The crystal diode is a p-n junction formed by P-type semiconductor and N-type semiconductor, which forms a space charge layer on both sides of its interface and has a self-built electric field. When there is no applied voltage, the diffusion current caused by the difference of carrier concentration on both sides of the p-n junction and the drift current caused by the self-built electric field are equal and in electric equilibrium. When there is a forward voltage bias, the mutual suppression of the external electric field and the self-built electric field increases the carrier diffusion current and causes the forward current. When there is reverse voltage bias outside, the external electric field and the self-built electric field are further strengthened, forming a reverse saturation current I0 which is independent of the reverse bias voltage in a certain reverse voltage range.