At the contact boundary between P-type semiconductor and N-type semiconductor, because of the free electron diffusion movement and the drift motion caused by the internal electric field (N points to P), a very thin charge region occurs in the middle of the PN junction (the interface between the P and N regions), which is the space charge region (that is, the PN junction). In this region, most of the carriers have spread to each other and have been recombined, or consumed, so the space charge region is also called the depletion layer.

The width of the space charge region determines the capacitance effect of the PN junction, and the larger the space charge region width is, the larger the capacitance is, which determines the frequency use of the tube.

If the reverse voltage accepted by the PN junction exceeds its critical value, resulting in the electric field strength exceeding the critical electric field strength, then bump ionization will be triggered, resulting in avalanche effect or carrier multiplication effect, that is, the number of carriers in the space charge region will be rapidly increased, that is, the original small reverse current will be rapidly added, resulting in the destruction of the semiconductor. This type of breakdown is called Type I breakdown.

The maximum reverse voltage that the diode can accept depends on the internal structure and doping parameters. In practice, in order to improve the reverse voltage resistance of the diode, a layer of low-doping N region is added between the P region and the N region, that is, the drift region, and the low-doping N region is close to the undoped pure semiconductor material (intrinsic semiconductor) because of the low doping concentration, which is called the P-I-N structure. Because of the low doping concentration, the low-doping N-region can accept a higher voltage without being broken down, and the thicker the low-doping N-region, the higher the reverse voltage the diode can accept.