An imaging device with excellent imaging performance is provided. The imaging device has a first circuit including a first photoelectric conversion element and a second circuit including a second photoelectric conversion element. The second circuit is shielded from light. In the imaging device, a current mirror circuit in which a transistor connected to the second photoelectric conversion element serves as an input transistor and a transistor connected to the first photoelectric conversion element serves as an output transistor is formed. With such a configuration, the amount of photocurrent in the first circuit from which the contribution of the dark current of the first photoelectric conversion element has been excluded can be detected.

An object is to provide a structure of a transistor which has a channel formation region formed using an oxide semiconductor and a positive threshold voltage value, which enables a so-called normally-on switching element. The transistor includes an oxide semiconductor stack in which at least a first oxide semiconductor layer and a second oxide semiconductor layer with different energy gaps are stacked and a region containing oxygen in excess of its stoichiometric composition ratio is provided.

An imaging device that has a structure where a transistor is used in common by a plurality of pixels and is capable of imaging with a global shutter system is provided. A transistor that resets the potential of a charge detection portion, a transistor that outputs a signal corresponding to the potential of the charge detection portion, and a transistor that selects a pixel are used in common by the plurality of pixels. A transistor is provided between a power supply line and a photoelectric conversion element. Exposure is performed by turning on the transistor. Imaging data is retained in a charge retention portion by turning off the transistor.

Variation in electrical characteristics of a semiconductor device including an oxide semiconductor is inhibited and the reliability thereof is improved. The oxide semiconductor is formed over a substrate. An insulator is formed over the oxide semiconductor. A metal oxide is formed over the insulator. A conductor is formed over the metal oxide. The conductor, the metal oxide, and the insulator over the oxide semiconductor are removed to expose a portion of the oxide semiconductor. Plasma treatment is performed on a surface of the exposed portion of the oxide semiconductor. A nitride insulator is formed over the exposed portion of the oxide semiconductor and the conductor.

When a semiconductor device including a transistor in which a gate electrode layer, a gate insulating film, and an oxide semiconductor film are stacked and a source and drain electrode layers are provided in contact with the oxide semiconductor film is manufactured, after the formation of the gate electrode layer or the source and drain electrode layers by an etching step, a step of removing a residue remaining by the etching step and existing on a surface of the gate electrode layer or a surface of the oxide semiconductor film and in the vicinity of the surface is performed. The surface density of the residue on the surface of the oxide semiconductor film or the gate electrode layer can be 110.sup.13 atoms/cm.sup.2 or lower.

An image sensor includes a photoelectric conversion element including a first impurity region and a second impurity region, wherein the first impurity region contacts a first surface of a substrate, wherein the second impurity region has conductivity complementary to the first impurity region and is formed in the substrate and below the first impurity region; a pillar formed over the photoelectric conversion element; a transfer gate formed over the photoelectric conversion element to surround the pillar; and a channel layer formed between the transfer gate and the pillar and contacting the photoelectric conversion element, wherein the channel layer contacts the first impurity region and has the same conductivity as the second impurity region.

A semiconductor device is provided that includes an array of imaging cells realized from a plurality of layers formed on a substrate, wherein the plurality of layers includes at least one modulation doped quantum well structure spaced from at least one quantum dot structure. Each respective imaging cell includes an imaging region spaced from a corresponding charge storage region. The at least one quantum dot structure of the imaging region generates photocurrent arising from absorption of incident electromagnetic radiation. The at least one modulation doped quantum well structure defines a buried channel for lateral transfer of the photocurrent for charge accumulation in the charge storage region and output therefrom. The at least one modulation doped quantum well structure and the at least one quantum dot structure of each imaging cell can be disposed within a resonant cavity that receives the incident electromagnetic radiation or below a structured metal film having a periodic array of holes.

A backside illumination image sensor and a method for reducing a dark current of the backside illumination image sensor. The backside illumination image sensor comprises: a photodiode, a first conductive type isolated layer (120); a gate structure of a pass transistor, corresponding to the first conductive type isolated layer (120) and formed on an upper surface of a first conductive type semiconductor substrate (100), the gate structure (130) comprising: gate oxide (131), a gate layer (132), and a gate sidewall (133), and the gate structure (130) correspondingly covering the photodiode; and a floating diffusion zone (140), formed in the first conductive type semiconductor substrate (100) and having second conductive type heavy doping. In the backside illumination image sensor, a defect does not easily appear on a surface, right above the photodiode, of the first conductive type semiconductor substrate (100), so that a dark current is effectively prevented from being produced.

A solid-state imaging device with high productivity and improved dynamic range is provided. In the imaging device including a photoelectric conversion element having an i-type semiconductor layer, functional elements, and a wiring, an area where the functional elements and the wiring overlap with the i-type semiconductor in a plane view is preferably less than or equal to 35%, further preferably less than or equal to 15%, and still further preferably less than or equal to 10% of the area of the i-type semiconductor in a plane view. Plural photoelectric conversion elements are provided in the same semiconductor layer, whereby a process for separating the respective photoelectric conversion elements can be reduced. The respective i-type semiconductor layers in the plural photoelectric conversion elements are separated by a p-type semiconductor layer or an n-type semiconductor layer.

A solid-state imaging device which includes a plurality of pixels in an arrangement, each of the pixels including a photoelectric conversion element, pixel transistors including a transfer transistor, and a floating diffusion region, in which the channel width of transfer gate of the transfer transistor is formed to be larger on a side of the floating diffusion region than on a side of the photoelectric conversion element.