The invention discloses a transistor and a formation method thereof. The formation method of the transistor comprises: providing a semiconductor substrate, wherein the surface of the semiconductor substrate is provided with a threshold voltage adjusting film, the surface of the threshold voltage adjusting film is provided with a barrier film, the surface of the barrier film is provided with a channel film, the threshold voltage adjusting film is internally provided with doped irons, the channel film is at an intrinsic state, and the barrier film is used for preventing penetration by the doped irons in the threshold voltage adjusting film; forming a grid structure on the surface of the channel film; forming a first side wall on the surface of the channel film at the two sides of the grid structure; by taking the grid structure and the first side wall as masks, etching the channel film, the barrier film, the threshold voltage adjusting film and a part of the semiconductor substrate to form a channel layer, a barrier layer and a threshold voltage adjusting layer; and forming a doping layer on the surface of the semiconductor substrate at the two sides of the threshold voltage adjusting layer, the barrier layer, the channel layer and the grid structure, wherein the surface of the doping layer is not lower than the surface of the channel layer. The performance of the formed transistor is improved.