A device includes a wafer substrate having at least two isolation features, a buffer layer embedded between the two isolation features and a fin disposed over the buffer layer. The buffer layer includes a first lattice constant. The fin includes at least one pair of alternating layers having a compressive strained layer and a tensile strained layer such that the pair of alternating layer has a second lattice constant matching to the first lattice constant and remains strained at edge of the fin. The device further includes a gate disposed over the fin. The buffer layer, the compressive strained layer, and the tensile strained layer include element in Group III-V, or combination thereof. A thickness of the compressive strained layer or a thickness of the tensile strained layer is a function of the first lattice constant.