In an isolated atom, the electrons outside the nucleus are arranged in a certain shell, and each shell contains a certain number of electrons. The electrons on each shell have discrete energy values, that is, the electrons are distributed according to energy levels. For the sake of brevity, on the graph showing the level of energy, the energy levels of electrons are represented by horizontal lines of different heights. This graph is called the electron energy level diagram.
(1) enegy band
A large number of atoms in the crystal are gathered together, and the distance between the atoms is very close. Taking silicon as an example, there are 5X 1022 atoms per cubic centimeter of volume, and the shortest distance between atoms is 0.235nm. This causes the shells farther from the nucleus to overlap. The overlap of the shells makes the electrons no longer confined to a certain atom, and may transfer to the similar shells of adjacent atoms, or move from adjacent atoms to more. A far away atomic shell goes up. This phenomenon is called the commonality of electrons. As a result, electrons that are originally in the same energy state have a slight energy difference, and the corresponding energy level is expanded into an energy band.
(2) Forbidden band
The energy band allowed to be occupied by electrons is called the allowed band, and the range between the allowed bands is not allowed to be occupied by electrons. This range is called the forbidden band. The inner permissible band in the atomic shell is always occupied by electrons first, and then the permissible band of the outer layer with higher energy is occupied. The allowable band occupied by electrons is called the full band, and the band without electrons at each energy level is called the empty band.
(3) Valence band
The outermost electron in an atom is called a valence electron, and a valence band.
(4) Conduction band
The allowable band with the lowest energy above the valence band is called the conduction band.
The bottom energy level of the conduction band is denoted as Ec, the top energy level of the valence band is denoted as Ev, and the energy interval between Ec and Ev is the band gap Eg. The conductive effect of a conductor or semiconductor is realized by the movement of charged particles (to form an electric current), and the carrier of this electric current is called a carrier. The carriers in a conductor are free electrons, and the carriers in a semiconductor are negatively charged electrons and positively charged holes. For different materials, the forbidden band width is different, and the number of electrons in the conduction band is also different, thus having different conductivity.
As shown in Figure 1, the Eg of the insulating material SiO2 is about 5.2eV, and there are very few electrons in the conduction band, so the conductivity is not good, and the resistivity is greater than 1012Ω·cm. The Eg of semiconductor Si is about 1.1 eV, and there are a certain number of electrons in the conduction band, so it has certain conductivity, and the resistivity is 10-3~1012Ω.cm. The conduction band of the metal overlaps with the valence band to a certain extent, Eg=0, valence electrons can move freely in the metal, so the conductivity is good, and the resistivity is 10-6~10-3Ω. cm.
When the minimum conduction band and the maximum valence band of a semiconductor correspond to the same position in the wave vector space, they are called direct band gap semiconductors, such as CdTe, Cu(InGa)Se2 and GaInP, etc. The photon absorption process is shown in Figure 2(a). The transition of valence band electrons to the conduction band does not require the participation of phonons, but only needs to absorb energy. Indirect band gap semiconductor materials (such as Si and Ge) conduction band minimum (conduction band bottom) and full band maximum are at different positions in the wave vector space, forming a half-full energy band not only requires energy absorption, but also changes momentum, photon absorption The process is shown in Figure 2(b), and only the participation of phonons can achieve the conservation of electron momentum. The difference between the two is: the electrons on the conduction band of the direct band gap semiconductor are caused by the direct transition of the valence band, while the electrons on the conduction band of the indirect band gap semiconductor are excited by the valence band and return to the conduction band. It takes a relaxing process to reach the bottom of the conduction band. Part of the energy is wasted in the form of phonons in this process. From the perspective of energy utilization, semiconductors with direct band gaps have better light utilization. Amorphous materials can also present a similar energy band structure. Atoms are arranged according to a certain rule. Due to the overlap of electron wave functions, a mobility band gap is formed. Electrons above the mobility band form a conduction band. The lower holes constitute a valence band, but unlike crystal materials, there are a large number of local energy states in the co-mobility band gap, so the transition situation is more difficult to analyze.
During irradiation or current injection, the semiconductor deviates from the thermal balance, and the concentration of electrons and holes will be rebalanced through the recombination process. The electrons return to the valence band from the conduction band and offset part of the holes in the valence band. There are three main types. Recombination mechanism: Recombination through traps in the band gap, radiation recombination and Auger recombination (as shown in Figure 3).