Field distribution near the surface of beveled P-N junctions in high voltage devices

Experience with beveled p-n junctions does not fully agree with the predictions of Davies and Gentry. Specifically no optimum negative angle as claimed by Huth and Davies was found. A new method for determining the form of the space charge region near the surface shall be presented which avoids the insufficiencies of the model cited. Apart from more accurate results, this paper gives insight into an entirely new phenomenon. In contrast to conventional theory p-n junctions with small negative bevel angles exhibit an absolute electric field maximum underneath the surface. This effect causes a reduction of the breakdown voltage even when the electric field on the surface is sufficiently small. Results will be presented which show the dependency of the absolute field maximum on bevel angle, doping profile and material resistivity. It will be shown that a limit for the use of negative bevel angles exists at about 4 kV breakdown voltage. These theoretical predictions are in excellent agreement with potential probing experiments. Results will also be presented on the influence of dielectrics or charges on the surface.