The invention relates to a method of manufacturing a (horizontal) MOST, as used, for example, in (BI)CMOS ICs. On either side of a gate electrode (2), the surface of a silicon substrate (10, 11) which is positioned above a gate oxide (IA) is provided with a dielectric layer (1B) at the location where a source (3) and drain (4) are to be formed, which dielectric layer includes a thermal oxide layer (1B) to be formed as the starting layer. The source (3) and/or drain (4) is/are provided with LDD regions (3A, 4A) and the remaining parts (3B, 4B) of the source (3) and drain (4) are provided by an ion implantation (I1) of doping atoms into the silicon substrate (10, 11). A MOST obtained in this way still suffers from so-called short-channel effects, resulting in a substantial dependence of the threshold voltage upon the length of the gate electrode (2), in particular in the case of very short lengths of the gate electrode (2). In a method according to the invention, the LDD regions (3A, 4A) are made as follows: in a first step, suitable doping atoms (D) are implanted into the dielectric layer (1B), in a second ion implantation (I2), and subsequently in a second step, a part of the doping atoms (D) is diffused from the dielectric layer (1B) into the silicon substrate (10, 11), whereby the LDD regions (3A, 4A) are formed. This method enables a MOST with excellent properties to be obtained, for example with a flatter profile of the threshold voltage versus the gate-electrode (2) length (curve 130) than in conventionally made MOSTs (curve 131). This result is obtained in a simple and reproducible manner.