Active matrix electroluminescent display with two TFTs and storage capacitor in each pixel

Abstract: A display apparatus comprising, arranged on a substrate, a plurality of scanning lines, a plurality of data lines intersecting said scanning lines, a plurality of common power supply lines, a plurality of pixels formed by said scanning lines and said data lines in matrix form, each of said pixels comprising a first thin film transistor having a first gate electrode which is supplied with a scanning signal through said scanning line, a holding capacitor which holds an image signal supplied through said data lines via said first thin film transistor, a second thin film transistor having a second gate electrode which is supplied with said image signal held by said holding capacitor, and a luminescent element having an emitting thin film which is provided between a pixel electrode formed each by each of said pixels and an opposite electrode opposed to said pixel electrode and emits light due to the driving current which flows between said pixel electrode and said opposite electrode when said pixel electrode is electrically connected to said common power supply line through said second thin film transistor, wherein one of the electrodes of said holding capacitor, opposite to that electrically connected to said second gate electrode of said second thin film transistor, is supplied with a pulse, a potential polarity of which is opposite to the selecting pulse of said scanning signal with a delay behind said selecting pulse.

Description: Technical Field The present invention relates to an active matrix type display apparatus including luminescent elements such as EL (Electro-luminescence) elements or LED (Light Emitting Diode) elements which emit light by driving current flowing in thin films of organic semiconductors or the like, and also including thin film transistors (hereinafter TFT's) to control the emitting operation of these luminescent elements. More particularly, the present invention relates to a technique of driving each element formed in this type of display apparatus. Background Art Active matrix type display apparatuses incorporating luminescent elements of a current controlling type, such as EL elements and LED elements, have been proposed. Since any luminescent element employed in this type of display apparatus emits by itself, there are advantages in using no back-light and in having a minimal dependence on the viewing angle and the like. Fig. 18 is a block diagram illustrating an active matrix type display apparatus incorporating organic thin film EL-elements of an electric charge filling type, as an example of these types of display apparatuses. In the display apparatus 1A shown in this figure, a plurality of scanning lines "gate", a plurality of data lines "sig" extending in a direction that intersects the direction in which the scanning line "gate" extend, a plurality of common power supply lines "com" extending parallel to the data lines "sig", and a plurality of pixels 7 located at the intersections of the data lines "sig" and the scanning lines "gate" which are formed on a transparent substrate. Each pixel 7 comprises a first TFT 20 in which a scanning signal is supplied to the gate electrode (a first gate electrode) through the scanning gate, a holding capacitor "cap" which holds an image signal supplied from the data line "sig" via the first TFT 20, a second TFT 30 in which the image signal held by the holding capacitor "cap" is supplied to the gate electrode (a second gate electrode), and an luminescent element 40 (indicated as a resistor) into which the driving current flows from the common power supply line "com" when the element 40 is electrically connected to the common power supply line "com" through the second TFT 30. In the above display apparatus 1A, both the first TFT 20 and the second TFT 30 are conventionally formed, as with an N channel type TFT or a P channel type TFT, as shown in an equivalent circuit diagram of Fig. 19 from the viewpoint of simplifying the production process, for example, in the case of an N channel type. Taking the N channel type as an example, as shown in Figs. 20 (A) and (B), when the high potential image signal "data" is written into the holding capacitor "cap" from the data line "sig", while the scanning signal "Sgate" supplied through the scanning line "gate" has become higher in potential to turn the first TFT 20 "on", the second TFT 30 is held in the "on" state. Consequently, in the luminescent element 40, the driving current keeps flowing from a pixel electrode 41 to an opposite electrode "op" in the direction indicated by the arrow "E" and consequently, the luminescent element 40 keeps emitting (the "on" state). On the other hand, when the image signal "data", which is lower than the intermediate between the potential of the common power supply line "com" and the potential of the opposite electrode "op", is written into the holding capacitor "cap" from the data line "sig", while the scanning signal "Sgate" supplied through the scanning line "gate" has become higher in its potential to turn the first TFT 20 "on", the second TFT 30 is turned "off" and consequently, the luminescent element 40 is turned "off" (the "off" state). In the above display apparatus 1A, a semiconductor thin film, an insulating thin film, an electrode, etc., which constitute each element, are formed by thin films deposited on the substrate. Considering the heat resistance of the substrate, a low-temperature process is often used to form the thin films. Therefore the quality of the thin film is poor, as is evidenced by the frequent defects caused by a physical-property difference between a thin film and a bulk, which result in problems such as an electrical breakdown, and wherein time-degradation is apt to arise in the TFT and similar devices. In the case of a liquid crystal display apparatus incorporating liquid crystals as light modulation elements, although it also uses the thin films, time-degradation can be suppressed not only in the liquid crystal but also in the TFT, because the light modulation element is driven by AC power. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, time-degradation is more often encountered in the TFT than in the liquid crystal display apparatus insofar as the apparatus is essentially driven by D.C. power. Although improvements have been made in the structure of the TFT and the process techniques in the display apparatus 1A, incorporating luminescent elements of the current controlling type, they do not yet seem to be improved enough. In the case of incorporating the liquid crystals as the light modulation elements, the power consumption is small because the light modulation element is controlled by the voltage which causes the current flow in each element to be only momentary. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, a constant driving current is required to keep the luminescent element "on", and this results in high power consumption and the risk of the frequent occurrence of electrical breakdown and time-degradation. Further, in the liquid crystal display apparatus, the liquid crystal can be AC-driven by one TFT per one pixel. On the other hand, in the display apparatus 1A incorporating luminescent elements of the current controlling type, the luminescent element 40 is DC-driven by two TFTs 20, 30 per one pixel. This raises the driving voltage, and exacerbates the aforementioned problems, such as electrical breakdown and time-degradation. For example, as shown in Fig. 20 (A), the gate voltage "Vgsw" of the first TFT, when selecting a pixel, corresponds to the potential difference between the potential equal to the higher potential of the scanning signal "Sgate" and the potential of the potential-holding electrode "st" (the potential of the holding capacitor "cap" or the potential of the gate electrode of the second TFT 30). Therefore when the potential of the potential-holding electrode "st" and, hence, the gate voltage "Vgcur" of the second TFT 30 are raised to make the luminescent element 40 emit in a high luminance, the gate voltage "Vgsw" of the first TFT 20 is lowered correspondingly. Therefore, the greater amplitude of the scanning signal "Sgate" has to be employed, requiring the higher driving voltage in the display apparatus 1A. Besides, in the aforementioned display apparatus 1A, since when the luminescent element 40 is "off", the potential of the image signal "data" is made lower than the intermediate potential between the potential of the common power supply line "com" and the potential of the opposite electrode "op" in order to turn the second TFT 30 "off", there is another problem of increased amplitude of the image signal "data". Accordingly, in this display apparatus 1A, special consideration for the power consumption and the withstanding of voltage of the TFT, etc. is needed compared to the liquid crystal display apparatus. However, the conventional display apparatus 1A has not been provided with sufficient consideration of these factors. Accordingly, an object of the present invention is to provide a display apparatus, which improves display image quality as well as suppresses power consumption, electric breakdown and deterioration with time by reducing the driving voltage, relying upon a driving method which takes into account the conduction types of TFTs used for controlling emission operations of the current-driven light-luminescent elements so as to reduce the driving voltage, which improves both the display image quality and characteristics such as power consumption, breakdown and time-degradation. EP 0112700A discloses a display device in which each pixel comprises first and second thin film transistors, a capacitor and a liquid crystal element. The features of the preamble of claim 1 are disclosed by this document. According to the present invention, there is provided a display apparatus comprising a substrate having arranged thereon: a plurality of scanning lines; a plurality of data lines intersecting the scanning lines; a plurality of common power supply lines; and a plurality of pixels formed in a matrix at the intersections of the scanning lines and the data lines, each of the pixels comprising: a first transistor having a first gate electrode which is arranged to be supplied with a scanning signal through a said scanning line; a holding capacitor which is arranged to hold an image signal supplied through a said data line via the first transistor; a second transistor having a second gate electrode which is arranged to be supplied with the image signal held by the holding capacitor; and a luminescent element having an emitting thin film which is provided between a pixel electrode provided for each of said pixels and an opposite electrode opposed to said pixel electrode, wherein the second transistor is arranged to be controlled by the image signal to electrically connect the pixel electrode to a said common power supply line so that a driving current flows between the pixel electrode and the opposite electrode and the luminescent element emits light; characterised by comprising: means arranged to supply an electrode of the holding capacitor, opposite to that electrically connected to the second gate electrode of the second transistor, with a pulse having a potential polarity opposite to the selecting pulse of the scanning signal and being delayed with respect to the application of the selecting pulse of the scanning signal. In this structure, since the writing of the image signals into the holding capacitor can be supplemented, the potential of the image signal applied to the gate electrode of the second transistor can be shifted in the direction to increase a luminance, without increasing the amplitude of the image signal. An organic semiconductor film can be used as the emitting thin films for example. The second transistor can be arranged to be driven in the saturated region to prevent an abnormal current from being generated in the luminescent element, which would result in the generation of a cross-talk, etc., at another pixel because of the voltage drop, or the like. Further, it is possible to prevent unevenness of the threshold voltage from influencing a display operation by arranging the second TFT to be driven in the linear region. Brief Description of the Drawings Fig. 1 is a plan view schematically illustrating a display apparatus in accordance with the present invention. Fig. 2 is a block diagram illustrating a basic construction of a display apparatus in accordance with the present invention. Fig. 3 is an exploded plan view illustrating a pixel in the display apparatus shown in the Fig. 2. Fig. 4 is a cross-sectional view taken on line A-A' of Fig. 3. Fig. 5 is a cross-sectional view taken on line B-B' of Fig. 3. Fig. 6(A) is a cross-sectional view taken on line C-C' of Fig. 3, and Fig. 6(B) is a schematic representation indicating an effect when the apparatus is constructed as shown in Fig. 6 (A). Fig. 7(A) and Fig. 7(B) are respective cross-sectional views of the luminescent elements utilized in the display apparatus shown in Fig. 2. Fig. 8(A) and Fig. 8(B) are respective cross-sectional views of the luminescent elements having a different structure from the pixels shown in Fig. 7. Fig. 9 is a graph showing a current-voltage characteristic of the luminescent elements shown in Fig. 7(A) and Fig. 8(B). Fig. 10 is a graph showing a current-voltage characteristic of the luminescent elements shown in Fig. 7(B) and Fig. 8(A). Fig. 11 is a graph showing current-voltage characteristics of an N channel type of TFT. Fig. 12 is a graph showing current-voltage characteristics of a P channel type of TFT. Fig. 13 is a flow sectional view illustrating a method for producing a display apparatus in accordance with the present invention. Figs. 14(A) and (B) are respectively a plan view and a cross-sectional view of the pixels having a different structure from the pixels shown in Figs. 3 through 6. Figs 15(A) and (B) are an equivalent circuit diagram illustrating a pixel of a display apparatus in accordance with the present invention, and a waveform chart indicating the signals to drive the pixel, respectively. Fig. 16 is a block diagram of the driving circuit at the scanning side to generate the signals shown in Fig. 15. Fig. 17 is a waveform chart indicating each signal generated from the driving circuit at the scanning side shown in Fig. 16. Fig. 18 is a block diagram of a display apparatus. Fig. 19 is an equivalent circuit diagram illustrating a conventional pixel construction in the display apparatus shown in Fig. 18. Figs. 20(A) and (B) are a waveform chart indicating the signals to drive the pixel shown in Fig. 19, and a schematic representation indicating correspondences between these signals and the equivalent circuit, respectively. Figs. 21 (A) and (B) are a block diagram indicating a construction to form a capacitor by using an adjacent gate line and a waveform of the gate voltage signals, respectively. Best Mode for Carrying Out of the Invention An embodiment of the invention will be described with reference to the drawings. Before showing the embodiment of the invention, reference will be made to the common structures , wherein the portions having common functions are designated by the same reference numerals, respectively, to avoid duplicate description. (The general construction of an active matrix substrate) Fig. 1 is a schematic block diagram schematically illustrating a general layout of a display apparatus, and Fig. 2 is an equivalent circuit diagram of an active matrix formed therein. As illustrated in Fig. 1, the center portion of a transparent substrate 10 as a basic body is a display portion 2 . In the peripheral region of the transparent substrate 10, at the upper and lower sides as viewed on the figure, an inspection circuit 5, and a driving circuit 3 at the data side, which outputs an image signal to a data line "sig", are formed respectively. And in the right and left sides as viewed on the figure, driving circuits 4 at the scanning side, which output scanning signals to a scanning line "gate", are formed. In each of the driving circuits 3,4, a complementary type TFT comprising a shift register circuit, a level shifter circuit, an analogue switch circuit, etc. is formed by an N type TFT and a P type TFT. A packaging pad 6, which is a terminal group for inputting the image signals, the various electric potentials and the pulse signals, is formed in the peripheral region on the transparent substrate 10, outside of the data side driving circuit 3. In the display apparatus 1, a plurality of the scanning lines "gate" and a plurality of the data lines "sig", which extend in a direction that intersects the direction in which the scanning lines "gate" extend, are formed on the transparent substrate 10, in the same way as in an active matrix substrate of a liquid crystal display apparatus. As shown in Fig. 2, many pixels 7 are formed in a matrix form by the crossing of the data lines "sig" and the scanning lines "gate". In any of the pixels 7, a first TFT 20 is formed, in which a scanning signal is supplied to a gate electrode 21 (a first gate electrode) through the scanning line "gate". One side of a source-drain region of the TFT 20 is electrically connected to the data line "sig", while the other side of the source-drain region is electrically connected to a potential-holding electrode "st". That is, a capacitor line "dine" is formed parallel to the scanning line "gate" and a holding capacitor "cap" is formed between the capacitor line "cline" and the potential-holding electrode "st". Accordingly, when the first TFT 20 is selected by the scanning signal and turned to "on", an image signal, which is supplied from the data line "sig" and forwarded through the first TFT 20, is written in the holding capacitor "cap". A gate electrode 31 (a second gate electrode) of the second TFT 30 is electrically connected to the potential-holding electrode "st". While one side of the source-drain region of the second TFT 30 is connected to a common power supply line "com", the other side of the source-drain region is electrically connected to one of electrodes (a pixel electrode, as will be referred to below) of a luminescent element 40. The common power supply line "com" is held at a constant potential. When the second TFT 30 is turned "on", the electric current from the common power supply line "com" flows to illuminate the luminescent element 40 through the second TFT 30, and makes the luminescent element 40 emit. In the display apparatus 1 constructed as stated above, since the driving current flows through the current route formed by the luminescent element 40, the second TFT 30 and the common power supply line "com", the flow of the current stops when the second TFT 30 is turned "off" . However, in the display apparatus 1, when the first TFT 20 is selected by the scanning signal and turned "on", the image signal, which is supplied from the data line "sig" and forwarded through the first TFT, is written into the holding capacitor "cap". Accordingly, since the potential of the gate electrode of the second TFT 30 is held at equal to that of the image signal by the holding capacitor "cap", even if the first TFT 20 is turned "off", the second TFT 30 remains "on". Therefore, the driving current in the luminescent element 40 keeps flowing and this pixel remains illuminated. This state will be maintained until new image data is written into the holding capacitor "cap" and the second TFT 30 turns "off". In the display apparatus 1, various arrangements are possible with the common power supply line "com", the pixel 7 and the data line "sig". As an example, a plurality of the pixels 7, having the luminescent elements 40 power supply, in which the driving current is supplied via common power supply line "com", are disposed on both sides of the common power supply line "com". Two data lines "sig" are arranged at the side of these pixels 7 opposite to the common power supply line "com". That is, a unit including data line "sig", a pixel group connected to this data line, a piece of common power supply line "com", another pixel group connected to this common power supply line and another data line "sig" which supplies the pixel signals to this pixel group, is repeated in the direction that the scanning line "gate" extends. Each common power supply line "com" supplies driving currents to two lines of the pixels 7. In this example, in the each of two pixels 7 provided with the common power supply line "com" therebetween, the first TFTs 20, the second TFTs 30 and the luminescent elements 40 are disposed symmetrically with respect to the common power supply line "com" in order to make the electrical connection between these elements and each of wiring layers easier. Thus, in this example, since a piece of the common power supply line "com" drives two lines of the pixels, only one half of the number of the common power supply lines "com" is needed compared with the number of the common power supply lines "com" when each of them is formed per one line of the pixels, and also the space reserved between the common power supply line "com" and the data line "sig" formed in the same layer can be eliminated. Therefore, the area for wiring can be reduced on the transparent substrate 10, and consequently, the display performance can be improved in terms of luminance, contrast ratio and so forth. In addition, since the common power supply lines are arranged such that each common power supply line "com" is connected to two lines of the pixels, each two data lines "sig" are arranged side-by-side and supply the image signal to each line of the pixels group. (The pixel construction) The structure of each pixel 7 in the display apparatus 1 formed as described above will be explained in detail with reference to Figs. 3 to 6. Fig. 3 is an enlarged plan view showing three pixels 7 of a plurality of pixels 7 formed in the display apparatus 1 . Fig. 4, Fig. 5 and Fig. 6 are section views taken on line A-A', B-B' and C-C' of Fig. 3, respectively. First, at the position taken on line A-A' in Fig. 3, a silicon film 200, shaped like islands, is formed on the transparent substrate 10 per each pixel 7 to form the first TFT 20, as shown in Fig. 4. A gate insulating film 50 is formed on the surface of the silicon film 200. The gate electrode 21 (a portion of the scanning line "gate") is formed on the surface of the gate-insulating film 50. The source-drain regions 22 and 23 are formed by self-alignment with respect to the gate electrode 21. On the front side of the gate insulating film 50, a first inner-layer-insulating film 51 is formed. The source drain-regions 22 and 23 are electrically connected to the data line "sig" and the potential-holding electrode "st", respectively, through contact holes 61 and 62 formed in the inner-layer-insulating film. In each pixel 7, in parallel with the scanning line "gate", the capacitor line "dine" is formed in the same inner-layer as the scanning line "gate" and the gate electrode 21 (between the gate-insulating film 50 and the first inner-layer-insulation film 51). An extension "st1" of the potential-holding electrode "st" overlays on this capacitor line "dine" through the first inner-layer-insulation thin film 51. In this way, the capacitor line "cline" and the extension "st1" of the potential holding electrode "st" form the holding capacitor "cap" incorporating the first inner-layer-insulation film 51 as a dielectric film. On the front side of the potential-holding electrode "st" and on that of the data line "sig", a second inner-layer-insulation thin film 52 is formed. At the position indicated by line B-B' in Fig. 3, as shown in Fig. 5, two of the data lines "sig", which correspond to each pixel 7, are arranged in parallel on the surfaces of the first and the second inner-layer-insulation films 51 and 52 formed on the transparent substrate 10. At the position indicated by line C-C' in Fig. 3, as shown in Fig. 6 (A), a silicon film 300, shaped like islands, is formed on the transparent substrate 10 to form the second TFT 30, spreading across two pixels, which sandwich the common power supply line "com" therebetween. The gate insulating film 50 is formed on the surface of the silicon film 300. The gate electrodes 31, corresponding to each pixel 7, are formed on the surface of the gate insulating film 50, sandwiching the common power supply line "com", and the source-drain regions 32 and 33 are formed by self-alignment with respect to the gate electrodes 31. On the front side of the gate insulating film 50, the first inner-layer-insulation film 51 is formed. A source drain-region 32 is electrically connected to a junction electrode 35 through a contact hole 63 formed on the first inner-layer-insulation film 51. On the other hand, the common power supply line "com" is electrically connected to a portion of the source-drain region 33, which is common between two of the pixels provided at the center of the silicon film 300, through a contact hole 64 of the first inner-layer-insulation film 51. On the surfaces of the common power supply line "com" and the junction electrode 35, the second inner-layer-insulation film 52 is formed. On the surface of the second inner-layer-insulation film 52, a pixel electrode 41 is formed that includes an ITO film. This pixel electrode 41 is electrically connected to the junction electrode 35 through a contact hole 65 formed on the second inner-layer-insulation film 52, and then electrically connected to the source drain region 32 of the second TFT 30 through the junction electrode 35. (Characteristics of the luminescent element) Since any type of structures of the luminescent element 40 can be used in the apparatus of the invention, a typical structure will be described below. First, the pixel electrode 41 comprising the ITO film constitutes one electrode (the positive electrode) of the luminescent element 40, as shown in Fig. 7 (A). On the surface of the pixel electrode 41, a hole injection layer 42 and an organic semiconductor film 43, as an emitting thin film, are laminated. Further, an opposite electrode "op" (the negative electrode) comprising a metal film, such as a lithium-containing aluminum film or a calcium film, is formed on the surface of the organic semiconductor film 43. This opposite electrode "op" is to be a common electrode formed entirely, or in striped patterns, on the transparent substrate 10, and is held at a constant potential. In contrast, when the driving current flows in the reverse direction to the luminescent element 40 that is shown in Fig. 7 (A), the luminescent element 40 may be formed as shown in Fig. 7 (B). In this structure, the pixel electrode 41 (the negative electrode) comprising the ITO film, the lithium-containing aluminum electrode 45, which is very thin to be almost transparent, the organic semiconductor layer 43, the hole injection layer 42, the ITO film layer 46 and the opposite electrode "op" (the positive electrode), comprising a metal film such as the aluminum containing lithium film or the calcium film, are laminated in this order from bottom to top layer. In this structure, even in the case that the driving current of opposite polarity flows in each of the luminescent elements 40 shown in Figs. 7 (A) and (B), the emitting characteristics of the elements 40 are not changed since the structure of the electrode layers, with which the hole injection layer 42 and the organic semiconductor layer 43 contact directly, are the same structures as the former. Any of the luminescent elements 40 shown in Figs. 7 (A) and (B) has the pixel electrode 41 comprising the ITO film in the under-layer side (the substrate side). Light is emitted from the backside of the transparent substrate 10 through the pixel electrode 41 and the transparent substrate 10, as shown by the arrow "hν". In contrast, when the luminescent element 40 is formed as shown in Figs. 8 (A) and (B), light is emitted from the front side of the transparent substrate 10 through the opposite electrode "op", as shown by the arrow "hν". That is, as shown in Fig. 8 (A), the organic semiconductor layer 43 and the hole injection layer 42 are laminated on the surface of the pixel electrode 41 (the negative electrode) comprising a metal film, such as aluminum containing lithium. Further, the opposite electrode "op" comprising the ITO film (the positive electrode) is formed on the surface of the hole injection layer 42. This opposite electrode "op" is also a common electrode formed entirely, or in striped patterns, and is held at a constant potential. In contrast, in order to flow the driving current in the reverse direction of the luminescent element that is shown in Fig. 8 (A), the luminescent element 40 may be formed as shown in Fig. 8 (B). This luminescent element 40 is formed by the pixel electrode 41 (the positive electrode) comprising the metal thin film such as aluminum containing lithium, the ITO film layer 46, the hole injection layer 42, the organic semiconductor layer 43, the lithium-containing aluminum electrode 45, which is very thin to be almost transparent, and the opposite electrode "op" (the negative electrode), comprising the ITO film, which are laminated from bottom to top in this order. When forming any type of structures of the luminescent element 40, the manufacturing process is not complicated, even if the top-and-bottom positional relationship is reversed, provided that the hole injection layer 42 and the organic semiconductor layer 43 are formed inside of a bank layer "bank" by an ink jet method as described below. Further, in the case that the lithium-containing aluminum electrode 45, which is very thin to be almost transparent, and the ITO film layer 46 are added, there is no obstacle to displaying images, even if the lithium-containing aluminum electrode 45 is laminated in the same region of the pixel electrode 41 or if the ITO film 46 is laminated in the same region of the opposite electrode "op". Therefore, the lithium-containing aluminum electrode 45 and the pixel electrode 41 can be patterned, either separately or simultaneously, using the same resist-mask. Similarly, the ITO film layer 46 and the opposite electrode "op" can be patterned, either separately or simultaneously, using the same resist-mask. The lithium-containing aluminum electrode 45 and the ITO film layer 46 may be formed only at the inside region of the bank layer "bank" as a matter of course. Further, the opposite electrode "op" may be formed by the ITO film, and the pixel electrode 41 may be formed by the metal film. In any case, light is emitted from the transparent ITO film. The voltage is applied across the opposite electrode "op" as the positive electrode, and the pixel electrode 41 as the negative electrode, of the luminescent element 40 formed as described above. As shown in Fig. 9 (ampere-volt characteristics of the luminescent element 40 shown in Fig. 7 (A) and Fig. 8(B)), and Fig. 10 (ampere-volt characteristics of the luminescent element 40 shown in Fig. 7 (B) and Fig. 8(A)), the current through the organic semiconductor layer 43 (the driving current) increases suddenly in the region where the applied voltage (x-axis/the potential of the opposite electrode "op" to the pixel electrode 41) rises above the threshold value and there is "on-state", i.e., the low resistance state. Consequently, the luminescent element 40 emits light as the electro-luminescence element or as the LED element. This emitting light from the luminescent element 40 is reflected by the opposite electrode "op" and is emitted through the transparent pixel electrode 41 and the transparent substrate 10. In contrast, in the region where the applied voltage (x-axis/the potential of the opposite electrode "op" to the pixel electrode 41) drops below the threshold voltage, the "off-state", i.e., the high resistance state is provided, and the current through the organic semiconductor layer 43 (the driving current) stops. Consequently the light luminescent element 40 is turned "off". The threshold voltages in the examples shown in Figs. 9 and 10 are approximately +2 V and approximately -2 V, respectively. Although the light emitting efficiency tends to somewhat decline, the hole injection layer 42 may be omitted. There may be a case that without incorporating the hole injection layer 42, an electron injection layer is formed at the opposite position to where the hole injection layer 42 is formed with respect to the organic semiconductor layer 43. Further, both the hole injection layer 42 and the electron injection layer may be incorporated. (TFT Characteristics) As the TFTs (the first TFT 20 and the second TFT 30 shown in Fig. 2) for controlling light emission of the luminescent element 40 that is formed as described above, the ampere-volt characteristics of N channel type and P channel type TFTs are shown in Figs. 11 and 12, respectively (in any of the Figures, examples of the drain voltages are 4V and 8V are shown). As understood by these Figures, the TFT operates "ON-OFF" control action depending on the gate voltage applied across the gate electrode. That is, when the gate voltage rises over the threshold voltage, the TFT will be in "on-state" (the low resistance state) to increase the drain current. In contrast, when the gate voltage decreases below the threshold voltage, the TFT will be "off-state" (the high resistance state) to reduce the drain current. (A Method of Producing the Display Apparatus) In a method of producing the display apparatus 1 that is formed as described above, the steps up to the formation of the first TFT 20 and the second TFT 30 on the transparent substrate 10 are almost the same as the steps for producing the active matrix substrate of the liquid crystal display apparatus 1. Accordingly, a general description will be made simply with reference to Fig. 13. Fig. 13 is a diagrammatic flow sectional view illustrating the steps for forming each component of the display apparatus 1 under a temperature condition below 600 °C. As shown in Fig. 13 (A), a groundwork protection film (not shown in figure) comprising a silicon oxide film ranging about 2000 to 5000 Å in thickness is formed, as needed, on the transparent substrate 10 by a plasma enhanced CVD method utilizing TEOS (tetraethoxysilane) or oxygen gas, etc., as a material. Then, after the substrate temperature is set at 350 °C, a semiconductor film 100 comprising an amorphous silicon film ranging about 300 to 700 Å thick is formed on the surface of the groundwork protection film by a plasma enhanced CVD method. Next, the semiconductor film 100 comprising the amorphous silicon film is subjected to crystallization such as laser annealing or a solid-phase growth method to crystallize the semiconductor film 100 into a poly-silicon film. The laser annealing utilizes, for example, an excimer laser line beam having a long side of 400 mm, and its output power is, for example, 200 mJ/cm 2 . The line beams are scanned so that the line beams overlap with each other at portions corresponding to 90% of the peak laser power in the short side. Then, as shown in Fig. 13 (B), by a patterning, the semiconductor film 100 is formed into semiconductor films 200, 300, shaped as islands, and on the surface of them, the gate insulating film 50 comprising a silicon oxide film or a silicon nitride film, ranging about 600 to 1500 Å in thickness, is formed by a plasma enhanced CVD method utilizing TEOS (tetraethoxysilane) or oxygen gas, etc., as a material. Next, as shown in Fig.13 (C), after a conductive film comprising a metal film, such as aluminum, tantalum, molybdenum, titanium, tungsten, etc., is formed by a sputtering method, the gate electrodes 21 and 31, as portions of the scanning lines "gate", are formed by a patterning. In this step, the capacitor line "cline" is also formed. In the figure, reference number 310 indicates an extensional part of the gate electrode 31. At this state, by an implantation of impurities, such as high concentration phosphorus ions or boron ions, etc., the source-drain regions 22, 23, 32, and 33 are formed by self-alignment with respect to the gate electrodes 21 and 31 on the silicon films 200 and 300. The portions where the impurity is not implanted are channel regions 27 and 37. A different conduction type TFT may be formed on the same substrate, as will be described later. In this case, in the impurity implantation step, the impurity implantation will be performed masking a region to form the opposite conduction type TFT. Then, as shown in Fig. 13 (D), after the inner-layer-insulation film 51 is formed, the contact holes 61,62,63,64 and 69 are formed, and then the data line "sig", the potential-holding electrode "st" having the extended portion "st1" overlapped with the capacitor line "cline" and with the extended portion 310 of the gate electrode 31, the common power supply line "com", and the junction electrode 35 are formed. Consequently, the potential-holding electrode "st" is electrically connected to the gate electrode 31 through the contact hole 69 and the extended portion 310. As mentioned above, the first TFT 20 and the second TFT 30 are formed. Further, the holding capacitor "cap" is formed by the capacitor line "cline" and the extended portion "st1" of the potential-holding electrode "st". Next as shown in Fig. 13 (E), the second inner-layer-insulation film 52 is formed, and the contact hole 65 is formed at the place corresponding to the junction electrode 35 in this inner-layer insulation film. Then, after the conductive film is formed all over the surface of the second inner-layer insulation film 52, patterning is performed, and the pixel electrode 41 is formed to electrically connect the conductive film to the source-drain region 32 of the second TFT 30 through the contact hole 65. Next as shown in Fig. 13 (F), after a black resist layer is formed on the front side of the second inner-layer insulation film 52, a bank layer "bank" is formed, leaving this resist to surround the regions for forming the organic semiconductor film 43 of the luminescent element 40 and the hole injection layer 42. In either case, where the organic semiconductor film 43 is formed in a box shape independently per pixel, or formed in a stripe shape along the data line "sig", this producing process can be applied only by forming the bank layer "bank" in a shape adapted thereto. Liquid material(a precursor), for forming the organic semiconductor film 43, is injected into an inner region of the bank layer "bank" from an inkjet-head "IJ" to form the organic semiconductor film 43 in the inner region of the bank layer "bank". Similarly, liquid material (a precursor) for forming the hole injection layer 42 is injected into an inner region of the bank layer "bank" from the inkjet-head "IJ" to form the hole injection layer 42. As seen from the description presented above concerning the construction of the luminescent element with reference to Figs. 7 (A) and (B) and Figs. 8 (A) and (B), an order of steps to form the organic semiconductor film 43 and the hole injection layer 42 may be interchangeable depending on the structure. Since the bank layer "bank" comprises the resist, the layer is water repellent. In contrast, since the precursors of the organic semiconductor film 43 and the hole injection layer 42 utilize a hydrophilic solvent, the coating region of the organic semiconductor film 43 is strictly defined by the bank layer "bank", and the region cannot extend off to an adjacent pixel. When the bank layer "bank" is formed at a sufficient height, the organic semiconductor film 43 or the hole injection layer 42 can be formed within a predetermined region by a coating method, such as a spin coating method, even if the ink jet method is not employed. In this example, in order to improve the production efficiency in forming the organic semiconductor film 43 or the hole injection layer 42 by the ink jet method, as shown in Fig. 3, the forming regions of the organic semiconductor films 43 have the same inter-center pitch P between adjacent pixels 7 lying along the extending direction of the scanning line "gate". Therefore, as indicated in the arrow "Q", there is an advantage that the material of the organic semiconductor film 43, etc. can be injected by an ink jet head "IJ" simply with the same pitch along the extending direction of the scanning line "gate". The same pitch injection also simplifies a device for transferring the ink jet head "IJ" while facilitating the improvement of injection accuracy. Afterward, as shown in Fig. 13 (G), the opposite electrode "op" is formed on the front side of the transparent substrate 10. The opposite electrode "op" may be formed either on the entire surface or in a striped shape. In the latter, the patterning will be performed after the film is formed on the entire front side of the transparent substrate 10, and then patterning it into the striped shape. The TFTs are also formed in the driving circuit at the data side 3 or the driving circuit at the scanning side 4, as shown in Fig. 1. This forming process of the TFTs employs all or a part of the steps for the TFT formation in the above described pixel 7. Therefore, the TFTs of the driving circuit are provided in the same inner-layer that the TFTs of the pixel 7 are formed in. In this example, since the bank layer "bank" comprises a black and insulating resist, the resist is left as it is to be utilized as a black matrix "BM" and an insulating layer for reducing a parasitic capacitance. As shown in Fig. 1, the bank layer "bank" is also formed in the peripheral region of the transparent substrate 10 (hatched area in the figure). Hence, as the driving circuit at the data side 3 as well as the driving circuit at the scanning side 4 is overlaid by the bank layer "bank", the bank layer "bank" is disposed between the wiring layer of the driving circuit and the opposite electrode "op", even if the opposite electrode "op" and the forming regions of these driving circuits are overlapped. Therefore, the prevention of the driving circuits 3,4 from the parasitic capacitance can be achieved so as to reduce the load of the driving circuit at the data side 3, resulting in providing reduced electric consumption or speeding up the display operation. In this example, as shown in Figs. 3 through 5, the bank layer "bank" is formed so as to overlap with the data line "sig". Thus, the bank layer "bank" is disposed between the data line "sig" and the opposite electrode "op", and consequently, it is possible to prevent the parasitic capacitance in the data line "sig". This results in the reduction of the load of the driving circuit, providing a reduction of the electric consumption or a speeding up of the display operation. Further, in this example, as shown in Fig. 3, Fig. 4 and Fig. 6(A), the bank layer "bank" is preferably also formed in the region where the pixel electrode 41 and the junction electrode 35 overlap. That is, as shown in Fig. 6(B), if the bank layer "bank" is not formed at the region where the pixel electrode and the junction electrode 35 overlap, even when the organic semiconductor film 43 emits light by the driving current across the pixel electrode 51 and the opposite electrode op, the light cannot be emitted and does not contribute to the display operation. This is because the light gets in between the junction electrode 35 and the opposite electrode "op". The driving current equivalent to the light, that does not contribute to the display operation, may be called an ineffective current with respect to display. In this example, however, since the bank layer "bank" is formed in the region where the ineffective current is to flow to prevent the ineffective current, the waste current in the common power supply line "com" can be prevented. Hence, the width of the common power supply line "com" can be reduced accordingly. As discussed above, if the bank layer "bank" that includes a black resist is reserved, the bank layer "bank" works as a black matrix to improve the display image quality, such as a luminance and a contrast ratio. That is, in the display apparatus according to this example, since the opposite electrode "op" is formed in a striped shape on the entire surface, or on a broad region, of the front side of the transparent substrate 10, reflected light from the opposite electrode "op" reduces the contrast ratio. However, in this example, since the bank layer "bank", which prevents the parasitic capacitance, includes a black resist, with the forming region of the organic semiconductor film 43 defined, there is an advantage that the bank layer "bank", working also as the black matrix, blocks useless light reflected from the opposite electrode "op", and it results in increasing the contrast ratio. Further, since the emitting region can be defined by self-alignment utilizing the bank layer "bank", a margin for alignment required for the emitting region is not necessary. This margin has been the problem when another metal layer, etc., is used as the black matrix instead of the bank layer "bank". (Another structure of the active matrix substrate) The present invention can be applied to various types of active matrix substrates as well as the above described structure. For example, the invention can be applied to the display apparatus 1A, wherein, as described in reference to Fig. 18, a unit comprising a data line "sig", a common power supply line "com" and a line of the pixels 7 is repeated in the direction of the scanning line "gate" on a transparent substrate 1. The holding capacitor "cap" may be formed between the common power supply line "com" and the potential-holding electrode "st" without the capacitor line. In this case, as shown in Figs. 14 (A) and (B), an extended portion 310 of the gate electrode 31 to connect electrically the potential-holding electrode "st" and the gate electrode 31, is expanded to the under-layer of the common power supply line "com" to form the holding capacitor "cap". This holding capacitor "cap" has the first inner-layer-insulation film 51 which is located between the extended portion 310 and the common power supply line "com" for a dielectric film. As for the holding capacitor "cap" further, Figure 14 is abbreviated, it may be formed utilizing a poly-silicon film for forming the TFT, and it also may be formed with the ahead scanning line other than the capacitor line and the common power supply line. As will be described with reference to Figs. 15(A) and (B), a pulse may be supplied to one of electrodes of the holding capacitor "cap". The electrode receiving the pulse is opposite to the other, which is electrically connected to the gate electrode of the second TFT 30. The potential of this pulse is opposite to the selecting pulse of the scanning signal "gate", and the pulse is supplied to the electrode with a delay behind the selecting pulse. In this embodiment of the invention, as shown in Fig. 15(A), one of the electrodes of the holding capacitor "cap", which is opposite to the one which is electrically connected to the gate electrode of the second TFT 30 through the potential-holding electrode "st", is formed by the capacitor line "cline", which is extended in parallel with the scanning line "gate". As shown in Fig. 15(B), this capacitor line "cline" is formed such that the potential "stg" is supplied to the capacitor line "cline" with a delay behind the selecting pulse "Pgate" of the scanning signal "Sgate". The potential "stg" supplied to the capacitor line "cline" includes the pulse signal "Pstg", and the polarity of which is opposite to the polarity of the selecting pulse. After the selecting pulse becomes non-selective, the pulse signal "Pstg" shifts the potential of the image signal "data" utilising a capacitive coupling of the holding capacitor "cap". Therefore, signals are held in the holding capacitor "cap" at the "off-state" pixel 7, corresponding to the potential obtained by adding the potential of the pulse signal "Pstg" to the potential of the image signal "data". Due to the high "on" resistance of the first TFT 20, it is difficult to completely write the signals in the higher potential side of the image signals "data" within a limited time. In the case of the embodiment, a shortage of the writing capacity results in no emitting of the pixel. However, in accordance with the embodiment 4, it is possible to supplement the writing of the image signal "data" to the holding capacitor "cap", even though the maximum range of the potential in the driving signal is not expanded. When the pulse signal "Pstg" is stored in the capacitor line "cline", as shown in Fig. 16 the "cline" is extracted from the driving circuit 4 in the scanning side. At the same time, in the driving circuit 4 of the scanning side, the output signal from a shift register 401 is outputted to any of gate stages, as the scanning signal "Sgate", through NAND gate circuit and an inverter. On the other hand, the output signal from the shift resistor 401 is outputted to the capacitor line "cline", through the NAND gate circuit and the two staged inverter, with a delay behind the scanning signal, shifting the power level in the higher potential side from "Vddy" to "Vccy", as shown in Fig. 17 In the above mentioned embodiment the type of luminescent element having the capacitor line "cline" was described. However, since this embodiment is not limited to this structure having the capacitor line "cline", it is also possible to form one of the electrodes of the holding capacitor by the adjacent gate line. As an example of these structures, Figs. 21 (A) and (B) illustrate a circuit block diagram and a voltage waveform chart of the gate electrode in the direction of the scanning of the gate line respectively. There is an advantage that it is possible to avoid taking the trouble to form the capacitor line "cline", by forming the gate line, which is adjacent to the pixels, as the one of the electrodes of the holding capacitor. [Other Embodiments for Carrying Out the Invention] In the aforementioned embodiment, a region in the ampere-volt characteristic where the second TFT 30 is operated has not been described. If the second TFT 30 is operated at the saturated region, it is possible to prevent an abnormal current flow in the luminescent element 40, utilizing a weak constant-current characteristic. For example, the organic semiconductor film, etc., forming the luminescent element 40 can possibly have pinhole defects, even though this does not cause a complete short circuit across the electrodes of the luminescent element 40, due to the restricted current in the luminescent element with the defect. If the second TFT 30 is operated at the linear region, it is possible to prevent the display operation from being affected by unevenness of the threshold voltage. In addition, the TFT may be formed in a bottom gate type as well as in a top gate type, and the production method is not limited to a low temperature process in producing the TFT.

Claims: A display apparatus (1) comprising a substrate having arranged thereon: a plurality of scanning lines (gate); a plurality of data lines (sig) intersecting the scanning lines; a plurality of common power supply lines (com); and a plurality of pixels formed in a matrix at the intersections of the scanning lines and the data lines, each of the pixels comprising: a first transistor (20) having a first gate electrode (21) which is arranged to be supplied with a scanning signal through a said scanning line; a holding capacitor (cap) which is arranged to hold an image signal supplied through a said data line via the first transistor; a second transistor (30) having a second gate electrode (31) which is arranged to be supplied with the image signal held by the holding capacitor; and a luminescent element (40) having an emitting thin film which is provided between a pixel electrode provided for each of said pixels and an opposite electrode opposed to said pixel electrode, wherein the second transistor is arranged to be controlled by the image signal to electrically connect the pixel electrode to a said common power supply line so that a driving current flows between the pixel electrode and the opposite electrode and the luminescent element emits light; characterised by comprising: means (401) arranged to supply an electrode of the holding capacitor, opposite to that electrically connected to the second gate electrode of the second transistor, with a pulse having a potential polarity opposite to the selecting pulse of the scanning signal and being delayed with respect to the application of the selecting pulse of the scanning signal. A display apparatus according to claim 1, wherein the emitting thin film is an organic semiconductor film. A display apparatus according to claim 1 or claim 2, wherein the second transistor is arranged to be driven in a saturated region. A display apparatus according to any one of the preceding claims, wherein the second transistor is arranged to be driven in a linear region.

Inventors: KIMURA MUTSUMI, OZAWA TOKUROH, KIMURA, MUTSUMI, OZAWA, TOKUROH
Applicants: SEIKO EPSON CORP, SEIKO EPSON CORPORATION
Application year: 1998
Main classification: G09G3
IPC codes: G09G3, H01L51, H01L27