Semiconductor device and method for fabricating the same

A semiconductor device includes: a plurality of stacked semiconductor layers; a plurality of composite doped regions separately and parallelly disposed in a portion of the semiconductor layers along a first direction; a gate structure disposed over a portion of the semiconductor layers along a second direction, wherein the gate structure covers a portion of the composite doped regions; a first doped region formed in the most top semiconductor layer along the second direction and being adjacent to a first side of the gate structure; and a second doped region formed in the most top semiconductor layer along the second direction and being adjacent to a second side of the gate structure opposite to the first side thereof.