Design and optimization of junction termination extension (JTE) for 4H-SiC high voltage Schottky diodes

摘要

This paper analyzes single- and double-zone junction termination extension (JTE) structures for 4H-SiC Schottky diodes using numerical simulations. In the single-zone case, we study the effects of JTE dose, depth, length, metal/JTE overlap length, and surface or interface charge. In the double-zone case, we systematically vary the inner and outer doses over about 80 possible combinations for each of three sets of inner and outer zone widths. The total JTE width is constrained to be that necessary for optimum breakdown voltage in the single-zone case. The results are presented as contour plots of breakdown voltage and maximum surface field as a function of the two doses, with the locations of peak bulk and surface fields also indicated at each point. The resulting tolerance to variations in activated dose can then be visualized directly. The physics underlying the shapes of the contours is explained in some detail. We show that JTE behavior is significantly different for Schottky diodes compared to the better-known case of p(i)n junction diodes. The peak surface field is increased for Schottky diodes when the single-zone dose is reduced below its optimum value, which is opposite to the behavior of pn junctions. Moreover, double-zone JTE is not effective in reducing peak surface field for the Schottky case, unlike pn junctions, although tolerance to dose variations can be improved with two zones. The usual rule of thumb for double-zone JTE design for pn junctions is not appropriate for Schottky diodes, because of the field crowding near the sharp metal edge. We recommend an inner dose of 95-105% of the ideal single-zone and an outer dose of 70-80% of the single-zone value, with a width ratio of ～1:1 for the inner and outer zones and a total width similar to the optimal value in a single-zone design.