In making bipolar transistors, an interfacial oxide layer (5) is formed over ther monocrystalline region (1), and polysilicon (6) is formed both thereon as an extrinsic emitter region. After doping the polysilicon a monocrystalline emitter region (4) is produced in the base region by diffusion from the extrinsic polysilicon emitter region. The oxide layer (5) acts as a diffusion barrier to ensure that excessive dopant does not reach the monocrystalline region.After the above operation, a thermal treatment is effected at a higher temperature, e.g. 1100.degree. C., for a few seconds, which breaks down the interfacial oxide layer referred to above. This temporary use of the interfacial oxide layer leads to better and more consistant transistor characteristics.