Solid state physics is a science that studies the physical properties, microstructure, motion forms and their relationships of various particles that make up solid matter. It is a branch of physics, rich in content and widely used. Solid-state physics is the foundation of microelectronics, optoelectronics, energy technology, material science and other technical disciplines, and the research papers on solid-state physics account for more than one third of the research papers on physics.

The achievements of solid-state physics and experimental methods have more and more influence on chemical physics, catalysis, life science and earth science, and are forming a new cross-field. Solid state physics is actually a multi-body problem. In solids, the coupling modes between particles have their own characteristics, which leads to the specific collective motion form and individual motion form of particles, thus producing different physical properties of different solids.

Energy band theory;

The state and behavior of electrons in solids are the basis for understanding the physical and chemical properties of solids. 1853, Weidemann and Franz determined the empirical law of the relationship between thermal conductivity and electrical conductivity of metals through experiments.

Lorenz established the classical statistical theory of free electrons in 1905, which can explain the above empirical laws of solid physics, but can't explain the reason why metal electron gas contributes little to specific heat capacity at room temperature. Pauli successfully calculated the paramagnetism of free electron gas with quantum statistics for the first time in 1927, and Sommerfeld obtained the specific heat capacity and transport phenomenon of electron gas with quantum statistics in 1928, which solved the difficulties of classical theory.

The motion state of electrons in solids obeys the laws of quantum mechanics and quantum electrodynamics. In crystals, the outer electrons of atoms may have the energy to form energy bands. Electrons cannot have energy values outside the band. Crystals can be divided into metals, insulators and semiconductors according to the different filling methods of electrons in the energy band. The energy band theory is combined with the basic research of semiconductor germanium and silicon, and the high-quality semiconductor single crystal growth and doping technology lays a theoretical foundation for the preparation of transistors.