A semiconductor integrated circuit includes a substrate, a multi-gate transistor device formed on the substrate, and an n-well resistor formed in the substrate. The substrate includes a plurality of first isolation structures and at least a second isolation structure formed therein. A depth of the first isolation structures is smaller than a depth of the second isolation structure. The multi-gate transistor device includes a plurality of fin structures, and the fin structures are parallel with each other and spaced apart from each other by the first isolation structures. The n-well resistor includes at least one first isolation structure. The n-well resistor and the multi-gate transistor device are electrically isolated from each other by the second isolation structure.