Polycrystalline Silicon Thin-Film Transistors: a continuous evolving technology

Polysilicon thin-film transistors (TFTs) are of great interest for their application in large area microelectronics and in particular for active matrix liquid crystal displays. This is owing to the improved performances of polysilicon TFTs that allow the fabrication of active matrix devices with integrated driving circuitry. The progressive improvement of this technology has been related to the optimization of the polysilicon active layer, first deposited by LPCVD, then obtained by solid phase crystallization of amorphous silicon and, more recently, by excimer laser crystallization of amorphous silicon. For circuit application there is a strong demand for high performances, high stability and low noise devices. Some specific aspects of the electrical characteristics of the polysilicon TFTs and related to their circuit application are also discussed, including the electrical instabilities induced by hot-carrier effects, the anomalous drain current increase occurring at high source-drain voltages (called "kink" effect) and the low-frequency noise performances.