METHOD OF MANUFACTURING A SEMICONDUCTOR DEVICE AND SEMICONDUCTOR DEVICE MANUFACTURED BY THE METHOD

12 A method of manufacturing a semiconductor device, in which semiconductor circuit elements are provided in regions (6A,7A,11A) formed by diffusion from one or more buried layers (6,7,11) in an epitaxial layer (13). According to the invention, the diffusion is carried out in a manner such that a surface layer (13A) having substantially the same doping as the original epitaxial layer (13) is left above the buried layer or layers, which surface layer serves as a reference doping for the insulated gate field effect transistors to be formed. This is of particular important for threshold voltage determination in CMOS circuits having adjoining "twin tub" regions (6A,11A) diffused from buried layers of opposite conductivity types.