A device includes a substrate and a fin isolation structure between a first gate structure and a second gate structure. The first gate structure wraps around a first vertical stack of nanostructure channels overlying a first fin. The second gate structure wraps around a second vertical stack of nanostructure channels overlying a second fin. The fin isolation structure extends from an upper surface of the first gate structure to an upper surface of the substrate. A trench isolation structure is between the first fin and the fin isolation structure, and has different etch selectivity than the fin isolation structure.