A short channel semiconductor device is formed with halo regions that are separated from the bottom of the gate electrode and from each other. Embodiments include implanting halo regions after forming source/drain regions and source/drain extension regions. An embodiment includes forming source/drain extension regions in a substrate, forming source/drain regions in the substrate, forming halo regions under the source/drain extension regions, after forming the source drain regions, and forming a gate electrode on the substrate between the source/drain regions. By forming the halo regions after the high temperature processing involved informing the source/drain and source/drain extension regions, halo diffusion is minimized, thereby maintaining sufficient distance between halo regions and reducing short channel NMOS Vt roll-off.