A semiconductor device includes a first transistor and a second transistor. The first transistor includes: a first source and a first drain separated by a first distance, a first semiconductor structure disposed between the first source and first drain, a first gate electrode disposed over the first semiconductor structure, and a first dielectric structure disposed over the first gate electrode. The first dielectric structure has a lower portion and an upper portion disposed over the lower portion and wider than the lower portion. The second transistor includes: a second source and a second drain separated by a second distance greater than the first distance, a second semiconductor structure disposed between the second source and second drain, a second gate electrode disposed over the second semiconductor structure, and a second dielectric structure disposed over the second gate electrode. The second dielectric structure and the first dielectric structure have different material compositions.