Avalanche breakdown enhancement by mesa contouring

Summary

In a thyristor there are two junctions that have to support high breakdown voltages. One supports the maximum forward-blocking potential and the other supports the reverse-blocking potential. For an idealized case in which the device shows perfect symmetry and has an almost perfect cathode-emitter short, the two blocking potentials are equal. In real-life devices this is rather an exceptional situation because the avalanche breakdown is limited by surface rather than by bulk breakdown and the two junctions have different edge contours. The reverse-blocking junction is usually beveled with a positive angle, whereas the forward-blocking junction is beveled, for practical reasons, with a negative angle. It is of interest to fully utilize the material’s properties and to obtain full breakdown symmetry, with the bulk breakdown as the final limit.

The results reported recently by a few authors demonstrate by numerical calculations and by experimentation that a negatively beveled diffused junction, with a small bevel angle minimizing the surface field, develops a much higher field underneath and in the vicinity of the surface on the higher-doped side. Therefore, although the surface breakdown will not occur for small-angle negative bevels, silicon will still break down below the ultimately possible bulk breakdown due to the presence of this high field. The positively beveled junction does not exhibit this kind of limitation and presents a more desirable approach for mesa contouring.

It has been shown also that the smaller the negative angle, the smaller the surface field, and there is no optimum angle of 6 degrees as was thought until recently.

The normally existing positive surface charges increase the absolute maximum field underneath the surface and, therefore, lower the breakdown voltage. From the practical (mechanical and economical) point of view, the use of very small negative angles is highly undesirable. Some novel approaches avoiding the shallow-angle pitfalls were suggested recently and are reviewed in the last section of this chapter.