Diodes can be divided into four categories by type : germanium diodes, silicon diodes, electrical contact diodes, and surface contact diodes.

      Germanium diode: Germanium diode is a diode made of germanium material. Semiconductor diodes are used in almost all electronic circuits. It plays an important role in many circuits. It is one of the earliest semiconductor devices and its applications are also very extensive.

　　 Silicon diode: Germanium diode is a diode made of germanium material.

　　Electrical contact type diode: Point contact type diode is formed by pressing a metal pin on a single wafer of germanium or silicon material, and then formed by current method. Therefore, its PN junction has a small electrostatic capacity and is suitable for high-frequency circuits. Compared with the junction type, the point contact type is less used for large current and rectification. Because the structure is simple, the price is cheap. For general purposes such as detection, rectification, modulation, mixing, and limiting of small signals, it is a type with a wide range of applications.

　　 Surface contact diode: Surface contact diode is a special silicon diode, which can not only pass a large current, but also has stable and reliable performance. It is mostly used in switching, pulse and low frequency circuits.