If the objects in the world are distinguished by their conductivity, some are easy to conduct electricity and are called conductors, such as gold, silver, copper, aluminum, lead, tin and other metals; some are not easy to conduct electricity and are called insulators. There are glass, rubber, plastic, quartz, etc.; objects with electrical conductivity between the two are called semiconductors, mainly germanium, silicon, gallium arsenide, cadmium sulfide, etc. As we all know, an atom is composed of a nucleus and its surrounding electrons. When some electrons are freed from the shackles of the nucleus and can move freely, they are called free electrons. The reason why the metal is easy to conduct electricity is because there are a large number of electrons that can move freely in the metal body. Under the action of the electric field, these electrons regularly flow in the opposite direction to the electric field, forming an electric current. The greater the number of free electrons or the higher the average speed at which they flow regularly under the influence of an electric field, the greater the current flow. The flow of electrons carries electricity, and we call the particles that carry electricity as carriers. At room temperature, there are only a very small amount of free electrons in the insulator, so it does not exhibit electrical conductivity to the outside. There are a small number of free electrons in the semiconductor, which can conduct electricity under some specific conditions.
Semiconductors can be elements such as silicon (Si) and germanium (Ge), compounds such as cadmium sulfide (CdS) and gallium arsenide (GaAs), or alloys such as GaxAl1-xAs, where x is any number between 0 and 1. Many organic compounds, such as anthracene, are also semiconductors.
The resistivity of semiconductors is large (10-5Ω•m≤ρ≤107Ω•m), while the resistivity of metals is very small (about 10-8~10-6Ω•m), and the resistivity of insulators is very large ( p>108Ω•m). The resistivity of semiconductors is sensitive to temperature. For example, when the temperature of the semiconductor increases from 20°C to 30°C, the resistivity will decrease by about half. The resistivity of metal changes less with temperature. For example, when the temperature of copper increases by 100 °C, ρ increases by about 40%. The resistivity is significantly affected by impurities. When the metal contains a small amount of impurities, the resistivity does not change much, but when a trace amount of impurities is doped into the semiconductor, it can cause a large change in the resistivity. For example, when pure silicon is doped with parts per million One of the bricks, the resistivity of silicon is reduced from 2.14 × 103Ω·m to about 0.004Ω·m. The resistivity of metals is not affected by light, but the resistivity of semiconductors can change significantly under appropriate light exposure.