Disclosed is a thin film transistor, an array substrate, a method for manufacturing the same, and a display device. The method includes: forming a source and drain on a base substrate and forming a semiconductor layer. Between the step of forming the source and drain and the step of forming the semiconductor layer, the method further includes: forming a diffusion barrier layer. Metal atoms diffused from the source and drain and passing through the diffusion barrier layer react with a part of the semiconductor layer near the source and drain, and a metal transition layer containing metal silicide is formed.