Surface ions contaminate negative charges and repel electrons to attract holes, forming P-type inverted layer channels, thus increasing leakage current; while positive charges attract electrons to repel holes, which is equivalent to reducing surface resistivity, narrowing surface space charge region and forming low breakdown on PN junction surface.

When the surface of PN junction only produces the effect of current, the waveform is still avalanche breakdown, and VB is basically unchanged. The generated current is proportional to the surface area of PN junction. Therefore, inadequate corrosion, rough surface, a large number of composite centers, make the surface leakage current larger.

The first purpose of corrosion (pickling) is to remove all kinds of contamination on the grain table, such as flux, dust particles, metal atoms or ions... The method is to dissolve it with strong oxidizing acid to form soluble substance.

The second is to remove the damage caused by cutting and splitting on the grain surface (the damage layer may be 50-100 microns thick), and to form a smooth surface. Flat and smooth surfaces help to reduce leakage current and prevent the formation of local strong electric fields. Local strong electric field easily causes leakage current to rise with voltage before breakdown, forming rounded corners or backrest chairs (channel breakdown).

Thirdly, the edges and corners of polygons (square, rectangle, hexagon) are passivated or even rounded, because the chemical reaction at the edges and corners is faster. The passivation edge can reduce the angular electric field of the p-n junction in reverse operation and increase the breakdown voltage of the diode. For circular grains, there is no such requirement.

Determined resistivity, diffusion concentration and junction depth are equivalent to an ideal breakdown voltage BVpp (assumed to be a planar junction). In order to improve the voltage withstand of devices, it is necessary to make the surface of p-n junction reach or approach the breakdown voltage of p-n junction. Two problems need to be solved: first, the contamination on the surface of p-n junction and the structural defects of single crystal must be eliminated, and the surface of p-n junction must be effectively insulated; second, the surface of p-n junction must have a reasonable shape to make p-n junction table. The surface electric field is not higher or close to the internal electric field intensity.

Because of the principle of electrical neutrality, the space charge region must bend at the surface of p-n junction for bevel modeling of mesa. Positive oblique angle (for ordinary P+N type diodes, the area of N plane is small, the area of P plane is large, and the negative oblique angle is opposite) is advantageous to the broadening of depletion zone, while the negative oblique angle can achieve satisfactory results only at a very small angle (3 6). The maximum field intensity of negative oblique angle is on the side of high concentration, while the maximum field intensity of positive oblique angle is on the side of low concentration.