In some embodiments, the concentration of germanium in the silicon germanium layer 53 is formed to be non-uniform to compensate for the effect of dopant diffusion from doped source/drain regions into the channel region of the FinFET device 100. For example, in subsequent processing, the source/drain regions 82 (see FIG. 17B) doped with a dopant (e.g., boron) are formed, and the dopant of the source/drain regions may diffuse into the channel region of the FinFET device 100. Since the concentration of the diffused dopant (e.g., boron) in the channel region may be non-uniform, diffusion of the dopant into the channel region, if left uncompensated for, may cause a non-uniform threshold voltage for the FinFET device 100. In other words, different areas of the channel region (e.g., different areas disposed along the vertical direction from the top of the fin 58 toward the substrate 50) of the FinFET device 100 may have different threshold voltages, thus may be turned on (e.g., forming an electrically conducting path) at different voltages. A non-uniform threshold voltage may make it difficult to control the ON and OFF state of the FinFET device 100 effectively, and may degrade the performance of the FinFET device.
