Bipolar transistor having thin intrinsic base with low base resistance and method for fabricating the same

A method for fabricating a semiconductor device, said method comprising the steps of:

• providing a first semiconductor region of a first conductivity type;
• providing a first insulting film surrounding said first semiconductor region;
• providing a second insulating film on said first semiconductor region and said first insulating film;
• providing a polycrystalline semiconductor layer over at least said second insulating film such that it extends over said first semiconductor region;
• forming, on said polycrystalline semiconductor layer and said second insulating film, a third insulating film of an insulating material having etching characteristics different from those of said second insulating film;
• etching a hole in said third insulating film and said polycrystalline semiconductor layer to expose a portion of a surface of said second insulating film;
• depositing an insulating material having etching characteristics different from those of said second insulating film over said polycrystalline semiconductor layer and on said hole to increase the thickness of said third insulating film;
• removing a part of said third insulating film in the vicinity of said hole to expose a portion of the surface of said second insulating film;
• etching said second insulating film through the hole formed by the removed portion of said third insulating film to expose a portion of a surface of said first semiconductor region and to undercut a portion of said third insulating film under said third insulating film and said polycrystalline semiconductor layer such that there is formed an opening and a cantilever structure consisting of said third insulating film and said polycrystalline semiconductor layer extending partly over said opening;
• forming a second semiconductor region by growing a semiconductor layer containing impurities of said second conductivity type in a high concentration within said opening;
• etching said second semiconductor region with said third insulating film used as a mask to expose a part of the surface of said first semiconductor region in said opening;
• forming an intrinsic base film of the second conductivity type having an impurity concentration lower than that of said second semiconductor region on the surface of said first semiconductor region and on the inner periphery of said second semiconductor region to a thickness not exceeding that of said semiconductor region;
• forming a sidewall insulating film over inner periphery portions of side walls of said intrinsic base film;
• and
• forming an emitter region of the first conductivity type on said intrinsic base film and surrounded by said sidewall insulating film.