each other and are distributed in a certain energy region.
For this reason, the dense energy levels in this energy region are vividly called energy bands. Since the energy difference between the energy levels in the energy band is very small, the energy levels in the energy band can usually be regarded as continuous. In ordinary atoms, the energy levels of inner electrons are filled with electrons. When atoms form a crystal, the energy bands corresponding to these inner energy levels are also filled with electrons. Under ideal absolute zero, in covalently bonded crystals such as silicon, germanium, and diamond, the electrons from the innermost layer to the outermost valence electrons just fill the corresponding energy band. The energy band with the highest energy is the band filled with valence electrons, called the valence band. The energy band above the valence band is basically empty, and the band with the lowest energy is often called the conduction band. The area between the valence band and the conduction band is called the band gap. The energy bands of insulators, semiconductors, and conductors are shown in Figure 1.
Generally, the forbidden band of an insulator is relatively wide, the valence band is filled with electrons, and the conduction band is generally empty. The energy band of a semiconductor is similar to that of an insulator. Under the ideal absolute zero, there are also a valence band filled with electrons and a completely empty conduction band, but the forbidden band is relatively narrow. Because of this, under certain conditions, electrons in the valence band are easily excited into the conduction band. Many important characteristics of semiconductors are caused by this. There are two types of energy bands of a conductor: one is that its valence band is not filled by electrons, that is, electrons with the highest energy can only fill the lower half of the valence band, and the upper half is empty; the other is that its valence band is The conduction bands overlap.
The above argument on the formation of energy bands is not strict. The correspondence between energy bands and atomic energy levels is not as simple as the above explanation, and it is not always that one atomic energy level corresponds to one energy band. Moreover, the energy band diagram does not actually exist. The energy band theory of insulators, semiconductors, and conductors is only used to illustrate the energy distribution of electrons.