There is provided a method of manufacturing a semiconductor device having
a TFT with sufficient characteristics and little fluctuation by
accurately controlling the addition amount of impurity ions to the
semiconductor layer using an ion doping device. A semiconductor device
having a TFT showing sufficient and stable characteristics may be
obtained by increasing the ratio of the dopant amount in the doping gas
and decreasing the ambient atmosphere components (C, N, O) and hydrogen
to be simultaneously added with the impurity ions at the time of doping.