A semiconductor device includes a pair of fin structures on a semiconductor substrate, each including a vertically stacked plurality of channel layers, a dielectric fin extending in parallel to and between the fin structures, and a gate structure on and extending perpendicularly to the fin structures, the gate structure engaging with the plurality of channel layers. The dielectric fin includes a fin bottom and a fin top over the fin bottom. The fin bottom has a top surface extending above a bottom surface of a topmost channel layer. The fin top includes a core and a shell, the core having a first dielectric material, the shell surrounding the core and having a second dielectric material different from the first dielectric material.