Silicon-carbide semiconductor device and method for manufacturing the same

A silicon carbide semiconductor device, comprising:

• a drift region of a first conductivity type that is formed on a first main surface of a silicon carbide semiconductor substrate having an off-angle and is made of silicon carbide;
• a well region of a second conductivity type that is formed on a surface of said drift region and is made of silicon carbide;
• a source region of the first conductivity type that is selectively formed in a surface layer portion of said well region and is made of silicon carbide;
• a trench that penetrates said well region from a surface of said source region and reaches said drift region;
• a gate electrode that is formed inside said trench through a gate insulating film;
• a source electrode that is connected to said well region and said source region;
• a drain electrode that is formed on a second main surface while being in contact with said silicon carbide semiconductor substrate, the second main surface being a surface opposite to the first main surface of said silicon carbide semiconductor substrate;
• a high-concentration well region of the second conductivity type that is formed in said well region and has an impurity concentration higher than that of said well region, wherein a distance from a first sidewall surface of said trench to said high-concentration well region is shorter than a distance from a second sidewall surface of said trench to said high-concentration well region, the second sidewall surface facing the first sidewall surface of said trench through said gate electrode in said trench,
• wherein a threshold voltage of a field-effect transistor formed in said first sidewall surface is lower than a threshold voltage of a field-effect transistor formed in said second sidewall surface when the distance from said first sidewall surface to said high-concentration well region is the same as the distance from said second sidewall surface to said high-concentration well region.

A method for manufacturing a silicon carbide semiconductor device, comprising:

• forming a drift region of a first conductivity type made of silicon carbide on a first main surface of a silicon carbide semiconductor substrate having an off-angle;
• forming a well region of a second conductivity type made of silicon carbide on a surface of said drift region;
• forming a source region of the first conductivity type made of silicon carbide selectively in a surface layer portion of said well region;
• forming a trench that penetrates said well region from a surface of said source region and reaches said drift region;
• forming a gate electrode inside said trench through a gate insulating film;
• forming a source electrode in contact with said well region and said source region;
• forming a drain electrode on a second main surface being a surface opposite to the first main surface of said silicon carbide semiconductor substrate;
• and
• forming, in said well region, a high-concentration well region of the second conductivity type having a second conductivity-type impurity concentration higher than that of said well region such that a distance from a first sidewall surface of said trench is shorter than a distance from a second sidewall surface of said trench, the second sidewall surface facing the first sidewall surface of said trench through said gate electrode in said trench,
• wherein the high-concentration well region is formed with reference to a mark provided in said forming a trench,
• said first main surface has an off-angle tilted from a (0001) plane in an [11-20] axis direction,
• said first sidewall surface is a surface close to a (11-20) plane, and
• said second sidewall surface is a surface close to a (−1-120) plane.