In the method of producing bipolar transistor structures in a semiconductor process, an advanced epitaxial trisilane process can be used without the risk of poly stringers being formed. A base window is structured in a polycrystalline silicon layer covered with an oxide layer, and a further step is epitaxial growing of a silicon layer in the base window from trisilane. The window structuring is performed in a sequence of anisotropic etch and isotropic ash steps, thereby creating stepped and inwardly sloping window edges. Due to the inwardly sloping side walls of the window, the epitaxially grown silicon layer is formed without inwardly overhanging structures, and the cause of poly stringers forming is thus eliminated.