A thin film transistor, a method for fabricating the same, an array substrate, and a display device are provided. The method comprises forming an active layer on a substrate, wherein source-and-drain-to-be-formed regions of the active layer are thicker than a semiconductor region between the source-and-drain-to-be-formed regions, and by a patterning process, forming a gate on the active layer, and forming a pattern of source and drain in the source-and-drain-to-be-formed regions of the active layer.