A transistor with small parasitic capacitance can be provided. A transistor with high frequency characteristics can be provided. A semiconductor device including the transistor can be provided. Provided is a transistor including an oxide semiconductor, a first conductor, a second conductor, a third conductor, a first insulator, and a second insulator. The first conductor has a first region where the first conductor overlaps with the oxide semiconductor with the first insulator positioned therebetween; a second region where the first conductor overlaps with the second conductor with the first and second insulators positioned therebetween; and a third region where the first conductor overlaps with the third conductor with the first and second insulators positioned therebetween. The oxide semiconductor including a fourth region where the oxide semiconductor is in contact with the second conductor; and a fifth region where the oxide semiconductor is in contact with the third conductor.

To provide a semiconductor device in which a large current can flow. To provide a semiconductor device which can be driven stably at a high driving voltage. The semiconductor device includes a semiconductor layer, a first electrode and a second electrode electrically connected to the semiconductor layer and apart from each other in a region overlapping with the semiconductor layer, a first gate electrode and a second gate electrode with the semiconductor layer therebetween, a first gate insulating layer between the semiconductor layer and the first gate electrode, and a second gate insulating layer between the semiconductor layer and the second gate electrode. The first gate electrode overlaps with part of the first electrode, the semiconductor layer, and part of the second electrode. The second gate electrode overlaps with the semiconductor layer and part of the first electrode, and does not overlap with the second electrode.

To provide a semiconductor device with a novel structure. The semiconductor device includes an accelerator. The accelerator includes a first memory circuit, a second memory circuit, and an arithmetic circuit. The first memory circuit includes a first transistor. The second memory circuit includes a second transistor. Each of the first transistor and the second transistor includes a semiconductor layer including a metal oxide in a channel formation region. The arithmetic circuit includes a third transistor. The third transistor includes a semiconductor layer including silicon in a channel formation region. The first transistor and the second transistor are provided in different layers. The layer including the first transistor is provided over a layer including the third transistor. The layer including the second transistor is provided over the layer including the first transistor. The data retention characteristics of the first memory circuit are different from those of the second memory circuit.

To improve field-effect mobility and reliability of a transistor including an oxide semiconductor film. Provided is a semiconductor device including an oxide semiconductor film. The semiconductor device includes a first insulating film, the oxide semiconductor film over the first insulating film, a second insulating film and a third insulating film over the oxide semiconductor film, and a gate electrode over the second insulating film. The oxide semiconductor film includes a first oxide semiconductor film, a second oxide semiconductor film over the first oxide semiconductor film, and a third oxide semiconductor film over the second oxide semiconductor film. The first to third oxide semiconductor films contain the same element. The second oxide semiconductor film includes a region where the crystallinity is lower than the crystallinity of one or both of the first oxide semiconductor film and the third oxide semiconductor film.

An object is to control composition and a defect of an oxide semiconductor, another object is to increase a field effect mobility of a thin film transistor and to obtain a sufficient on-off ratio with a reduced off current. A solution is to employ an oxide semiconductor whose composition is represented by InMO.sub.3(ZnO).sub.m, where M is one or a plurality of elements selected from Ga, Fe, Ni, Mn, Co, and Al, and m is preferably a non-integer number of greater than 0 and less than 1. The concentration of Zn is lower than the concentrations of In and M. The oxide semiconductor has an amorphous structure. Oxide and nitride layers can be provided to prevent pollution and degradation of the oxide semiconductor.

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.

A novel semiconductor device in which a metal film containing copper (Cu) is used for a wiring, a signal line, or the like in a transistor including an oxide semiconductor film is provided. The semiconductor device includes an oxide semiconductor film having conductivity on an insulating surface and a conductive film in contact with the oxide semiconductor film having conductivity. The conductive film includes a Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti).

A transistor includes a multilayer film in which an oxide semiconductor film and an oxide film are stacked, a gate electrode, and a gate insulating film. The multilayer film overlaps with the gate electrode with the gate insulating film interposed therebetween. The multilayer film has a shape having a first angle between a bottom surface of the oxide semiconductor film and a side surface of the oxide semiconductor film and a second angle between a bottom surface of the oxide film and a side surface of the oxide film. The first angle is acute and smaller than the second angle. Further, a semiconductor device including such a transistor is manufactured.

An embodiment is a semiconductor device which includes a first oxide semiconductor layer over a substrate having an insulating surface and including a crystalline region formed by growth from a surface of the first oxide semiconductor layer toward an inside; a second oxide semiconductor layer over the first oxide semiconductor layer; a source electrode layer and a drain electrode layer which are in contact with the second oxide semiconductor layer; a gate insulating layer covering the second oxide semiconductor layer, the source electrode layer, and the drain electrode layer; and a gate electrode layer over the gate insulating layer and in a region overlapping with the second oxide semiconductor layer. The second oxide semiconductor layer is a layer including a crystal formed by growth from the crystalline region.

A novel metal oxide is provided. A semiconductor device with favorable electrical characteristics is provided. The metal oxide has a plurality of energy gaps, and includes a first region having a high energy level of a conduction band minimum and a second region having an energy level of a conduction band minimum lower than that of the first region. The second region includes more carriers than the first region. A difference between the energy level of the conduction band minimum of the first region and the energy level of the conduction band minimum of the second region is greater than or equal to 0.2 eV.