the present invention relates to a thin film transistor , in a low - temperature poly - si thin film becoming an elemental material of the thin film transistor , an object of the invention is to provide the thin film transistor suitable for realizing an image display device having a high performance and a large area at low cost by realizing a poly - crystalline thin film having a crystal structure restraining current scattering in a grain boundary , lessening surface roughness , and capable of realizing high mobility even to a positive hole current . the object described above is achieved by realizing a tft with high mobility by restraining a current scattering factor in a grain boundary of crystal with an introduction of ge into the poly - crystalline si thin film and with a difference in ratios of ge compositions between an interior grain of crystal and a grain boundary of crystal resulted from a phase separation involved in crystallization , and by restraining surface roughness using a difference in volumes in a crystal .