Narrow channel MOS devices and method of manufacturing

High performance MOS transistors are provided by a method that includes providing an oxygen-impermeable mask on a selected region of a semiconductor substrate, oxidising the unmasked portion of the surface to provide a thick oxide layer at least partially recessed in the substrate. This layer includes a smoothly tapered beak that extends between the margin of the mask and the underlying silicon surface, and, after removing the mask, implanting a selected impurity in the substrate beneath the selected region to form a thin impurity layer that terminates at a sloped portion of the substrate formed by the oxidation step. (U.K.)