A semiconductor device having a transistor structure is described. The transistor structure comprises a fin active region vertically protruding from a substrate and extending in a first horizontal direction; and a gate structure crossing the fin active region and extending in a second horizontal direction. The gate structure includes an upper gate structure disposed over the fin active region, the upper gate structure having a line shape extending in the second horizontal direction; and a lower gate structure disposed on both sides of the fin active region. The lower gate structure has a first horizontal width in the first horizontal direction. The upper gate structure has a second horizontal width in the first horizontal direction. The first horizontal width is less than the second horizontal width.