Provided is a semiconductor device including a semiconductor substrate which includes a first semiconductor layer of a first conductivity and a pair of second semiconductor layers disposed on the first semiconductor layer and spaced apart from each other to form a trench therebetween, wherein the second semiconductor layer includes a first impurity-diffused region of the first conductivity extending from a lower surface toward an upper surface of the second semiconductor layer, and a second impurity-diffused region of a second conductivity which extends from the lower surface toward the upper surface and is adjacent to the first impurity-diffused region, an insulating layer covering a sidewall of the trench, and a cap layer which is in contact with the semiconductor substrate and covers an opening of the trench to form an enclosed space in the trench, a material of the cap layer being almost the same as that of the semiconductor substrate.