Semiconductor device having trench-gate transistor with parallel channel regions in gate trench

A semiconductor device includes, on a semiconductor substrate, an active region surrounded by an STI region, a gate trench formed in one direction transverse to the active region, a gate insulating film formed on a side surface of the gate trench, an insulating film formed on a bottom of the gate trench and thicker than the gate insulating film, and a gate electrode having at least a part of the gate electrode formed in the gate trench. Portions of the semiconductor substrate present in the active region and located on both sides of the gate trench in an extension direction of the gate trench function as a source region and a drain region, respectively. A portion of the semiconductor substrate located between the side surface of the active region (the side of the STI region) and the side surface of the gate trench functions as a channel region.