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.

If avalanche breakdown occurs in a very small part of the PN junction, the rapidly increasing current will be concentrated in this area, and it will be burned down due to the concentration of heat. This destructive breakdown is called thermal breakdown, which is irreversible. This is mostly due to the uneven surface or residual mechanical damage of PN junctions.