A semiconductor device that can be miniaturized or highly integrated is provided. The semiconductor device includes a first transistor including a first oxide, a second transistor including a second oxide, and a third oxide. The first oxide includes a channel formation region of the first transistor. The second oxide includes a channel formation region of the second transistor. The third oxide contains the same material as the first oxide and the second oxide. The third oxide is separated from the first oxide and the second oxide. In a top view, the third oxide is positioned between the first oxide and the second oxide. The third oxide is placed in the same layer as the first oxide and the second oxide.

An object is to provide a semiconductor device with a novel structure. The semiconductor device includes a first wiring; a second wiring; a third wiring; a fourth wiring; a first transistor having a first gate electrode, a first source electrode, and a first drain electrode; and a second transistor having a second gate electrode, a second source electrode, and a second drain electrode. The first transistor is provided in a substrate including a semiconductor material. The second transistor includes an oxide semiconductor layer.

In a thin film transistor, an increase in off current or negative shift of the threshold voltage is prevented. In the thin film transistor, a buffer layer is provided between an oxide semiconductor layer and each of a source electrode layer and a drain electrode layer. The buffer layer includes a metal oxide layer which is an insulator or a semiconductor over a middle portion of the oxide semiconductor layer. The metal oxide layer functions as a protective layer for suppressing incorporation of impurities into the oxide semiconductor layer. Therefore, in the thin film transistor, an increase in off current or negative shift of the threshold voltage can be prevented.

Provided is a highly reliable semiconductor device which includes a transistor including an oxide semiconductor. The semiconductor device includes an oxide semiconductor layer; a gate insulating layer provided over the oxide semiconductor layer; a gate electrode layer overlapping with the oxide semiconductor layer with the gate insulating layer provided therebetween; an insulating layer being in contact with part of an upper surface of the oxide semiconductor layer, covering a side surface of the gate insulating layer and a side surface and an upper surface of the gate electrode layer, and having a lower oxygen-transmitting property than the gate insulating layer; a sidewall insulating layer provided on the side surface of the gate electrode layer with the insulating layer provided therebetween; a source electrode layer and a drain electrode layer which are electrically connected to the oxide semiconductor layer.

The reliability of a transistor including an oxide semiconductor is improved. The transistor in a semiconductor device includes a first oxide semiconductor film over a first insulating film, a gate insulating film over the first oxide semiconductor film, a second oxide semiconductor film over the gate insulating film, and a second insulating film over the first oxide semiconductor film and the second oxide semiconductor film. The first oxide semiconductor film includes a channel region overlapping with the second oxide semiconductor film, a source region and a drain region each in contact with the second insulating film. The channel region includes a first layer and a second layer in contact with a top surface of the first layer and covering a side surface of the first layer in the channel width direction. The second oxide semiconductor film has a higher carrier density than the first oxide semiconductor film.

A highly reliable semiconductor device including, an oxide semiconductor is provided. Provided is a semiconductor device including an oxide semiconductor layer, an insulating layer in contact with the oxide semiconductor layer, a gate electrode layer overlapping with the oxide semiconductor layer, and a source electrode layer and a drain electrode layer electrically connected to the oxide semiconductor layer. The oxide semiconductor layer includes a first region having a crystal whose size is less than or equal to 10 nm and a second region which overlaps with the insulating layer with the first region provided therebetween and which includes a crystal part whose c-axis is aligned in a direction parallel to a normal vector of the surface of the oxide semiconductor layer.

An oxide semiconductor layer which is intrinsic or substantially intrinsic and includes a crystalline region in a surface portion of the oxide semiconductor layer is used for the transistors. An intrinsic or substantially intrinsic semiconductor from which an impurity which is to be an electron donor (donor) is removed from an oxide semiconductor and which has a larger energy gap than a silicon semiconductor is used. Electrical characteristics of the transistors can be controlled by controlling the potential of a pair of conductive films which are provided on opposite sides from each other with respect to the oxide semiconductor layer, each with an insulating film arranged therebetween, so that the position of a channel formed in the oxide semiconductor layer is determined.

High field-effect mobility is provided for a transistor including an oxide semiconductor. Further, a highly reliable semiconductor device including the transistor is provided. In a bottom-gate transistor including an oxide semiconductor layer, an oxide semiconductor layer functioning as a current path (channel) of the transistor is sandwiched between oxide semiconductor layers having lower carrier densities than the oxide semiconductor layer. In such a structure, the channel is formed away from the interface of the oxide semiconductor stacked layer with an insulating layer in contact with the oxide semiconductor stacked layer, i.e., a buried channel is formed.

A method of making a display device includes, providing a substrate having a display area and a pad area in a periphery of the display area, the display area including a plurality of pixel regions; forming a thin film transistor having a channel layer on the substrate; arranging a gate link line and a first common voltage line to cross each other, and having a first insulation film be interposed therebetween; arranging a second common voltage line and a data link line to cross each other, and having second insulation film be interposed therebetween; disposing a first pattern on the first insulation film; and disposing a second pattern on the second insulation film, wherein the channel layer, the first pattern and the second pattern are formed of the same material.

An object is to improve field effect mobility of a thin film transistor using an oxide semiconductor. Another object is to suppress increase in off current even in a thin film transistor with improved field effect mobility. In a thin film transistor using an oxide semiconductor layer, by forming a semiconductor layer having higher electrical conductivity and a smaller thickness than the oxide semiconductor layer between the oxide semiconductor layer and a gate insulating layer, field effect mobility of the thin film transistor can be improved, and increase in off current can be suppressed.