A semiconductor device is provided. The semiconductor device includes source/drain (S/D) epitaxy, a gate stack adjacent to the S/D epitaxy, a semiconductor layer underlying the gate stack and including a semiconductor material surrounded by an inner spacer, an etch stop layer underlying the semiconductor layer, back trench S/D epitaxy and a self-aligned backside contact. The backside trench S/D epitaxy contacts the S/D epitaxy and is insulated from the semiconductor material by the inner spacer. The self-aligned backside contact contacts the backside trench S/D epitaxy and is insulated from the semiconductor material by the etch stop layer.

A manufacturing method of a gate structure includes the following steps. A semiconductor substrate is provided. An isolation structure is formed in the semiconductor substrate and surrounds an active region in the semiconductor substrate. A gate pattern is formed on the active region and the isolation structure. The gate pattern includes a first gate structure and a first capping layer disposed on the first gate structure. A part of the first capping layer located above an interface between the active region and the isolation structure is removed for exposing a part of the first gate structure located above the interface between the active region and the isolation structure. A removing process is performed for reducing a thickness of the part of the first gate structure located above the interface between the active region and the isolation structure.