Regions of the substrate which are to be the collector sinker and the active area of a bipolar transistor are isolated by forming a trench about them and filling it with a dielectric. The dielectric can be oxide formed in a LOCOS process. A dielectric body, which may be nitride, is formed on part of the active area, and base contacts implanted using it as a mask. Polysilicon is deposited over the whole and then cut to form future metallisation-to-base contacts. The dielectric body is removed and the base implanted through the resulting aperture. Oxide spacers are formed on the sidewall of the aperture and polysilicon deposited. The polysilicon is doped and used to produce the emitter by driving the dopant into the substrate between the oxide spacers.