A representative fin field effect transistor (FinFET) includes a substrate having a major surface; a fin structure protruding from the major surface having a lower portion comprising a first semiconductor material having a first lattice constant; an upper portion comprising the first semiconductor material. A bottom portion of the upper portion comprises a dopant with a first peak concentration. A middle portion is disposed between the lower portion and upper portion, where the middle portion comprises a second semiconductor material having a second lattice constant different from the first lattice constant. An isolation structure surrounds the fin structure, where a portion of the isolation structure adjacent to the bottom portion of the upper portion comprises the dopant with a second peak concentration equal to or greater than the first peak concentration.