Method of fabricating a semiconductor device of high-voltage CMOS structure

A method of fabaricating a semiconductor device having an SOI substrate composed of a semiconductor substrate of a first conductivity type on an element forming surface to form a first transistor of a MOS structure and a second transistor of said MOS structure disposed on said SOI substrate, said method comprising the steps of:

• (a) diffusing an impurity of a second conductivity type opposite to said first conductivity type to a gate and a source forming layer of said first transistor of the MOS structure and a drain forming layer of said second transistor of the MOS structure, said diffusion taking place as far as a bottom surface of said semiconductor substrate, said diffusing leaving a drain forming layer of said first transistor which is not diffused and has a depth approximately equal to the depth of said source forming layer;
• (b) diffusing an impurity of said first conductivity type to said drain forming layer and said source forming layer of said first transistor of the MOS structure and a source forming layer of said second transistor of the MOS structure, such that said drain forming layer of said first transistor is approximately equal in depth to the drain forming layer of the second transistor;
• and
• thereafter forming gate oxide films and gate electrodes of said first transistor of the MOS structure and said second transistor MOS structure respectively on an upper surface of said semiconductor substrate.