A semiconductor device and a manufacturing method thereof are disclosed in the present invention. The semiconductor device includes a source structure; a gate structure disposed above the source structure; a first opening penetrates through the gate structure in a vertical direction; a semiconductor structure; a gate dielectric layer; an insulation structure; and a void. The semiconductor structure is partially disposed in the first opening, and at least a portion of the gate structure is located at two opposite sides of the semiconductor structure in a horizontal direction. The gate dielectric layer is disposed in the first opening and located between the semiconductor structure and the gate structure. At least a portion of the insulation structure is disposed in the first opening, and the void is located in the insulation structure.