The inventive method provides improved semiconductor devices, such as MOSFET's with raised source/drain extensions on a substrate with isolation trenches etched into the surface of the substrate. The inventive method provides thin first dielectric spacers on the side of a gate and gate oxide and extend from the top of the gate to the surface of the substrate. Raised source/drain extensions are placed on the surface of a substrate, which extend from the first dielectric spacers to the isolation trenches. Thicker second dielectric spacers are placed adjacent to the first dielectric spacers and extend from the top of the first dielectric spacers to the raised source/drain extensions. Raised source/drain regions are placed on the raised source/drain extensions, and extend from the isolation trenches to the second dielectric spacers. The inventive semiconductor devices provide for very shallow source drain extensions which results in a reduced short channel effect.