PURPOSE: To obtain a semiconductor device having excellent high frequency characteristics at a high speed by covering a region out of a region to be formed with a base layer with a conductive film, then heat treating it, and implanting to diffuse a first conductivity type impurity in the region to be formed with the base layer with the conductive film as an impurity shut-off film.

CONSTITUTION: Phosphorus is predeposited in polysilicon films 23, 25, and heat treated to automatically dope a single crystalline silicon layer 5 with the phosphorus. In this case, a region 36 is not doped with the phosphorus with a silicon oxide film 21 as a barrier. Then, a width of an opening 34 becomes narrower from a width d1 to a width d2 by heat treating. Boron is ion implanted in the entire substrate, and annealed to form a base layer 11, an emitter layer 13 and a collector layer 15 in a self-alignment manner. Accordingly, the base layer having a small width can be formed, and the base layer and a conductor film can be insulated therebetween.