A method for fabricating microminiature, planar semiconductor devices in which the number of defects, in particular, pipes, is minimized. The thicknesses of the thermally grown silicon dioxide and of the silicon nitride masking layers which are used for the formation of limited impurity regions by high temperature diffusion processes within the semiconductor substrate have a specified, limited range. The thickness of the silicon dioxide is between 800A - 3000A and the thickness of the silicon nitride is between around 250A and 600A, preferably 500A. The method is particularly useful in forming extremely small emitter regions in bipolar transistors.