Thin film transistor, method for manufacturing the same, array substrate, and display device

A method for manufacturing a thin film transistor (TFT), comprising steps of:

• forming a pattern of a gate electrode on a base substrate;
• forming a gate insulation layer with an even surface;
• forming a pattern of a polysilicon semiconductor layer;
• and
• forming patterns of a source electrode and a drain electrode,
• wherein the step of forming the pattern of the polysilicon semiconductor layer comprises:
• depositing an amorphous silicon layer;
• crystallizing the amorphous silicon layer, so as to form the polysilicon semiconductor layer;
• and
• patterning the polysilicon semiconductor layer to form the pattern of the polysilicon semiconductor layer;
• and
• wherein the forming the gate insulation layer with the even surface comprises:
• depositing a gate insulation layer film on the base substrate and the pattern of the gate electrode;
• and
• removing a protrusion on the gate insulation layer film to form the gate insulation layer with the even surface, wherein a thickness of the gate insulation layer is greater than a thickness of the pattern of the gate electrode.