When DC bias occurs, the mutual inhibition of external electric field and self-built electric field increases the diffusion current of carriers, resulting in forward current (that is, the cause of conduction).

When the reverse bias voltage is generated, the external electric field and the self-built electric field are further strengthened, and the reverse saturation current I0 independent of the reverse bias voltage is formed in a certain reverse voltage range (that is, the reason of non-conduction).

When the external reverse bias voltage exists, the external electric field and the self-built electric field are further strengthened, and the reverse saturation current I0 independent of the reverse bias voltage is formed in a certain reverse voltage range.