An OS transistor is described. The carrier density of the oxide semiconductor is preferably reduced in order to suppress the negative shift of the threshold voltage of the transistor or reduce the off-state current of the OS transistor. In order to reduce the carrier density of the oxide semiconductor, the impurity concentration in the oxide semiconductor is reduced so that the density of defect states can be reduced. In this specification and the like, the state in which the impurity concentration is low and the density of defect states is low is referred to as “highly purified intrinsic” or “substantially highly purified intrinsic”. The carrier density of a highly purified intrinsic oxide semiconductor is lower than 8×1015 cm?3, preferably lower than 1×1011 cm?3, further preferably lower than 1×1010 cm?3 and is higher than or equal to 1×10?9 cm?3.