A method to form a TFT includes providing an insulating substrate. An amorphous silicon layer, a first polysilicon layer, an oxide layer, a second polysilicon layer are sequentially formed on the insulating substrate and are patterned to form an active region. On the active region, there are a desired gate region, and a desired interchangeable source/drain region. A portion of the second polysilicon, the oxide layer, the first polysilicon layer above the desired interchangeable source/drain region are removed, and a top portion of the amorphous-Si layer with a depth is also removed. The remaining portion of the a-Si layer is converted into a conductive layer through, for example, laser crystallization method so as to serve as the interchangeable source/drain region.