Abstract: A method of manufacturing a semiconductor device, in which a silicon slice (1) is locally provided with field oxide (10, 30) with a subjacent channel stopper zone (12, 32), which are formed during the same oxidizing heat treatment. The formed field oxide layer (10, 30) is removed in part by an etching treatment with a thinner and smaller field oxide layer (11, 31) being formed. The temperature at which the heat treatment is carried out is chosen so that lateral diffusion (15, 35) of the dopant forming the channel stopper zone (12, 32) extends in lateral direction practically over the same distance as the reduced field oxide layer (11, 31). Thus, it is achieved that, for example, for the manufacture of a MOS transistor an oxidation mask (7) having substantially the same width as a channel zone (17) to be formed can be used.