Bipolar transistor with particular base structure

A bipolar transistor comprising:

• a collector layer of a first conductivity type surrounded by an oxide film for separating elements;
• a first base layer comprising a non-doped epitaxial layer formed on said collector layer;
• a second base layer comprising an epitaxial layer of a second conductivity type formed on said first base layer;
• a polysilicon film of the second conductivity type formed at a first area of a surface of said second base layer;
• an emitter layer of the first conductivity type formed at a second area of a surface of said second base layer, said polysilicon film having an upper surface and a lateral surface;
• a base polysilicon electrode of the second conductivity type formed on the upper surface and lateral surface of said polysilicon film and on said oxide film for separating elements;
• a sidewall comprising an insulating film formed over a lateral wall of said base polysilicon electrode and a lateral wall of said polysilicon film;
• and
• an emitter polysilicon electrode of the first conductivity type formed over said emitter layer and said side wall, wherein
• an energy band gap of said first base layer is made narrower than that of said collector layer, an energy band gap of said second base layer being aligned with that of said first base layer at a portion where it contacts said first base layer and being gradually widened towards the emitter layer away from said first base layer.