The disclosure relates to a fin field effect transistor (FinFET). An exemplary FinFET comprises a substrate comprising a major surface; a fin structure protruding from the major surface comprising a lower portion comprising a first semiconductor material having a first lattice constant; an upper portion comprising the first semiconductor material, wherein a bottom portion of the upper portion comprises a dopant with a first peak concentration; a middle portion between the lower portion and upper portion, wherein the middle portion comprises a second semiconductor material having a second lattice constant different from the first lattice constant; and an isolation structure surrounding the fin structure, wherein 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.