An embodiment method includes forming a semiconductor liner layer on a first fin structure and on a second fin structure and forming a first capping layer on the semiconductor liner layer disposed on the first fin structure. The method further includes forming a second capping layer on the semiconductor liner layer disposed on the first fin structure, where a composition of the first capping layer is different from a composition of the second capping layer. The method additionally includes performing a thermal process on the first capping layer, the second capping layer, and the semiconductor liner layer to form a first channel region in the first fin structure and a second channel region in the second fin structure. A concentration profile of a material of the first channel region is different from a concentration profile of a material of the second channel region.