A semiconductor device, an array substrate, and a display device, and their fabrication methods are provided. An exemplary semiconductor device includes a first electrode, an insulating layer, and a second electrode, over a substrate. A conductive layer is on the insulating layer. A semiconductor layer is on the first electrode, on a first sidewall of the insulating layer, on the conductive layer, on the second sidewall of the insulating layer, and on the second electrode. A first gate electrode is over a portion of the semiconductor layer that is on the first sidewall of the insulating layer. A second gate electrode is over a portion of the semiconductor layer that is on the second sidewall of the insulating layer.