Method of fabricating semiconductor device by forming doped regions that limit width of the base

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

• forming a first semiconductor region of a first conduction type, the first semiconductor region forming a transistor collector;
• forming a non-crystalline conductor film on the first semiconductor region;
• forming an opening through the conductor film;
• forming a second semiconductor region on the first semiconductor region, the second semiconductor region being of a second conduction type and forming a transistor base, the second semiconductor region being electrically connected to the conductor film;
• forming a third semiconductor region in an emitter-forming area within the second semiconductor region, the third semiconductor region being of the first type and forming a transistor emitter;
• and
• implanting ions into the first and second semiconductor regions via the opening along a direction inclined with respect to a surface of the second semiconductor region and thereby forming a fourth semiconductor region of the first conduction type, the fourth semiconductor region having a main portion and an upper portion, wherein the main portion extends in a part of the first semiconductor region below an edge of the emitter-forming area within the second semiconductor region, the upper portion extends from the main portion into a part of the second semiconductor region , the upper portion not reaching the upper surface of the second semiconductor region, and the surface of the upper portion is shaped so as to conform to the edge of the emitter-forming area to limit a width of the transistor base.