To provide a transistor with favorable electrical characteristics, a transistor with stable electrical characteristics, or a highly integrated semiconductor device. An electrode is provided over an oxide semiconductor layer A, the oxide semiconductor layer A and the electrode are covered with a layer C, and then heat treatment is performed; thus, oxidation of the electrode which is caused in the heat treatment is prevented. For the layer C, for example, an oxide semiconductor can be used. By covering a side surface of the oxide semiconductor layer A where a channel is formed with the layer C and the oxide semiconductor layer B, diffusion of impurities from the side surface of the oxide semiconductor layer A into the oxide semiconductor layer A is prevented.

An object is to provide a semiconductor device having electrical characteristics such as high withstand voltage, low reverse saturation current, and high on-state current. In particular, an object is to provide a power diode and a rectifier which include non-linear elements. An embodiment of the present invention is a semiconductor device including a first electrode, a gate insulating layer covering the first electrode, an oxide semiconductor layer in contact with the gate insulating layer and overlapping with the first electrode, a pair of second electrodes covering end portions of the oxide semiconductor layer, an insulating layer covering the pair of second electrodes and the oxide semiconductor layer, and a third electrode in contact with the insulating layer and between the pair of second electrodes. The pair of second electrodes are in contact with end surfaces of the oxide semiconductor layer.

A semiconductor device with a transistor having favorable electrical characteristics is provided. The semiconductor device has a memory circuit and a circuit that are over the same substrate. The memory circuit includes a capacitor, a first transistor, and a second transistor. A gate of the first transistor is electrically connected to the capacitor and one of a source and a drain of the second transistor. The circuit includes a third transistor and a fourth transistor that are electrically connected to each other in series. The first transistor and the third transistor each include an active layer including silicon, and the second transistor and the fourth transistor each include an active layer including an oxide semiconductor.

To provide an imaging device capable of high-speed reading. The imaging device includes a photodiode, a first transistor, a second transistor, a third transistor, and a fourth transistor. The back gate electrode of the first transistor is electrically connected to a wiring that can supply a potential higher than a source potential of the first transistor and a potential lower than the source potential of the first transistor. The back gate electrode of the second transistor is electrically connected to a wiring that can supply a potential higher than a source potential of the second transistor. The back gate electrode of the third transistor is electrically connected to a wiring that can supply a potential higher than a source potential of the third transistor and a potential lower than the source potential of the third transistor.

An object is to provide a semiconductor device including an oxide semiconductor, which has stable electric characteristics and high reliability. In a transistor including an oxide semiconductor film, the oxide semiconductor film is subjected to dehydration or dehydrogenation performed by heat treatment. In addition, as a gate insulating film in contact with the oxide semiconductor film, an insulating film containing oxygen, preferably, a gate insulating film including a region containing oxygen with a higher proportion than the stoichiometric composition is used. Thus, oxygen is supplied from the gate insulating film to the oxide semiconductor film. Further, a metal oxide film is used as part of the gate insulating film, whereby reincorporation of an impurity such as hydrogen or water into the oxide semiconductor is suppressed.

As a display device has a higher definition, the number of pixels, gate lines, and signal lines are increased. When the number of the gate lines and the signal lines are increased, a problem of higher manufacturing cost, because it is difficult to mount an IC chip including a driver circuit for driving of the gate and signal lines by bonding or the like. A pixel portion and a driver circuit for driving the pixel portion are provided over the same substrate, and at least part of the driver circuit includes a thin film transistor using an oxide semiconductor interposed between gate electrodes provided above and below the oxide semiconductor. Therefore, when the pixel portion and the driver portion are provided over the same substrate, manufacturing cost can be reduced.

Provided is a display device with extremely high resolution, a display device with higher display quality, a display device with improved viewing angle characteristics, or a flexible display device. Same-color subpixels are arranged in a zigzag pattern in a predetermined direction. In other words, when attention is paid to a subpixel, another two subpixels exhibiting the same color as the subpixel are preferably located upper right and lower right or upper left and lower left. Each pixel includes three subpixels arranged in an L shape. In addition, two pixels are combined so that pixel units including subpixel are arranged in matrix of 32.

An oxide semiconductor film contains In, M (M is Al, Ga, Y, or Sn), and Zn and includes a region with a film density higher than or equal to 6.3 g/cm.sup.3 and lower than 6.5 g/cm.sup.3. Alternatively, the oxide semiconductor film contains In, M (M is Al, Ga, Y, or Sn), and Zn and includes a region with etching at an etching rate higher than or equal to 10 nm/min and lower than or equal to 45 nm/min when a phosphoric acid aqueous solution obtained by diluting 85 vol % phosphoric acid with water 100 times is used for etching.

A high-resolution liquid crystal display device is provided. A liquid crystal display device with high aperture ratio is provided. A display device includes a liquid crystal element, a transistor, and an insulating layer. The transistor includes a semiconductor layer that transmits visible light. The semiconductor layer that transmits visible light includes a channel region and a low-resistance region. The channel region overlaps with a gate with a gate insulating layer therebetween. The low-resistance region includes a first portion that is in contact with a pixel electrode of the liquid crystal element and a second portion that is in contact with a side surface of an opening in the insulating layer.

A semiconductor structure formed based on a buried oxide (BOX) layer configured as a gate dielectric; a substrate adjacent to the BOX layer configured as a first gate electrode; a first source structure and a first drain structure, each residing above the BOX layer; a first channel structure residing between the first drain and first source structures; a second gate electrode residing above the first channel structure; a first shallow trench isolation (STI) structure and a second STI structure, each residing coplanar with and at opposite ends of the first source and first drain structures; and a second gate dielectric residing between the first channel structure and the second gate electrode, wherein a thickness of the second gate dielectric is less than a thickness of the BOX layer.