Basic physical and chemical properties of crystalline silicon
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- The electrical properties of silicon
Silicon is a typical semiconductor material, and its resistivity is about 10-4~1010Ω·cm. The conductivity and conductivity type are related to the impurities in the silicon crystal. Intrinsic semiconductor silicon does not contain impurities and defects, and its resistivity is very high; when a small amount of impurities is added, its conductivity increases. When pure silicon is doped with donor impurities (group V elements: P, As, Sb, etc.), N-type silicon is formed, which is electronically conductive; when doped with acceptor impurities (group III elements: B, Al, Ga, etc.) At this time, P-type silicon is formed, which conducts electricity through holes. The interface between P-type silicon and N-type silicon forms a PN junction, which is the basic structure of crystalline silicon solar cells and the basis of solar cells.
- Optical properties of silicon
The light incident on the crystalline silicon obeys the laws of light reflection, refraction and absorption.
In crystalline silicon, the absorption of light is divided into intrinsic absorption, impurity absorption, exciton absorption and lattice vibration absorption, etc. The most important is intrinsic absorption. Intrinsic absorption refers to the transition of photon excited electrons from the valence band to the conduction band, which occurs within the limit wavelength λ0, which corresponds to the band gap width of 1.leV; all other absorptions are outside λ0. In the infrared range of 1~7um, the transmittance of silicon is as high as 90%~95%.
Silicon is an indirect band gap material, but if it is excited by a photon with a large enough energy, the electrons in the silicon can also undergo direct transitions. When the absorption coefficient is below the absorption limit λ0, the photon energy gradually rises, and when a reaches the range of 104~108/cm, a direct transition occurs.
- Mechanical and thermal properties of silicon
At room temperature, silicon is a brittle material; when the temperature is higher than 700°C, silicon has thermoplasticity. The tensile stress of silicon is much greater than the shear stress. Silicon shrinks in volume when it melts, and expands when it solidifies. The surface tension of molten silicon is 736mN/m and the density is 2.533g/cm³.
- The chemical properties of silicon
In nature, silicon mainly exists in the form of oxide. At room temperature, the chemical properties of crystalline silicon are very stable; but at high temperatures, silicon can chemically react with almost all substances.