Abstract:
A source of a driving transistor for driving an organic EL element functions as a first capacitance electrode layer. A second capacitance electrode layer is formed on the source through a gate insulating film of the driving transistor. The second capacitance electrode layer is formed with the same layer and by the same process as the gate electrode. A third capacitance electrode layer is formed extending over the second capacitance electrode layer through the interlayer insulating film. The third capacitance electrode layer is formed with the same layer as the drain electrode and the drain signal line. The third capacitance electrode layer is connected to the source of the driving transistor. The forming area of the storage capacitance element for holding the video signal supplied to the gate of the driving transistor can be thus reduced, improving display quality as well as extending life span of the organic El element.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to an electroluminescent display device, especially to an electroluminescent display device with a storage capacitance element for holding a video signal supplied to a gate of a driving transistor. 
   2. Description of the Related Art 
   An electroluminescent (referred to as EL hereinafter) display device with an EL element has been gathering attention as a display device substituting a CRT or an LCD. The development effort for the EL display device with a thin film transistor (referred to as TFT hereinafter) as a switching element for driving the EL element has been made accordingly. 
     FIG. 4  is an equivalent circuit diagram of one pixel of an organic EL display device. A gate signal line  51  for supplying a gate signal Gn and a drain signal line  52  for supplying a drain signal, a video signal Dm, cross each other. 
   An organic EL element  60 , a driving TFT  40  for driving the organic EL element  60 , and a pixel selecting TFT  30  for selecting the pixel are disposed near the crossing of the two signal lines. The TFT  40  is P-channel type and the TFT  30  is N-channel type. 
   A drain  43   d  of the organic El element driving TFT  40  is provided with a plus source voltage PVdd. A source  43   s  of the TFT  40  is connected to an anode  61  of the organic EL element  60 . 
   The gate signal line  51  is connected to a gate  31  of the pixel selecting TFT  30  and provided with the gate signal Gn. The drain signal line  52  is connected to a drain  33   d  of the pixel selecting TFT  30  and provided with the video signal Dm. A source  33   s  of the TFT  30  is connected to a gate  41  of the TFT  40 . The gate signal Gn is outputted from a gate driver circuit not shown in the figure, and the video signal Dm is outputted from a drain driver circuit not shown in the figure. 
   Also, the organic EL element  60  includes the anode  61 , a cathode  65 , and an emissive layer  63  inserted between the anode  61  and the cathode  65 . The cathode  65  is provided with a minus source voltage CV. 
   A storage capacitance element  130  is connected to the gate  41  of the TFT  40 . That is, one of the electrodes of the storage capacitance element  130  is connected to the gate  41 , and the other electrode is connected to a storage capacitance electrode  131 . The storage capacitance element  130  is disposed in order to hold the video signal Dm of the display pixel for one field period by keeping the electric charge corresponding to the video signal Dm. 
   The operation of the EL display device with the above configuration is as follows. The TFT  30  turns on when the gate signal Gn becomes high level for one horizontal period. Then, the video signal Dm is supplied from the drain signal line  52  to the gate  41  of the TFT  40  through the TFT  30 . The conductance of the TFT  40  changes according to the video signal Dm supplied to the gate  41  and the corresponding driving electric current is applied to the organic EL element  60  through the TFT  40 . Thus, the organic EL element  60  emits light. 
     FIG. 5  shows a cross-sectional view of the storage capacitance element  130  mentioned above. The TFT  30  is formed on an insulating substrate  10 . The TFT  30  has the source  33   s , the drain  33   d , and the gate  31  formed on a gate insulating film  12 . The storage capacitance electrode  131  is formed on the source  33   s  of the TFT  30  through the gate insulating film  12 . The storage capacitance electrode  131  is provided with a predetermined stable voltage. 
   The storage capacitance element  130  is disposed for each of the pixels, in the conventional organic El display device, in order to maintain the voltage applied to the gate of the driving transistor for controlling the quantity of the electric current, which determines the luminescence of the organic El element  60 . 
   When the voltage of the video signal Dm supplied to the pixel drops by a large quantity, it will affect the quality of the display. Therefore, a large capacitance value of the storage capacitance element  130  is required. That is, the area of the storage capacitance element  130  should be large. 
   There are a top emission type and a bottom emission type among the organic EL display devices. The light emitted from the organic EL element  60  radiates from the side of the organic EL element  60  opposite from the insulting substrate  10  in the top emission type display device. That is, the light radiates from the upper surface of the panel. On the other hand, the light emitted from the organic EL element  60  radiates from the side of the insulting substrate  10  in the bottom emission type display device. 
   When the area of the storage capacitance element  130  is large, it does not create any problem in the top emission type organic EL display device. However, the portion where the storage capacitance element is formed works as a blind in the bottom emission type display device, leading to decreased open aperture. In this configuration, the electric current supplied to the organic El element should be increased in order to acquire the necessary luminescence, compared to the case where the device has an enough open aperture. As a result, the life span of the organic EL element is shortened. 
   SUMMARY OF THE INVENTION 
   The invention provides an electroluminescent display device having a plurality of pixel portions. Each of the pixel portions includes an electroluminescent element, a driving transistor driving the electroluminescent element, a drain signal line, and a pixel selecting transistor supplying a signal from the drain signal line to a gate of the driving transistor. The pixel portion also includes a storage capacitance element holding the signal supplied to the gate of the driving transistor. The storage capacitance element includes an extension of a source of the pixel selecting transistor as a first capacitance electrode layer, a second capacitance electrode layer disposed above the extension of the source and a third capacitance electrode layer connected to the source and disposed above the second capacitance electrode layer. 
   The invention also provides an electroluminescent display device having a plurality of pixel portions. Each of the pixel portions includes an electroluminescent element having an anode layer, an emissive layer and a cathode layer, a driving transistor driving the electroluminescent element, a drain signal line, and a pixel selecting transistor supplying a signal from the drain signal line to a gate of the driving transistor. The pixel portion also includes a storage capacitance element holding the signal supplied to the gate of the driving transistor. The storage capacitance element includes an extension of a source of the pixel selecting transistor as a first capacitance electrode layer, a second capacitance electrode layer disposed above the extension of the source, a third capacitance electrode layer connected to the source and disposed above the second capacitance electrode layer, a fourth capacitance electrode layer connected to the third capacitance electrode layer and disposed above the third capacitance electrode layer and a fifth capacitance electrode layer disposed above the fourth capacitance electrode layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of one pixel portion of an organic EL display device of a first embodiment of this invention. 
       FIGS. 2A and 2B  are cross-sectional views of the pixel portion of  FIG. 1 . 
       FIGS. 3A and 3B  are cross-sectional views of a pixel portion of an organic EL display device of a second embodiment of this invention. 
       FIG. 4  is an equivalent circuit diagram of a pixel portion of a conventional organic El display device. 
       FIG. 5  is a cross-sectional view of the pixel portion of  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A first embodiment of this invention will be explained hereinafter.  FIG. 1  is a plan view showing a pixel portion of an organic EL display device.  FIG. 2A  is a cross-sectional view of one pixel portion along the A-A line and,  FIG. 2B  is a cross-sectional view of the pixel portion along the B-B line in  FIG. 1 . The equivalent circuit diagram of the pixel portion is the same as shown in  FIG. 4 . 
   The pixel portion  115  is formed in the region surrounded with a gate signal line  51  and a drain signal line  52 , as shown in  FIGS. 1 ,  2 A and  2 B. A plurality of pixel portions is disposed in a matrix configuration, forming a display region. 
   An organic EL element  60 , which is a self emissive element, a pixel election TFT  30  for controlling the timing of supplying electric current to the organic EL element  60 , an organic El element driving TFT  40  for supplying electric current to the organic EL element  60 , and a storage capacitance element  130 A are disposed in the pixel portion  115 . The organic EL element  60  includes an anode layer  61 , an emissive layer made of an emissive material and a cathode layer  65 . 
   The pixel selecting TFT  30  is disposed near the crossing of a gate signal line  51  a drain signal line  52 . A source  33   s  of the TFT  30  works also as a first capacitance electrode layer  55 , and is connected to a gate  41  of the TFT  40 . A second capacitance electrode layer  54  is formed above the source  33   s  of the TFT  30  through a gate insulating film  12 . The second capacitance electrode layer  54  is made of chrome or molybdenum, and disposed parallel to the gate signal line  51 . Also, a third capacitance electrode layer  70  is formed above the second capacitance electrode layer  54  through an interlayer insulating film  15 . 
   A source  43   s  of the organic EL element driving TFT  40  is connected to the anode layer  61  of the organic EL element  60 , and a drain  43   d  is connected to a driving source line  53 , which is an electric source supplied to the organic EL element  60 . 
   The organic EL display device includes the TFTs and the organic EL element deposited sequentially on an insulating substrate  10 , which is either a substrate made of a glass, a synthetic resin, a conductive material or a semiconductor, as shown in  FIGS. 2A and 2B . When a conductive substrate or a semiconductor substrate is used as the insulating substrate  10 , an insulating film  12  such a SiO 2  film or a SiN film should first be disposed before forming the TFTs  30 ,  40  and the organic EL element. Both TFTs have a top-gate configuration, where a gate electrode is disposed above an active layer through the gate insulating film  12 . 
   Next, the detailed configuration of the pixel selecting TFT  30  and the storage capacitance element  130 A will be explained. An amorphous silicon film (referred to as a-Si film hereinafter) is formed through a CVD method on the insulating substrate. The a-Si film is irradiated by a laser beam for re-crystallization from melt, forming a polycrystalline silicon film (referred to as a p-Si film, hereinafter). This layer functions as the active layer  33 . Single layer or multiple layers of a SiO 2  film and a SiN film are formed on the p-Si film as the gate insulating film  12 . 
   Then, the gate signal line  51  also working as the gate electrode  31  made of a metal with a high-melting point such as Cr and Mo as well as the drain signal line  52  made of Al are disposed. Also, the driving source line  53 , which is made of Al and is an electric source of the organic El element  60 , is disposed. 
   A SiO 2  film, a SiN film and a SiO 2  film are sequentially deposited to form the interlayer insulating film  15  on the entire surface of the gate insulating film  12  and the active layer  33 . A drain electrode  36 , which is formed by filling a contact hole formed at the location corresponding to the drain  33   d  with a metal such as Al, is disposed, and a first planarization film  17  made of an organic resin for flattening the surface is formed on the entire surface. 
   Next, the configuration of the storage capacitance element  130 A will be explained. The source  33   s  of the TFT  30  functions also as the first capacitance electrode layer  55  The second capacitance electrode layer  54  is formed above the source  33   s  of the TFT  30 , through the gate insulating film  12 . The second capacitance electrode layer  54  is made of Cr or Mo, and formed in the same layer as the gate electrode  31  and by the same process as the gate electrode  31 . The third capacitance electrode layer  70  extends over the second capacitance layer  54  through the interlayer insulating film  15 . The third capacitance layer  70  is formed in the same layer as, and by the same process as the drain electrode  36  and the drain signal line  52 . The third capacitance electrode layer  70  is connected to the source  33   s  of the TFT  30  through a contact hole. 
   That is, the storage capacitance element  130  has a multiple-layer configuration with the second capacitance electrode layer  54  sandwiched by the first capacitance electrode layer  55  and the third capacitance electrode layer  70  through the insulating films. Therefore, the storage capacitance element  130  can form a large capacitance in a relatively small area. 
   It is also possible to acquire the larger capacitance by extending the cathode layer  65  over the third capacitance electrode layer  70  through the first planarization film  17  and a second planarization film  19 . 
   Next, the organic EL element driving TFT  40  will be explained. The a-Si film is formed on the insulating substrate  10 . The a-Si film is irradiated by a laser beam for forming a poly-crystalline silicon film functioning as an active layer  43 . The gate insulating film  12 , and the gate electrode  41  made of a metal with a high-melting point such as Cr and Mo are deposited on the active layer  43 . Channels  43   c  are formed in the active layer  43 . The source  43   s  and the drain  43   d  are also formed at both sides of the channels  43   c . A SiO 2  film, a SiN film and a SiO 2  film are sequentially deposited to form the interlayer insulating film  15  on the entire surface of the gate insulating film  12  and the active layer  43 . The driving source line  53 , which is connected to the driving source by filling a contact hole formed at the location corresponding to the drain  43   d  with a metal such as Al, is disposed. A source electrode  56  is also formed by filling a contact hole formed at the location corresponding to the source  43   s  with a metal such as Al. 
   Furthermore, the first planarization film  17  made of an organic resin for flattening the surface is deposited on the entire surface. A contact hole is formed in the first planarization film  17  at the location corresponding to the source electrode  56 . The anode layer  61  of the organic EL element, which is a transparent electrode made of ITO, making contact with the source electrode  56  through the contact hole described above is formed on the first planarization film  17 . The second planarization film  19  is further disposed on the first planarization film  17 . This film is removed from the area above the anode layer  61 . 
   The organic EL element  60  includes the anode layer  61  made of the transparent electrode such as ITO (Indium Tin Oxide), a hole transportation layer  62  having a first hole transportation layer made of MTDATA (4,4-bis(3-mathylphenylphenylamino)biphenyl) and a second hole transportation layer made of TPD (4,4,4-tris (3-methylphenylphenylamino)triphenylanine), an emissive layer  63  made of Bebq2 (bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivative, an electron transportation layer  64  made of Bebq2, and the cathode layer  65  made of either magnesium-indium alloy, aluminum or aluminum alloy. 
   The holes inputted from the anode layer  61  and the electrons inputted from the cathode layer  65  are re-combined in the emissive layer of the organic EL element  60 , activating organic molecules in the emissive layer. When the activated molecules are deactivated due to radiation, light is emitted from the emissive layer, and light then reaches outside through the transparent anode layer  61  and the insulating substrate  10 . 
   Next, a second embodiment of this invention will be explained.  FIGS. 3A and 3B  are cross-sectional views of one pixel portion of this embodiment.  FIG. 3A  is a cross sectional view of the pixel portion along the A-A line of  FIG. 1 , and  FIG. 3B  is a cross sectional view along the B-B line of  FIG. 1 , respectively. The structure f the pixel portion of this embodiment is the same as that of the first embodiment except the capacitance electrode structure described below. The equivalent circuit diagram of the pixel portion of this embodiment is also the same as shown in  FIG. 4 . 
   The storage capacitance element  130 A has a multiple-layer configuration with the second capacitance electrode layer  54  sandwiched by the first capacitance electrode layer  55  and the third capacitance electrode layer  70  through the insulating films in the first embodiment. The storage capacitance element  130 B of the second embodiment has an additional electrode layer to increase capacitance per unit area. 
   A fourth capacitance electrode layer  71  is deposited extending over the third capacitance electrode layer  70  through the first planarization layer  17  in addition to the configuration of the first embodiment. The fourth capacitance electrode layer  71  is in the same layer as and formed by the same process as the anode layer  61 . 
   Additionally, the cathode layer  65  is deposited extending over the fourth capacitance electrode layer  71  through the second planarization layer  19 . The cathode layer  65  functions as a fifth capacitance electrode layer. 
   In the first embodiment, a capacitance is formed between the third capacitance electrode layer  70  and the cathode layer  65  when the cathode electrode is used as a fourth capacitance electrode layer. Both the first planarization film  17  and the second planarization film  19  function as the capacitance insulating film in this configuration. In the second embodiment, however, a capacitance is formed between the fourth capacitance electrode layer  71  and the cathode layer  65 , i.e., the fifth capacitance electrode. Since the second planarization layer  19  is the only layer working as the capacitance insulating layer in this configuration, the capacitance insulating film between the capacitance electrodes facing each other is thinner compared to that of the first embodiment. Accordingly, the corresponding capacitance increases.