A semiconductor structure includes: a semiconductor substrate; a first source/drain feature and a second source/drain feature over the semiconductor substrate; and semiconductor layers extending longitudinally in a first direction and connecting the first source/drain feature and the second source/drain feature. The semiconductor layers are spaced apart from each other in a second direction perpendicular to the first direction. The semiconductor structure further includes inner spacers each between two adjacent semiconductor layers; metal oxide layers interposing between the inner spacers and the semiconductor layers; and a gate structure wrapping around the semiconductor layers and the metal oxide layers.