Depletion layer characteristics at the surface of beveled high-voltage P-N junctions

The design of high-voltage p-n-junction devices used today is usually based on beveling of the p-n junctions in order to reduce the tangential surface field far enough below the bulk field and to secure thus that breakdown occurs in the bulk of the device rather than at the surface. Using relaxation methods, solutions of Poisson's equation in two dimensions have been found which reveal that for commonly used bevelings of the forward-blocking junction in a thyristor structure, the electric field in the region below the surface is larger than in the bulk and at the surface. This field is strongly dependent on surface charges. It is shown that bevelings avoid the surface breakdown but the body breakdown obtained is not the true bulk breakdown as expected but rather occurs in the immediate neighborhood to the surface. The calculations make it possible to obtain the breakdown voltage for beveled structures. Measurements of the tangential surface field obtained by probing the junctions, light-spot measurements of the space-charge-region width on the surface and capacitance measurements of the surface charge as well as breakdown measurements are in good agreement with the calculations. This approach has led to the design of high-voltage thyristor structures that exhibit true bulk breakdown and, at the same time, loss of semiconducting material due to beveling is reduced.