The present disclosure provides a thin film transistor, a fabricating method thereof, an array substrate, and a display device. The thin film transistor includes: a substrate; a channel region; a heavily doped first semiconductor pattern located on both sides of the channel region; a second semiconductor pattern disposed on the heavily doped first semiconductor pattern; a gate insulating layer covering the channel region and the second semiconductor pattern; a gate pattern disposed on the gate insulating layer, an orthographic projection of the gate pattern on the substrate being within an orthographic projection of the channel region on the substrate; and a source pattern and a drain pattern in contact with the heavily doped first semiconductor pattern through the first via and the second via, respectively.