Semiconductor device having spaced unit regions and heavily doped semiconductor layer

A semiconductor device, comprising:

• a semiconductor substrate;
• a semiconductor region on a surface of said semiconductor substrate;
• a first semiconductor layer region at a surface of said semiconductor region and coupled to a first electrode;
• a second semiconductor layer region being of a ring-shaped form and located in said semiconductor region, the second semiconductor layer region being spaced from said first semiconductor layer region and surrounding said first semiconductor layer region, the second semiconductor layer region being different in conductivity type from said semiconductor region;
• a third semiconductor layer region having a plurality of unit regions located in said second semiconductor layer region, the plurality of unit regions being spaced from each other at a predetermined interval and being different in conductivity type from said second semiconductor layer region, each of the unit regions having a rectangle-like form with a width larger than the predetermined interval, the third semiconductor layer region being different in conductivity type from said second semiconductor layer region;
• a heavily doped semiconductor layer located in said second semiconductor layer region, at least below said third semiconductor layer region, the heavily doped semiconductor layer being doped more heavily than said second semiconductor layer region and being the same in conductivity type as said second semiconductor layer region;
• a gate electrode layer above said semiconductor region and a part of said second semiconductor layer region and configured to form a channel, at a surface of the part of said second semiconductor layer region, extending so as to reach said semiconductor region from the third semiconductor layer region, for transferring charges between the semiconductor region and the third semiconductor layer region through said channel;
• and
• a base layer located in said second semiconductor layer region, the base layer having been doped less heavily than said heavily doped semiconductor layer and being the same conductivity type as said heavily doped semiconductor layer, the heavily doped semiconductor layer extending farther toward the surface of said semiconductor substrate than does said base layer extend toward the surface of said semiconductor substrate.