MOS transistor and fabrication method

A method of forming a MOS transistor, comprising:

• providing a semiconductor substrate;
• forming a gate structure on the semiconductor substrate, the gate structure including a gate dielectric layer and a gate on the gate dielectric layer;
• forming a lightly doped region by a light ion implantation in the semiconductor substrate on both sides of the gate structure;
• forming a first halo region by a first halo implantation to substantially cover the formed lightly doped region in the semiconductor substrate;
• forming a groove in the semiconductor substrate on the both sides of the gate structure;
• prior to forming a source and a drain in the groove, forming a second halo region in the semiconductor substrate by a second halo implantation performed into a groove sidewall that is adjacent to the gate structure, wherein the second halo region covers:
• substantially the lightly doped region and at least a portion of the first halo region, in the semiconductor substrate under the gate, and substantially covers the same groove sidewall that is adjacent to the gate structure;
• and
• forming the source and the drain in the groove.