PURPOSE: To reduce a base width, by forming base and emitter regions by impurity diffusion and using temperature and time to control the diffusion.

CONSTITUTION: Boron ions are implanted into undoped polycrystalline silicon 4. When annealing is performed afterwards, boron is diffused from boron doped polycrystalline silicon 5 into an n-type silicon single-crystal layer 1, and one part of the n-type silicon layer 1 is converted into a p-type so that a p-type silicon region 6 is formed and a remaining n-type region becomes a collector region 9. When phosphorus ions are implanted into the boron-doped polycrystalline silicon 5 and next annealing is performed, one part of a p-type silicon region 6 is converted into an n-type silicon region 7. This n-type region 7 serves as an emitter region, and the remaining p-type region serves as a base region 8. At that time, temperature and time are used to control heat diffusion of boron and phosphorus from the polycrystalline silicon. Hence, a base width can be reduced.