Semiconductor device having epitaxial structures

A semiconductor device having epitaxial structures comprising:

• a gate structure positioned on a substrate;
• epitaxial structures formed in the substrate at two sides of the gate structure, the epitaxial structure comprising a dopant, a first semiconductor material having a first lattice constant, and a second semiconductor material having a second lattice constant, the second lattice constant being larger than the first lattice constant, and the second semiconductor material in the epitaxial structure having a first concentration;
• a first undoped cap layer formed on the epitaxial structures, the first undoped cap layer comprising the first semiconductor material and the second semiconductor material, the second semiconductor material in the first undoped cap layer having a second concentration, wherein the second concentration is lower than the first concentration and upwardly decreased;
• and
• a second undoped cap layer formed on the first undoped cap layer, the second undoped cap layer comprising the first semiconductor material and the second semiconductor material, the second semiconductor material in the second undoped layer having a third concentration, and the third concentration is equal to 0%.