A thin film transistor using a metal oxide semiconductor containing In, Ga, and Zn is disclosed as one of this type of TFT (Patent Document 1).
Characteristics vary among a plurality of TFTs due to damage to a semiconductor layer which is caused when forming a protective layer over the semiconductor layer by a sputtering method etc. In the TFT of Patent Document 1, in order to decrease such a variation in characteristics, the semiconductor layer is formed by two layers having different physical properties. Specifically, a first layer is formed by a sputtering method, and then a second layer is formed thereon by a pulsed laser deposition method that forms a low density film. Thus, the semiconductor layer is oxidized to a depth greater than that to which the semiconductor layer is damaged when forming a protective film in an oxygen atmosphere. This increases the resistance in the damaged region so that electric characteristics of the semiconductor layer are not affected.
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