Various kinds of flat panel displays such as liquid crystal displays (LCDs), plasma display panels and organic light emitting diode (OLED) displays are briefly evaluated from the perspective of applicability to ubiquitous products. It is clarified that the LCDs and OLED displays are suitable for realizing mobile electronic products with a high quality display, since these displays can use active devices on the backplanes to form active matrix displays and can integrate peripheral circuits of the displays and functional circuits of mobile electronics for a ubiquitous era. It is clarified further that the low temperature polycrystalline silicon (LTPS) thin film transistor (TFT) is the most promising active device for the backplane of such active matrix displays because the LTPS TFT has the possibility to enhance its performance without raising the cost. The low temperature poly-Si TFT fabrication process is introduced, and its key technologies such as crystallization, gate oxide formation, and impurity doping are surveyed. As the property of polycrystalline silicon (poly-Si) influences not only the TFT performance itself but also the efficiency of impurity doping and the integrity of the gate oxide, the crystallinity of the poly-Si is reviewed. After that, the history of the development and the state of the art in impurity doping technology and its issues are addressed in detail. Finally, foreseeing the application of LTPS TFT, the realization of OLED displays, and the progress of LTPS TFT for integrating higher functional circuits for ubiquitous applications, the requirements for impurity doping in such progress are addressed. In particular, the single grain silicon technology and the scaling down of the TFT size, which are thought to be highly effective to enhance the performance of TFTs, and issues of impurity doping technology relating to them are discussed.