1. Field of the Invention
The present invention relates to an EL (electro luminescence) display device having a semiconductor element (an element using a semiconductor thin film) formed on a substrate, and electric equipment using the EL display device as a display (display portion). The EL (electroluminescent) devices referred to in this specification include triplet-based light emission devices and/or singlet-based light emission device, for example.
2. Description of the Related Art
In recent years, a technique by which a TFT is formed on a substrate has greatly progressed, and the application of that technique to an active matrix display device has been increasingly developed. In particular, a TFT using a polysilicon film enables high-speed operation since it is higher in field effect mobility (also called mobility) than a conventional TFT using an amorphous silicon film.
Attention has been paid to the above active matrix display device because various advantages such as a reduction in manufacturing costs, a downsizing of the display device, an improvement in yield, and a reduction in through-put, are obtained by forming various circuits and elements on the same substrate.
The active matrix EL display device provides a switching element formed of a TFT (hereinafter referred to as a switching element) on each of pixels and activates a driver element that conducts a current control by the switching TFT, to thereby make an EL layer (speaking rigidly, a light emitting layer) emit light. For example, Japanese Patent Application Laid-open No. Hei 10-189252 discloses the EL display device.
As the active matrix EL display device, there are proposed two EL element structures depending on light radiating directions. One of those structures is that a light emitted from the EL element penetrates an opposing substrate and is then radiated so as to enter eyes of an observer. In this case, the observer can recognize an image from the opposing substrate side. The other structure is that a light emitted from the EL element penetrates an element substrate and is then radiated so as to enter eyes of the observer. In this case, the observer can recognize an image from the element substrate side.
In the former structure, the light from the outside penetrates the opposing substrate and is then irradiated onto the TFTs existing in gaps between the respective pixel electrodes, to thereby deteriorate the TFTs. However, because the light from the outside is not high in intensity, the deterioration of the TFTs is not large.
On the other hand, in the latter structure generally frequently employed, because the light emitted from the EL element penetrates the element substrate and is then radiated, the light emitted from the EL element is irradiated onto the TFTs, resulting in such a serious problem in that the TFTs are deteriorated.
Also, a storage capacitor is provided in the pixel and a high aperture ratio is demanded for the pixel from the viewpoint of the display performance. If the respective pixels have the high aperture ratio, the light application efficiency is improved, thereby being capable of achieving the power saving and the downsizing of the display device.
In recent years, the fine pixel size is developed, and a higher definition image is demanded. The fine pixel size increases an area of one pixel on which the TFT and the wirings are formed, to thereby reduce the pixel aperture ratio.
Under the above circumstances, in order to obtain the high aperture ratio of each pixel within the limit of a regular pixel size, it is essential to efficiently layout circuit elements necessary for the circuit structure of the pixel.
As described above, in order to realize the active matrix EL display device high in pixel aperture ratio with a small number of masks, an entirely novel pixel structure that has not existed up to now is demanded.
The present invention has been made to meet the above demands, and therefore an object of the present invention is to provide an EL display device having a pixel structure that realizes a high aperture ratio without increasing the number of masks and the number of processes.
In order to solve the problems with the conventional art, the present invention provides the following means.
The present invention is characterized by a pixel structure in which gaps between respective TFTs and gaps between respective pixels are shielded from a light without using a black mask. As one means for shielding the TFTs from the light, a gate electrode and source wirings are formed on a first insulating film, and most of a semiconductor layer that serves as an active layer is covered with gate wirings formed on a second insulating film different from the first insulating film. Also, as one means for shielding the gaps between the respective pixels from the light, pixel electrodes are so disposed as to be superimposed on the source wirings.
The above-mentioned TFTs are directed to switching TFT disposed on the respective pixels or current control TFTs.
According to the structure of the present invention disclosed in this specification, there is provided an electronic device comprising a plurality of source wirings, a plurality of gate wirings, a plurality of current supply lines and a plurality of pixels, characterized in that:
each of the plurality of pixels includes a switching TFT, a current control TFT, and a light-emitting element; and
the switching TFT includes a semiconductor layer (first semiconductor layer 200) having a source region and a drain region on an insulating surface, and a channel-forming region interposed between the source region and the drain region; a first insulating film (gate insulating film) formed on the semiconductor layer (first semiconductor layer 200); an electrode formed (first electrode 113) on the first insulating film so as to be superimposed on the channel-forming region; a source wiring (115) formed on the first insulating film; a second insulating film that covers the electrode (first electrode 113) and the source wirings; and a gate wiring (145) formed on the second insulating film and connected to the electrode (first electrode 113).
In the above structure, the electronic device is characterized in that the semiconductor layer (first semiconductor layer 200) has a region, which is superimposed on the gate wiring.
Further, the electronic device is characterized in that the region of the semiconductor layer which is superimposed on the gate wiring includes at least the channel-forming region, a region existing between the channel-forming region and the drain region, or a region existing between the channel-forming region and the source region, and is protected from light from the outside.
In case of the electronic device of a multi-gate structure in which a plurality of gate electrodes are on one semiconductor layer through an insulating film, it is characterized in that the semiconductor layer includes a plurality of channel-forming regions that the gate wiring is so disposed as to be superimposed on a region existing between one of the channel-forming regions and another channel-forming region.
Further, the electronic device is characterized in that the electrode and the source wirings are made of the same material on the first insulating film and that the pixel electrode, the connection electrode and the gate wiring are made of the same material on the second insulating film.
According to another structure of the present invention, there is provided an electronic device comprising a plurality of source wirings, a plurality of first gate wirings, a plurality of current supply lines, a plurality of second gate wirings and a plurality of pixels, characterized in that:
each of the plurality of pixels includes a switching TFT, a current control TFT, an erasing TFT and a light-emitting element; and
the switching TFT includes a semiconductor layer (first semiconductor layer 900) having a source region and a drain region formed on an insulating surface, and a channel-forming region interposed between the source region and the drain region; a first insulating film (gate insulating film) formed on the semiconductor layer (first semiconductor layer 900); an electrode (first electrode 805) formed on the first insulating film and superimposed on the channel-forming region; a source wiring (803) formed on the first insulating film; a second insulating film that covers the electrode (first electrode 805) and the source wiring (803); and a first gate wiring (801) formed on the second insulating film and connected to the electrode (first electrode 805).
Further, according to another structure of the present invention, there is provided an electronic device comprising a plurality of source wirings, a plurality of first gate wirings, a plurality of current supply lines, a plurality of second gate wirings and a plurality of pixels, characterized in that:
each of the plurality of pixels includes a switching TFT, a current control TFT, an erasing TFT, and a light-emitting element; and
the erasing TFT includes a semiconductor layer having a source region and a drain region formed on an insulating surface, and a channel-forming region interposed between the source region and the drain region; a first insulating film (gate insulating film) formed on the semiconductor layer; a first electrode (third electrode 807) formed on the first insulating film and superimposed on the channel-forming region; a second electrode (second electrode 806) formed on the first insulating film; a second insulating film that covers the first electrode (third electrode 807) and the second electrode (second electrode 806); and a second gate wiring (802) formed on the second insulating film and connected to the first electrode (third electrode 807).
In the above structure, the electronic device is characterized in that the semiconductor layer has a region which is superimposed on the second gate wiring (802) and that the second gate wiring (802) is superimposed on at least the channel-forming region.
Further, the electronic device is characterized in that the region of the semiconductor layer which is superimposed on the second gate wiring (802) includes at least the channel-forming region, a region existing between the channel-forming region and the drain region, or a region existing between the channel-forming region and the source region, and is protected from light from the outside.
In the above structure, the first electrode (third electrode 807) that is superimposed on the channel-forming region comprises a gate electrode of the erasing TFT.
In the above structure, the second electrode (second electrode 806) comprises a gate electrode of the current control TFT, which is connected to the drain region of the switching TFT.
Further, it is characterized in that the first gate wiring and the second gate wiring are made of the same material in order to suppress the increase in the number of masks.