In the method for fabricating a semiconductor device of the present invention, a collector layer (102) of a first conductivity type is formed in a region of a semiconductor substrate (100) sandwiched by device isolation. A collector opening (110) is formed through a first insulating layer (108) deposited on the semiconductor substrate (100) so that the range of the collector opening (110) covers the collector layer (102) and part of the device isolation. A semiconductor layer of a second conductivity type (111) as an external base is formed on a portion of the semiconductor substrate located inside the collector opening (110), while junction leak prevention layers of the same conductivity type as the external base are formed in the semiconductor substrate. Thus, the active region is narrower than the collector opening reducing the transistor area, while minimizing junction leak.