A semiconductor structure includes a substrate having first and second wells of first and second conductivity types respectively. From a top view, the first and second wells extend lengthwise along a first direction, the first and second wells each includes a protruding section that protrudes along a second direction perpendicular to the first direction and a recessed section that recedes along the second direction. The protruding section of the first well fits into the recessed section of the second well, and vice versa. The semiconductor structure further includes first source/drain features over the protruding section of the first well; second source/drain features over the second well; third source/drain features over the protruding section of the second well; and fourth source/drain features over the first well. The first and second source/drain features are of the first conductivity type. The third and fourth source/drain features are of the second conductivity type.