Control of electric field at the surface of P-N junctions

Both degradation resulting from ion migration and surface breakdown of junctions depend strongly on electric field. These problems can be minimized or avoided by designing junction devices so that the electric field at the surface is substantially lower than that within the body of the device. The shape, surface doping, and dielectric constant at the junction-surface interface have an appreciable influence on the electric field of the adjoining space charge layer. Using relaxation methods, solutions of Poisson\´s equation in two dimensions have been found which permit predictions of the surface fields as a function of surface geometry and dielectric coatings. Measurements of the surface field by probing the junctions are in good agreement with calculated values. This approach has led to the design of very high voltage junction devices which exhibit body breakdown only. With the breakdown confined to the body of the device, the reverse power dissipation capability is not only predictable but significantly greater (typically 30 a at 1200 v for a 100 µsec pulse applied to a large area device) than that achievable with surface limited devices.