Buffer layer for promoting electron mobility and thin film transistor having the same

A method for forming a thin film transistor, comprising:

• providing a substrate;
• forming a buffer layer on the substrate, wherein the buffer layer comprises an amorphous silicon layer on the substrate, and a buffer dielectric layer on the amorphous silicon layer;
• forming a amorphous silicon active layer on the buffer layer;
• performing a excimer laser annealing (ELA) process to crystallize the amorphous silicon active layer to a polysilicon active layer;
• forming a gate dielectric layer at least covering the polysilicon active layer;
• forming a gate electrode on the gate dielectric layer;
• and
• doping the polysilicon active layer uncovered by the gate electrode to form a source region and a drain region using the gate electrode as a mask.