Semiconductor device

In a semiconductor device of a high blocking voltage comprising:

• a semiconductor substrate having a pair of mutually opposing principal surfaces disposed substantially in parallel with each other and a side surface connecting the principal surfaces and including between said pair of principal surfaces a first layer of one conductivity type, a second layer of the other conductivity type having in a uniform distribution a lower impurity concentration than that of said first layer, disposed adjacent to and forming a first PN junction with said first layer, a third layer of said one conductivity type having a higher impurity concentration than that of said second layer, disposed adjacent to and forming a second PN junction with said second layer, and a fourth layer of said other conductivity type, having a higher impurity concentration than that of said third layer, embedded in and forming a third PN junction with said third layer, and having a surface aligned with a corresponding one of said principal surfaces so as to be exposed, said first and second PN junctions being exposed at said side surface, said side surface being formed into a pulley-shape the valley portion of which is located substantially at the middle of said second layer;
• a cover member of a high dielectric constant filling up the valley portion of the pulley-shape;
• and a pair of main electrodes provided in ohmic contact with said first and fourth layers on said principal surfaces, respectively, in which the greater part of a depletion layer produced when the first or second PN junction is reversely biased exists in said second layer and spreads beyond said valley portion;
• the improvement comprising the fact that the ratio d/w of the depth d of the valley portion to the thickness w of the semiconductor substrate is larger than the value corresponding to the line connecting points A, B, C and D of FIG. 3, and wherein the ratio d/w of the valley depth to the substrate thickness is not larger than unity.