1. Germanium, silicon, selenium, gallium arsenide and many metal oxides, metal sulfides and other objects, whose conductivity is between conductor and insulator, are called semiconductors. Semiconductors have some special properties. For example, the thermistor (thermistor) for automatic control can be made by using the relationship between the resistivity and temperature of the semiconductor; With its photosensitive characteristics, photosensitive elements for automatic control can be made, such as photocells, photocells and photoresistors.
2. Semiconductors also have one of the most important properties. If a small amount of impurities are properly mixed in the pure semiconductor material, its conductivity will increase by a million times. This feature can be used to manufacture various semiconductor devices for different purposes, such as semiconductor diodes, triodes, etc.
3. When one side of a semiconductor is made into a P-type region and the other side into a N-type region, a thin layer with special properties is formed near the junction, which is generally called a PN junction. The upper part of the figure shows the diffusion of carriers on both sides of the interface between P-type and N-type semiconductors (represented by black arrows). The middle part is the formation process of the PN junction, indicating that the diffusion effect of the carrier is greater than the drift effect (indicated by the blue arrow, and the red arrow indicates the direction of the built-in electric field). The lower part is the formation of PN junction. Indicates the dynamic balance of diffusion and drift.