A method for producing buried oxide layers in selected portions of a semiconductor substrate including the steps of applying a patterned mask made from a high-density material over a semiconductor substrate and selectively forming buried oxide layers by oxygen ion implantation. The high-density material of the mask is preferably tungsten, but can also be made from other suitable materials such as silicon nitride. A MOS transistor is made by the process of the present invention by applying the high-density mask material over the gate of the transistor, and forming buried oxide layers by ion implantation beneath only the source region and drain region of the transistor. The completed MOS transistor has the characteristics of reduced drain and source capacitance, reduced leakage, and faster response, but does not suffer from the floating-body effect of MOS transistors made by SOI processes.