Polycrystalline silicon thin film transistor device and method of fabricating the same

A polycrystalline silicon thin film transistor device, disposed on a substrate, the polycrystalline silicon thin film transistor device comprising:

• a buffer layer, disposed on the substrate, wherein the buffer layer has dopants;
• a polycrystalline silicon layer, disposed on the buffer layer, wherein the polycrystalline silicon layer comprises a channel, a source doped region, and a drain doped region, and the source doped region and the drain doped region are respectively located on two sides of the channel;
• a gate insulating layer, disposed on the polycrystalline silicon layer;
• and
• a gate electrode, disposed on the gate insulating layer, and corresponding to the channel of the polycrystalline silicon layer,
• wherein the polycrystalline silicon layer is formed by performing a thermal process to convert an amorphous silicon layer into the polycrystalline silicon layer by means of polycrystalization, and simultaneously to out-diffuse a portion of the dopants in the buffer layer into the polycrystalline silicon layer for adjusting a threshold voltage.