FIG. 10 illustrates a cross-sectional view of another embodiment of the silicon germanium layer 53 in the fin 58 of the FinFET device 100 of FIG. 5. In FIG. 10, the silicon germanium layer 53 has the segment A, which is the buffer layer with a thickness between about 0 nm and 5 nm and a germanium concentration between about 0 at % and about 5 at %. In addition, the silicon germanium layer 53 has a segment E with a uniform (e.g., uniform with manufacturing limitations) germanium concentration that is between about 20 at % and about 30 at %, and a segment F with a uniform (e.g., uniform with manufacturing limitations) germanium concentration that is between about 15 at % and about 25 at %. FIG. 10 further illustrates an interface 121 between the segment A and the segment E, and an interface 123 between the segment E and the segment F. In the illustrated example of FIG. 10, the segment E and the segment F have a same thickness, which is between about 25 nm and about 30 nm.

The example illustrated in FIGS. 6-10 are non-limiting examples. The silicon germanium layer 53 may have other numbers of segments or sublayers with other germanium concentrations, which may be determined based on the concentration of the diffused dopant from the source/drain regions to the channel region. These and other variations are fully intended to be included within the scope of the present disclosure.