A thin-film transistor includes an oxide semiconductor layer, a gate insulating film, a gate electrode, and a source-drain electrode. The oxide semiconductor layer includes a channel region and a low-resistance region that has an electric resistance lower than an electric resistance of the channel region. The gate insulating film is provided on the oxide semiconductor layer. The gate electrode is provided on the gate insulating film and opposed to the channel region of the oxide semiconductor layer. The gate electrode includes a first electrode layer and a second electrode layer in order from the gate insulating film. The first electrode layer has a first width that is along a channel length and greater than a second width of the second electrode layer along the channel length. The source-drain electrode is electrically coupled to the low-resistance region of the oxide semiconductor layer.

Disclosed is a semiconductor device with a transistor in which an oxide semiconductor is used. An insulating layer on a back channel side of the oxide semiconductor layer has capacitance of lower than or equal to 210.sup.4 F/m.sup.2. For example, in the case of a top-gate transistor, a base insulating layer has capacitance of lower than or equal to 210.sup.4 F/m.sup.2, whereby the adverse effect of an interface state between the substrate and the base insulating layer can be reduced. Thus, a semiconductor device where fluctuation in electrical characteristics is small and reliability is high can be manufactured.

The transistor includes a gate electrode, a gate insulating film over the gate electrode, an oxide semiconductor film over the gate insulating film, a source electrode and a drain electrode electrically connected to the oxide semiconductor film. The oxide semiconductor film includes a first oxide semiconductor film on the gate electrode side and a second oxide semiconductor film over the first oxide semiconductor film. The first oxide semiconductor film includes a first region in which an atomic proportion of In is larger than that of M (M is Ti, Ga, Sn, Y, Zr, La, Ce, Nd, or Hf). The second oxide semiconductor film includes a second region in which an atomic proportion of In is smaller than that of the first oxide semiconductor film. The second region includes a portion thinner than the first region.

An organic light emitting diode display comprises a substrate including a display area in which a pixel is disposed and a peripheral area surrounding the display area, a driving semiconductor layer disposed in the display area on the substrate, a driving gate electrode disposed in the display area on the driving semiconductor layer, a common voltage line disposed in the peripheral area on the substrate and disposed on a same layer as the driving gate electrode, a gate electrode anti-oxidation layer disposed on the driving gate electrode, a common voltage line anti-oxidation layer disposed on the common voltage line, an interlayer insulating layer disposed on the driving semiconductor layer, the driving gate electrode, the common voltage line, the gate electrode anti-oxidation layer, and the common voltage line anti-oxidation layer. A driving source electrode and a driving drain electrode are disposed in the display area on the interlayer insulating layer, and a common voltage applying electrode is disposed in the peripheral area on the interlayer insulating layer and on the same layer as the driving source electrode and the driving drain electrode.

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.

A transistor with stable electrical characteristics. A semiconductor device includes a first insulator over a substrate, a second insulator over the first insulator, an oxide semiconductor in contact with at least part of a top surface of the second insulator, a third insulator in contact with at least part of a top surface of the oxide semiconductor, a first conductor and a second conductor electrically connected to the oxide semiconductor, a fourth insulator over the third insulator, a third conductor which is over the fourth insulator and at least part of which is between the first conductor and the second conductor, and a fifth insulator over the third conductor. The first insulator contains a halogen element.

The present invention discloses a thin film transistor (TFT), a method for manufacturing the TFT, and a display substrate using the TFT that may prevent degradation of the characteristics of an oxide semiconductor contained in the TFT by blocking external light from entering a channel region of the oxide semiconductor. The TFT includes an oxide semiconductor layer; a protective layer overlapping a channel region of the oxide semiconductor layer; an opaque layer disposed between the oxide semiconductor layer and the protective layer; a source electrode contacting a first side of the oxide semiconductor layer; a drain electrode contacting a second side of the oxide semiconductor layer and facing the source electrode across the channel region; a gate electrode to apply an electric field to the oxide semiconductor layer; and a gate insulating layer disposed between the gate electrode and the oxide semiconductor layer.

As a display device has higher definition, the number of pixels is increased and thus, the number of gate lines and signal lines is increased. When the number of gate lines and signal lines is increased, it is difficult to mount IC chips including driver circuits for driving the gate lines and the signal lines by bonding or the like, whereby manufacturing cost is increased. A pixel portion and a driver circuit for driving the pixel portion are provided on the same substrate, and at least part of the driver circuit comprises a thin film transistor including an oxide semiconductor sandwiched between gate electrodes. A channel protective layer is provided between the oxide semiconductor and a gate electrode provided over the oxide semiconductor. The pixel portion and the driver circuit are provided on the same substrate, which leads to reduction of manufacturing cost.

In a semiconductor device including a transistor including an oxide semiconductor film and a protective film over the transistor, an oxide insulating film containing oxygen in excess of the stoichiometric composition is formed as the protective film under the following conditions: a substrate placed in a treatment chamber evacuated to a vacuum level is held at a temperature higher than or equal to 180 C. and lower than or equal to 260 C.; a source gas is introduced into the treatment chamber so that the pressure in the treatment chamber is set to be higher than or equal to 100 Pa and lower than or equal to 250 Pa; and a high-frequency power higher than or equal to 0.17 W/cm.sup.2 and lower than or equal to 0.5 W/cm.sup.2 is supplied to an electrode provided in the treatment chamber.

To provide a semiconductor device which can be miniaturized or highly integrated. To obtain a semiconductor device including an oxide semiconductor, which has favorable electrical characteristics. To provide a highly reliable semiconductor device including an oxide semiconductor, by suppression of a change in its electrical characteristics. The semiconductor device includes an island-like oxide semiconductor layer over an insulating surface; an insulating layer surrounding a side surface of the oxide semiconductor layer; a source electrode layer and a drain electrode layer in contact with top surfaces of the oxide semiconductor layer and the insulating layer; a gate electrode layer overlapping with the oxide semiconductor layer; and a gate insulating layer between the oxide semiconductor layer and the gate electrode layer. The source electrode layer and the drain electrode layer are provided above the top surface of the oxide semiconductor layer. The top surface of the insulating layer is planarized.