The electrical conductivity of a substance is determined by the atomic structure. Conductors are generally low-cost elements, and their outermost electrons are easily detached from the nucleus into free electrons, which produce directional movement under the action of an external electric field to form an electric current. High-priced elements (such as inert gases) or high-molecular substances (such as rubber), their outermost electrons are strongly affected by atoms and binding forces, and it is difficult to become free electrons, so the conductivity is extremely poor and becomes an insulator. The commonly used semiconductor materials, silicon (Si) and germanium (Ge), are tetravalent elements, and their outermost electrons are neither as easy to break away from the nucleus as conductors, nor as tight as the insulators are bound by the nucleus. Conductivity is somewhere in between.
The pure semiconductor is made into a single crystal through a certain process. This is the intrinsic semiconductor. The atoms in the crystal form a well-aligned pillow in space, and adjacent atoms form covalent bonds.
The covalent bond in the crystal has a strong binding force. Therefore, at normal temperature, only a very small number of valence electrons obtain sufficient energy due to thermal motion (thermal excitation), thereby breaking free from the covalent bond into a free electron. . My colleague, leaving a hole in the covalent bond. An atom is positively charged by the loss of a valence electron, or the hole is positively charged. In the intrinsic semiconductor, free electrons and holes appear to be accepted, that is, the number of free electrons and holes is equal.
If a free electron encounters a hole in the process of movement, it will fill the cavity and make both disappear at the same time. This phenomenon is called conformity. At a certain temperature, the intrinsic excitation produces a free electron and hole pair that is equal in number to the matching free electron and hole pairs, thus achieving dynamic equilibrium.
Band theory:
1. When electrons in a single atom move around a nucleus, the electrons in each orbit each have a specific energy;
2. The closer to the orbit of the core, the lower the electron energy;
3. According to the principle of minimum energy, electrons always have the lowest energy level.
4. The energy band occupied by valence electrons is called the valence band.
5. There is a forbidden band above the valence band. There is no energy level occupied by electrons in the forbidden band.
6. The forbidden zone IQ is the conduction band. The energy level in the conduction band is the energy level that the free electron can occupy when the price electron breaks away from the covalent bond.
7. The forbidden band width is expressed by Eg, and its value is related to factors such as the material of the semiconductor and the temperature at which it is placed.