Method of manufacturing a bipolar transistor having thin base region

A method of manufacturing a semiconductor device comprising the steps of:

• forming a first insulating film on a monocrystalline semiconductor layer of one conductivity type;
• covering said first insulating film with a first polycrystalline semiconductor film of an opposite conductivity type having a first aperture exposing a portion of said first insulating film;
• covering said first polycrystalline semiconductor film with a second insulating film with leaving a part of said portion of said first insulating film to be exposed;
• selectively removing said first insulating film by using said second insulating film and said first polycrystalline semiconductor film as a mask to form a second aperture bigger than said first aperture, said second aperture exposing a part of said monocrystalline semiconductor layer and a part of a bottom surface of said first polycrystalline semiconductor layer;
• and
• growing a semiconductor material to form a monocrystalline semiconductor film of said opposite conductivity type on said part of monocrystalline semiconductor layer and a second polycrystalline semiconductor film of said opposite conductivity type on said part of said first polycrystalline semiconductor film until said monocrystalline semiconductor film is in contact with said second polycrystalline film.