Method of manufacturing a semiconductor integrated circuit device comprising an MOS transistor and a bipolar transistor

A method of manufacturing a semiconductor integrated circuit device which comprises a bipolar transistor and a field effect transistor, the method comprising the steps of:

• forming, in a semiconductor substrate of a first conductivity type, first semiconductor regions of a second conductivity type;
• forming a semiconductor layer of said first conductivity type on said semiconductor substrate;
• forming, on said semiconductor layer, second semiconductor regions of said second conductivity type contacting said first semiconductor regions;
• forming, on said semiconductor layer, an insulation film having a contact hole formed therein leading to a portion of a predetermined one of said second semiconductor regions;
• forming an impurity diffusion source layer of said second conductivity type on both said insulation film and said portion of said last-mentioned second semiconductor region via said contact hole;
• doping said last-mentioned second semiconductor region with an impurity from said impurity diffusion source layer via said contact hole formed in said insulation film, to form, in said last-mentioned second semiconductor region, a third semiconductor region of said second conductivity type constituting a collector leading region, having a high impurity concentration and contacting the first semiconductor region contacting said last-mentioned second semiconductor region;
• and
• patterning said impurity diffusion source layer to form a gate electrode of a field effect transistor and a collector electrode of a bipolar transistor with said bipolar transistor being formed in said last-mentioned second semiconductor region and said field effect transistor being formed in said semiconductor layer where said second semiconductor region is not formed or in another predetermined one of said second semiconductor regions.