A semiconductor device includes a storage element layer and a selector. The selector is electrically coupled to the storage element layer, and includes a first insulating layer, a second insulating layer, a third insulating layer, a first conductive layer and a second conductive layer. The first insulating layer, the second insulating layer and the third insulating layer are stacked up in sequence, wherein the second insulating layer is sandwiched in between the first insulating layer and the third insulating layer, and the first insulating layer and the third insulating layer include materials with higher band gap as compared with a material of the second insulating layer. The first conductive layer is connected to the first insulting layer, and the second conductive layer is connected to the third insulating layer.