在主动矩阵型显示设备中，包含在电路中的薄膜晶体管的电气特性很重要，且该电气特性影响到显示设备的性能。借由对反交错型(inverted staggered)薄膜晶体管使用包含In、Ga及Zn的氧化物半导体膜，可以减少薄膜晶体管之电气特性的变动。为了解决目的，不接触于空气地进行溅射法来连续地形成闸极绝缘膜、氧化物半导体层、信道保护膜的三层。另外，采用在氧化物半导体层中的重叠于信道保护膜的区域的厚度比接触于导电膜的区域的厚度厚的结构。

In an active matrix display device, electric characteristics of thin film transistors included in a circuit are important, and performance of the display device depends on the electric characteristics. Thus, by using an oxide semiconductor film including In, Ga, and Zn for an inverted staggered thin film transistor, variation in electric characteristics of the thin film transistor can be reduced. Three layers of a gate insulating film, an oxide semiconductor layer and a channel protective layer are successively formed by a sputtering method without being exposed to air. Further, in the oxide semiconductor layer, the thickness of a region overlapping with the channel protective film is larger than that of a region in contact with a conductive film.