Semiconductor device and method of manufacturing the same

Abstract

A semiconductor device includes a first nitride semiconductor layer formed above a substrate, a second nitride semiconductor layer formed over the first nitride semiconductor layer and having a band gap larger than that of the first nitride semiconductor layer, a trench passing through the second nitride semiconductor layer and into the first nitride semiconductor layer, a gate insulation film formed in the trench, and a gate electrode disposed by way of the gate insulation film in an inside of the trench. The corner of the trench between a side wall of the trench and a bottom of the trench includes a rounded shape, and a corner of the gate insulation film in contact with the corner of the trench includes a rounded shape.

What is claimed is: 
     
         1 . A semiconductor device comprising:
 a first nitride semiconductor layer formed above a substrate;   a second nitride semiconductor layer formed over the first nitride semiconductor layer and having a band gap larger than that of the first nitride semiconductor layer;   a trench passing through the second nitride semiconductor layer and into the first nitride semiconductor layer;   a gate insulation film formed in the trench; and   a gate electrode disposed by way of the gate insulation film in an inside of the trench,   wherein a corner of the trench between a side wall of the trench and a bottom of the trench comprises a rounded shape, and a corner of the gate insulation film in contact with the corner of the trench comprises a rounded shape.   
     
     
         2 . The semiconductor device according to  claim 1 , wherein the side wall of the trench is substantially perpendicular to the bottom of the trench. 
     
     
         3 . The semiconductor device according to  claim 2 , wherein the rounded shape comprises a curved portion extending from the side wall of the trench to the bottom of the trench. 
     
     
         4 . The semiconductor device according to  claim 1 , wherein the side wall of the trench comprises a tapered side wall, and the corner of the trench between the tapered side wall and the bottom of the trench comprises a rounded shape. 
     
     
         5 . The semiconductor device according to  claim 4 , wherein the rounded shape comprises a curved portion extending from the tapered side wall of the trench to the bottom of the trench. 
     
     
         6 . The semiconductor device according to  claim 1 , wherein the trench comprises a concave portion having an arcuate cross section. 
     
     
         7 . The semiconductor device according to  claim 1 , wherein the trench is formed midway into the first nitride semiconductor layer or less. 
     
     
         8 . The semiconductor device according to  claim 1 , wherein a thickness of the gate insulation film situated at the corner of the trench is √2×Th/2 or greater and less than √2×Th, where Th is a thickness of the gate insulation film on the side wall of the trench. 
     
     
         9 . The semiconductor device according to  claim 1 , wherein a thickness of the gate insulation film situated at the corner of the trench is Th or greater and √2×Th×0.8 or less, where Th is a thickness of the gate insulation film on the side wall of the trench. 
     
     
         10 . The semiconductor device according to  claim 1 , further comprising:
 a first electrode and a second electrode formed respectively on both sides of the gate electrode over the second nitride semiconductor layer.   
     
     
         11 . The semiconductor device according to  claim 1 , wherein a bottom of the gate electrode above the trench is situated at a position higher than a position for a surface of the second nitride semiconductor layer. 
     
     
         12 . A method of manufacturing a semiconductor device comprising:
 forming a first nitride semiconductor layer, and forming a second nitride semiconductor layer having a band gap larger than that of the first nitride semiconductor layer over the first nitride semiconductor layer, thereby forming a stack;   etching the stack thereby forming a trench;   forming a gate insulation film in the inside of the trench; and   forming a gate electrode over the gate insulation film,   wherein the etching of the stack comprises forming the trench passing through the second nitride semiconductor layer and into the first nitride semiconductor layer, such that a corner of the trench between a side wall of the trench and a bottom of the trench comprises a rounded shape, and the forming of the gate insulation film comprises forming a corner of the gate insulation film in contact with the corner of the trench to have a rounded shape.   
     
     
         13 . The method of manufacturing the semiconductor device according to  claim 12 , wherein the forming of the gate insulation film comprises forming the gate insulation film by a chemical vapor deposition method. 
     
     
         14 . The method of manufacturing the semiconductor device according to  claim 12 , wherein a thickness of the gate insulation film situated at the corner of the trench is √2×Th/2 or greater and less than √2×Th, where Th is a thickness of the gate insulation film on the side wall of the trench. 
     
     
         15 . The method of manufacturing the semiconductor device according to  claim 12 , wherein a thickness of the gate insulation film situated at the corner of the trench is Th or greater and √2×Th×0.8 or less, where Th is a thickness of the gate insulation film on the side wall of the trench.