Semiconductor devices and methods are provided. A semiconductor device according to the present disclosure includes a substrate having a first area and a second area, a plurality of fin structures extending along a direction over the first area and the second area of the substrate, a first transistor and a second transistor in the first area, a first isolation structure disposed between the first transistor and the second transistor, a first isolation structure disposed between the first transistor and the second transistor, a third transistor and a fourth transistor in the second area, and a second isolation structure disposed between the third transistor and the fourth transistor. The first isolation structure includes a first width along the direction and the second isolation structure includes a second width along the direction. The second width is greater than the first width.