In order to produce an electrical connection to an inner layer such as a bottom diffusion (103), which has a good electrical conductivity and is located inside a bipolar semiconductor device isolated by trenches (119) and which for example forms a subcollector of a NPN-transistor, a hole (157) in a trench is used. The hole is filled with electrically conducting material and extends from the surface of the device to the bottom diffusion (103), so that the electrically conducting material in the hole is in contact therewith. The hole (157) is made aligned with a sidewall of the trench (119) by using selective etching. The hole can be made at the same time as contact holes for metallization are made and then also be filled in the metallization step, to contact the bottom diffusion. For a lateral PNP-transistor the hole can be made as a closed groove constituting the outer confinement of the base area, passing all around the transistor. The outer sidewall of such a closed trench can, as seen from above, be bevelled by 45°, so that no inner corners having too small angles are found in the trench, what facilitates the filling with oxide.
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