Semiconductor device including element isolation insulating film having thermal oxide film

A semiconductor device comprising:

• a semiconductor substrate having a main surface;
• a base region having a first conductivity type and disposed in a surface layer of the semiconductor substrate that is close to the main surface;
• an emitter region having a second conductivity type reverse to the first conductivity type and disposed in a surface layer of the base region;
• a collector region having the second conductivity type and disposed at a portion in the surface layer of the semiconductor substrate that is apart from the emitter region;
• and
• an element isolation insulating film disposed on the main surface, and having a thermal oxide film being in contact with a junction interface between the base region and the emitter region, wherein
• the semiconductor substrate includes an active layer of the second conductivity type that provides the surface layer of the semiconductor substrate,
• the base region includes a first region and a second region, the first region encloses the emitter region, and the second region extends from the first region toward the collector region and has an impurity concentration higher than an impurity concentration of the first region,
• a bottom surface of the second region is located at a depth shallower than a bottom surface of the first region with reference to the main surface, and
• the bottom surface of the second region is in contact with the active layer.