&Nbsp&Nbsp&Nbsp&Nbsp; Crystal diodes, abbreviated as diodes, are made of semiconductor materials, just like crystal transistors. The so-called semiconductor is a type of material that guides electrical properties between conductors and insulators. Commonly used semiconductor materials include silicon and germanium. The reason why we often hear about Silicon Valley in the United States is because there were many semiconductor manufacturers there at first. Semiconductor materials have two obvious characteristics: firstly, the size of their conductivity is greatly affected by the content of impurities. For example, as long as one millionth of boron is added to silicon, the conductivity can be increased by more than 500000 times; Secondly, the conductivity is greatly influenced by external conditions, such as changes in temperature and light, which can significantly alter its resistivity. By utilizing these characteristics, it is possible to create a variety of applications Semiconductor devices with different characteristics and functions Due to the vast majority of semiconductors being crystals, semiconductor materials are often referred to as crystals, and the names for crystal diodes and crystal transistors are derived from this. There are many types of crystal diodes, commonly used are ordinary diodes (used for rectification, detection, switching, etc.) and diodes with special performance (such as light-emitting diodes, voltage regulator diodes, photosensitive diodes, etc.). This article first introduces ordinary diodes that have many uses. The recognition of ordinary diodes is an early member of the semiconductor device family. In the field of modern electronic technology, it still plays a very important role. It is crucial for beginners to have a correct understanding of the identification and use of diodes for the smooth completion of various electronic production and maintenance