A semiconductor device including a transistor having a minute size is provided. In the semiconductor device, a second conductive layer is provided over a first conductive layer; the second conductive layer has a first opening overlapping with the first conductive layer; a third conductive layer is provided over the second conductive layer; the third conductive layer has a second opening overlapping with the first opening; a first insulating layer is in contact with a sidewall of the first opening in the second conductive layer; a semiconductor layer is in contact with a top surface of the first conductive layer, a side surface of the first insulating layer, and a top surface of the third conductive layer; a second insulating layer is provided over the semiconductor layer; a fourth conductive layer is provided over the second insulating layer; the first insulating layer includes a region sandwiched between the sidewall of the first opening in the second conductive layer and the semiconductor layer; and the semiconductor layer includes a region sandwiched between the sidewall of the first opening in the second conductive layer and the fourth conductive layer.

Provided is a field-effect transistor (FET) having small off-state current, which is used in a miniaturized semiconductor integrated circuit. The field-effect transistor includes a thin oxide semiconductor which is formed substantially perpendicular to an insulating surface, a gate insulating film formed to cover the oxide semiconductor, and a gate electrode which is formed to cover the gate insulating film. The gate electrode partly overlaps a source electrode and a drain electrode. The source electrode and the drain electrode are in contact with at least a top surface of the oxide semiconductor. In this structure, three surfaces of the thin oxide semiconductor are covered with the gate electrode, so that electrons injected from the source electrode or the drain electrode can be effectively removed, and most of the space between the source electrode and the drain electrode can be a depletion region; thus, off-state current can be reduced.

The present invention provides a thin-film transistor in which transistor characteristics such as drain current and threshold voltage are improved, and a method of manufacturing the same. The present invention provides a thin-film transistor provided with a source electrode (108), a drain electrode (109), a semiconductor layer (105), a gate electrode (103), and an insulating layer (104); wherein the semiconductor layer (105) contains a composite metal oxide obtained by adding to a first metal oxide an oxide having an oxygen dissociation energy that is at least 200 kJ/mol greater than the oxygen dissociation energy of the first metal oxide, whereby the amount of oxygen vacancy is controlled; and the insulating layer (104) is provided with an SiO.sub.2 layer, a high-permittivity first layer, and a high-permittivity second layer, whereby the dipoles generated at the boundary between the SiO.sub.2 layer and the high-permittivity layers are used to control the threshold voltage.

A liquid crystal display device is provided with a thin film transistor which includes a gate electrode film that is provided in a first electrode layer located over a first insulating layer, a semiconductor film that is disposed over the gate electrode film via a second insulating layer, a drain electrode and a source electrode that are provided in a second electrode layer located over the semiconductor film and are in contact with an upper surface of the semiconductor film, and a light blocking film that is disposed under the first insulating layer. At least a part thereof overlaps the semiconductor film and the gate electrode film in a plan view. One of the drain electrode and the source electrode is connected to a gate line, and the light blocking film is electrically connected to the source electrode.

The semiconductor device includes a transistor including an oxide semiconductor film having a channel formation region, a gate insulating film, and a gate electrode layer. In the transistor, the channel length is small (5 nm or more and less than 60 nm, preferably 10 nm or more and 40 nm or less), and the thickness of the gate insulating film is large (equivalent oxide thickness which is obtained by converting into a thickness of silicon oxide containing nitrogen is 5 nm or more and 50 nm or less, preferably 10 nm or more and 40 nm or less). Alternatively, the channel length is small (5 nm or more and less than 60 nm, preferably 10 nm or more and 40 nm or less), and the resistivity of the source region and the drain region is 1.910.sup.5 .Math.m or more and 4.810.sup.3 .Math.m or less.

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.

Provided is a co-planar oxide semiconductor TFT substrate structure, in which an active layer includes a main body and a plurality of short channels connected to the main body and are separated with a plurality of strip metal electrodes to make the active layer possess higher mobility and lower leak current. Also provided is a manufacture method of the co-planar oxide semiconductor TFT substrate structure, in which with the plurality of strip metal electrodes formed between the source and the drain, which are separately positioned, as deposing the oxide semiconductor layer, the plurality of short channels can be formed between the source and the drain. The method is simple and does not require additional mask or process to obtain the active layer structure different from prior art. The manufactured actively layer possesses higher mobility and lower leak current. Thus, the performance of the TFT element can be improved.

An object is to reduce leakage current and parasitic capacitance of a transistor used for an LSI, a CPU, or a memory. A semiconductor integrated circuit included in an LSI, a CPU, or a memory is manufactured using the transistor which is formed using an oxide semiconductor which is an intrinsic or substantially intrinsic semiconductor obtained by removal of impurities which serve as electron donors (donors) from the oxide semiconductor and has larger energy gap than a silicon semiconductor, and is formed over a semiconductor substrate. With the transistor which is formed over the semiconductor substrate and includes the highly purified oxide semiconductor layer with sufficiently reduced hydrogen concentration, a semiconductor device whose power consumption due to leakage current is low can be realized.

A thin-film transistor (TFT) switch includes a gate, a drain, a source, a semiconductor layer, and a fourth electrode. The drain is connected to a first signal. The gate is connected to a control signal to control the switch on or off. The source outputs the first signal when the switch turns on. The fourth electrode and the gate are respectively located at two sides of the semiconductor layer. The fourth electrode is conductive and is selectively coupled to different voltage levels, thereby reducing leakage current in a channel to improve switch characteristic when the switch turns off.

An oxide semiconductor device and a method for manufacturing the same are provided in the present invention. The oxide semiconductor device includes a back gate, an oxide semiconductor film, a pair of source and drain electrodes, agate insulating film, a gate electrode on the oxide semiconductor film with the gate insulating film therebetween, an insulating layer covering only over the gate electrode and the pair of source and drain electrodes, and a top blocking film over the insulating layer.