The main principle of diode is to make use of the unilateral conductivity of PN junction, and add leads and packaging on PN junction to form a diode. The crystal diode is a PN junction formed by a P-type semiconductor and an N-type semiconductor, and a space charge layer is formed on both sides of its interface, and a self-built electric field is built. When there is no applied voltage, the diffusion current caused by the carrier concentration difference between the two sides of the PN junction is equal to the drift current caused by the self-built electric field, and it is in an electric equilibrium state. When there is direct voltage bias in the outside world, the mutual suppression of the external electric field and the self-built electric field increases the diffusion current of carriers and causes the forward current. When there is an external reverse voltage bias, the external electric field and the self-built electric field are further strengthened, forming a reverse saturation current independent of the reverse bias voltage value in a certain reverse voltage range. When the applied reverse voltage is high to a certain extent, the electric field intensity in the space charge layer of PN junction reaches the critical value, resulting in the multiplication of carriers, a large number of electron-hole pairs, and a large reverse breakdown current, which is called diode breakdown phenomenon. The reverse breakdown of PN junction can be divided into Zener breakdown and avalanche breakdown.