An integrated circuit includes a stacked FinFET in a second area and a GAA transistor in a first area. The stacked FinFET includes two first source/drain, first and second semiconductor layers alternately stacked one over another and between the two first source/drain, a first gate dielectric layer over top and sidewalls of the first and second semiconductor layers, a first gate electrode layer over the first gate dielectric layer, and first spacer features laterally between the second semiconductor layers and the two first source/drain. The first and the second semiconductor layers include different materials. The GAA transistor includes two second source/drain, third semiconductor layers electrically connecting the two second source/drain, a second gate dielectric layer wrapping around the third semiconductor layers, a second gate electrode over the second gate dielectric layer, and second spacer features laterally between the second gate dielectric layer and the two second source/drain.