Abstract:
An insulator substrate ( 110 ) is provided with a display pixel region ( 200 ) comprising an electroluminescence element ( 160 ) having a cathode ( 167 ), emissive layer ( 166 ), and anode ( 161 ), and with first and second TFTs for driving the element. Surrounding the display pixel region ( 200 ), a peripheral drive circuit region ( 251 ) having a third TFT for driving each pixel is further provided on the insulator substrate ( 110 ). The cathode ( 167 ) is disposed in a region other than the peripheral drive circuit region ( 251 ). With this arrangement, generation of a back channel by applying the EL element potential to the cathode is prevented in a complementary TFT employed in the peripheral drive circuit region for controlling the display region, thereby suppressing changes in threshold values due to such back channel generation. As a result, an EL display device with reduced generation of penetration current and minimized increased current consumption is achieved.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electroluminescence display device employing an electroluminescence element and a thin film transistor. 
   2. Description of the Related Art 
   In recent years, electroluminescence (EL) display devices comprising EL elements have gained attention as potential replacement for CRT and LCD devices. Some research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as “TFT”) as switching elements to drive the EL elements. 
     FIG. 1  shows a plan view of a related organic EL display device. 
   As shown in the Figure, the organic EL display device comprises a display pixel region  200  having first and second TFTs for driving an organic EL element of the display pixel. The organic EL display device further comprises a peripheral drive circuit region  250  indicated by a single-dot broken line. The peripheral drive circuit region  250  includes vertical drive circuits  10  and horizontal drive circuit  20  for driving the TFT of the display pixel region. 
     FIG. 2  shows an equivalent circuit of a related single display pixel using an organic EL element. In the display pixel region  200 , a single display pixel is surrounded by a gate signal line  151  and a drain signal line  152 . A first TFT  130  is a switching element disposed near a junction of those lines. Source  131   s  of TFT  130  is connected to gate  142  of a second TFT  140  for driving the organic EL element  160 . A storage capacitor  170  is provided between source  131   s  and gate  142  for retaining for a predetermined period a voltage applied to gate  142 . Source  141   s  of the second TFT  140  is connected to the anode  161  of the organic EL element  160 . Drain  141   d  of TFT  140  is connected to the drive power line  153  that supplies a drive current to the organic EL element  160 . 
     FIG. 3  shows a cross-sectional structure including the second TFT  140  and the organic EL element  160  among components of a single display pixel. Gate electrodes  142  made of refractory metal such as chromium (Cr) or molybdenum (Mo) are formed on an insulator substrate  110  made of quartz glass, non-alkali glass, or a similar material. Sequentially formed over the gate electrodes  142  are a gate insulating film  112  and an active layer  141  using poly-silicon (referred to hereinafter as “p-Si”) film. The active layer  141  comprises intrinsic or substantially intrinsic channels  141   c  formed above the gate electrodes  142 , and the source  141   s  and drain  141   d  formed on respective sides of these channels  141   c  by ion doping. 
   An interlayer insulating film  115  formed by a sequential deposit of a SiO 2  film, a SiN film, and a SiO 2  film covers the gate insulating film  112  and the active layer  141 . A contact hole formed in the interlayer insulating film  115  in a region corresponding to the drain  141   d  is filled with metal such as aluminum (Al), forming the drive power line  153  connecting to a drive power supply  150 . Further, a planarizing insulating film  117  made of an organic resin is formed over the entire substrate to planarize the surface. In a region corresponding to the source  141   s,  a contact hole is formed penetrating through both the planarizing insulating film  117  and the interlayer insulating film  115 . A transparent electrode that contacts the source  141   s  through this contact hole is formed on the planarizing insulating film  117 . The transparent electrode is made of ITO (indium tin oxide), and functions as the anode  161  of the organic EL element  160 . 
   The organic EL element  160  is configured by sequentially forming, in order, the anode  161  made of ITO or similar material connected to the source  141   s  of the above-mentioned second TFT  140 , an element emissive layer  166  composed using an organic compound, and a cathode  167  composed using an alloy of magnesium and indium. In such an organic EL element  160 , a hole injected from the anode and an electron injected from the cathode recombines in an emissive layer within the element emissive layer  166 . As a result, organic compound molecules in the emissive layer are excited, generating excitons. Through a process of these excitons undergoing radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode  161  and the transparent insulator substrate  110 . 
   As shown in  FIG. 3 , the anode  161  is discretely formed for each display pixel, and the element emissive layer  166  is formed slightly larger than the anode  161  so as to cover the entire anode  161 . The cathode  167  can be formed as one common electrode over the entire substrate because the operation of the cathode can be electrically in common for all pixels. More specifically, as the cathode can be configured as a common electrode, according to the related art the cathode  167  can easily be provided by forming it in the region surrounded by a double-dot broken line in  FIG. 1 , this being the entire region of the substrate  110 . 
   A TFT using poly-silicon as the active layer can be employed not only as a pixel TFT within the display pixel region  200 , but also as a TFT for peripheral drive circuit to drive the display pixel region  200  on the substrate  110 . In other words, circuit for driving the display pixel region  200  may be formed on the same substrate  110  as the pixel region.  FIG. 4  illustrates a peripheral drive circuit disposed in a surrounding region of the display pixel region  200  as shown in  FIG. 1 , which is configured using the third TFT. The peripheral drive circuit is described below referring to  FIGS. 1 and 4 . Peripheral drive circuits configured using the third TFT comprise vertical drive circuits  10  and a horizontal drive circuit  20 . A vertical drive circuit  10  includes a vertical shift register (V-SR)  11  and a buffer circuit  12 , while a horizontal drive circuit  20  includes a horizontal shift register (H-SR)  21 , a buffer  22 , and a source line switch  23 . 
     FIG. 4  is a plan view showing the TFT of the buffer constituting the horizontal drive circuit.  FIG. 5  shows a cross-sectional view taken along line A—A of FIG.  4 . 
   As shown in  FIG. 4 , the buffer comprises inverters  400  and  500 . 
   The configuration of the respective TFT of the buffer is next described according to FIG.  5 . 
   Sequentially formed on an insulator substrate  510  composed of a material such as silica glass or non-alkali glass are gate electrodes  511  made of refractory metal such as chromium (Cr) or molybdenum (Mo), a gate insulating film  512 , and an active layer  513  composed of poly-silicon film. 
   The active layer  513  comprises channels  515 , 516  positioned above the gate electrodes  511 . Further within the active layer, sources  518 , 521  and drains  519 , 520  are formed on respective sides of these channels  515 , 516  by performing ion dope using stoppers  517  located above the channels  515 , 516  as masks. In this example, the TFT drawn towards the right of the figure is a n-type channel TFT having impurity ions such as phosphorus (P) implanted in source  518  and drain  519 , while the TFT on the left is a p-type channel TFT having impurity ions such as boron (B) implanted in source  521  and drain  520 . 
   An interlayer insulating film  522  formed by sequentially depositing a SiO 2  film, a SiN film, and a SiO 2  film is provided on the entire surface over the gate insulating film  512 , the active layer  513 , and the stoppers  517 . Contact holes formed in the interlayer insulating film  522  in regions corresponding to the sources  518 , 521  and the drains  519 , 520  are filled with metal such as Al, forming source electrodes  523 , 525  and a drain electrode  524 . The drain electrode  524  connected to the drains  519 , 520  is provided in common for the n-type channel TFT and the p-type channel TFT. A planarizing insulating film  526  made of an organic resin is formed over the entire surface for planarization. 
   Above this, the magnesium-indium alloy cathode  167  of the organic EL display element  161  illustrated in  FIG. 3  is formed over the entire surface. 
   The inverter  500  composed on an n-type channel TFT and a p-type channel TFT is configured as described above. The other inverter  400  has a similar structure. 
   In the manner described above, an organic EL display device comprising a horizontal drive circuit with inverters  400 , 500 , a vertical drive circuit, and a display pixel can be created. 
   However, when the cathode  167  of the organic EL element  161  is provided on the entire surface over the peripheral drive circuit region and the display pixel region of the organic EL display device as described above, a back channel is created in each TFT because of the cathode  167 . The existence of a back channel unduly influences the device, especially in the TFT of the peripheral drive circuit having a C-MOS structure. This is explained below. 
     FIG. 7  shows Vg-Id characteristics of n-type and p-type channel TFT. In the figure, the dotted lines indicate the initial characteristics, while the solid lines indicate characteristics after power is switched on. 
   As shown in the  FIG. 7 , there is at first no current leakage in either the n-type or p-type channel TFT when the gate voltage Vg is 0 V. However, when power is turned on, the potential applied to the cathode causes the characteristic of p-type channel TFT to shift to the right and the characteristic of n-type channel TFT to shift to the left. As a result, current leaks in both TFTs when Vg=0 V. 
   In a peripheral drive circuit, the TFT has a complementary structure composed with a p-type channel and an n-type channel. Accordingly, a change in the threshold voltage of the p-type channel TFT is caused when a high voltage is applied, while a change in the threshold voltage of n-type channel TFT is caused when the signal voltage is low. As a result, current flow, namely, a penetration current, is generated even when the gate voltage Vg=0. Generation of penetration current due to such changes disadvantageously causes an increase in power consumption. 
   SUMMARY OF THE INVENTION 
   The present invention was created in light of the above disadvantages. The purpose of the present invention is to stabilize operational threshold values of a thin film transistor for driving an emissive element such as an organic EL element so as to prevent characteristic changes in a peripheral drive circuit. 
   The present invention provides an electroluminescence display device comprising a display pixel region disposed on a substrate and having an electroluminescence element including an emissive layer between first and second electrodes and a drive circuit region disposed on the same substrate and having thin film transistors for driving said electroluminescence element wherein said first electrode entirely overlaps said display pixel region, but is absent in at least said drive circuit region. 
   In another aspect of the present invention, an emissive display device comprises a display pixel region disposed on a substrate and having an emissive element including an emissive layer between first and second electrodes and a drive circuit region disposed on the same substrate surrounding said display pixel region, said drive circuit region having thin film transistors for driving said emissive element, wherein said first electrode overlaps the entire display pixel region, but is absent from at least said drive circuit region. 
   According to a further aspect of the present invention, said first electrode is formed as a common electrode in said display pixel region. 
   In a still further aspect of the present invention, said display pixel region includes first and second thin film transistors for driving said electroluminescence element, an insulating film is formed overlapping said first and second thin film transistors and said thin film transistors of said drive circuit region, and said first electrode is formed over said insulating film in a position opposing said display pixel region. 
   According to another aspect of the present invention, said first electrode is a common cathode and said second electrode is a discrete anode. 
   In yet another of the present invention, an electroluminescence display device comprises a display pixel region disposed on a substrate, said display pixel region having an electroluminescence element including an emissive layer between an anode and a cathode, and further having first and second thin film transistors for driving said electroluminescence element, and a peripheral drive circuit region disposed surrounding said display pixel region and having third thin film transistors for driving said first and second thin film transistors, wherein said cathode is disposed in said display pixel region, but is absent from said drive circuit region. 
   According to another aspect of the present invention, said cathode on said substrate is formed over the entire display pixel region as a common electrode, but is absent at least from said peripheral drive circuit region. 
   According to the present invention, when a first electrode or cathode of an emissive element such as an electroluminescence element is formed as a common electrode in a display pixel region, the first electrode or cathode overlaps the display pixel region but is absent in the drive circuit region, as described above. Characteristic changes are easily caused in a drive circuit using a thin film transistor, and such changes tend to significantly influence power consumption and display quality of the device. By disallowing an electrode of the emissive element to overlap the drive circuit region according to the present invention, influences of the electrode of the emissive element on the drive circuit can be eliminated. Accordingly, the operational threshold values of the thin film transistor in the drive circuit can be stabilized, suppressing any increase in the devices power consumption and thereby achieving a high-performance device with low power consumption. 
   According to a further aspect of the present invention, a circuit in said drive circuit region includes a CMOS connection structure in which a p-type channel thin film transistor and an n-type channel thin film transistor are complementarily connected. 
   A circuit having a CMOS structure is favorable as a drive circuit. However, due to the presence of both p-type and n-type transistors, changes in the threshold voltage are likely to be caused in at least one of the types of the transistors during application of either high or low voltage to a gate of a CMOS transistor. Avoiding overlap of the drive circuit and an electrode of the emissive element can suppress characteristic changes of thin film transistors for a reliable drive circuit. 
   In a still further aspect of the present invention, said thin film transistors of said drive circuit region are bottom gate type transistors having gate electrodes located beneath an active layer, and said first electrode or said cathode is formed over an insulating layer extending on the entire substrate on the side of said active layer opposite from that which said gate electrodes are located, so as to overlap said display pixel region. 
   When an electrode such as the electrode of an emissive element of the present invention is located in an overlying layer of such a bottom gate type transistor, back channel tends to be generated in a position where the electrode of the element and the active layer of the transistor overlap. Avoiding overlap of the drive circuit having such a transistor and the electrode of the emissive element can suppress characteristic changes of thin film transistors for a reliable drive circuit. 
   In a further different aspect of the present invention, said emissive layer is a layer using an organic compound as an emissive material. 
   Forming the emissive layer using an organic compound can be extremely advantageous in a color display device because organic compounds can provide many variations in emitted colors and can be formed from a wide selection of possible materials. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view illustrating an EL display device of a related art. 
       FIG. 2  is a diagram showing an equivalent circuit in a display pixel of an EL display device of a related art. 
       FIG. 3  shows a cross-sectional view of an EL element and a transistor for supplying electric power to this element in an EL display device of a related art. 
       FIG. 4  is a schematic plan view of a buffer circuit among peripheral drive circuits of a related art EL display device. 
       FIG. 5  shows a cross-sectional view taken along line A—A of FIG.  4 . 
       FIG. 6  is a graph showing Vg-Ig characteristics of n-type and p-type channel TFTs. 
       FIG. 7  is a plan view illustrating an EL display device according to the present invention. 
       FIG. 8  is a plan view showing one pixel in the display pixel region of FIG.  7 . 
       FIG. 9A  shows a cross-sectional view taken along line A—A of FIG.  8 . 
       FIG. 9B  shows a cross-sectional view taken along line B—B of FIG.  8 . 
       FIG. 10  is a schematic plan view of a buffer circuit among peripheral drive circuits of an EL display device according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   An EL display device of the present invention will now be described. 
     FIG. 7  shows a plan view of an organic EL display device. An example in which the EL display device of the present invention is implemented in an organic EL display device is explained referring to this figure. 
   In the organic EL display device, as shown in  FIG. 7 , an insulator substrate  100  is provided with a peripheral drive circuit  251  including horizontal drive circuit  120  and vertical drive circuits  101  configured using third TFTs. Also formed on the insulator substrate  100  is a display pixel region  200  including display pixels of the organic EL display. The third TFTs are formed within the peripheral drive circuit region. Each vertical drive circuit  101  includes a vertical shift register (V-SR)  102  and a buffer circuit  103 . A horizontal drive circuit  120  includes a horizontal shift register (H-SR)  104 , a buffer  105 , and a source line switch  106 . 
   A pixel in the display pixel region  200  has a circuit configuration similar to the above-described  FIG. 2 and a  plan configuration shown in the example in FIG.  8 .  FIG. 9A  shows a cross-sectional view taken along line A—A of FIG.  8 .  FIG. 9B  shows a cross-sectional view taken along line B—B of FIG.  8 . 
   Each pixel comprises a gate signal line  151 , a drain signal line  152 , a first TFT  130  formed near the junction of these signal lines  151 , 152 , a storage capacitor  170 , a second TFT  140 , and an organic EL element  160 . The pixels are formed in a matrix arrangement within region  200  on the substrate  110 . 
   Source  131   s  of the first TFT  130  simultaneously serves as a capacitor electrode  155  that, together with the opposing storage capacitor electrode  154 , forms a capacitor. Gate electrode  142  of the second TFT  140  that drives the organic EL element  160  is connected to source  141   s  of the second TFT  140 , and the source  141   s  contacts with the anode  161  of the organic EL element  160 . The drain  141   d  of the second TFT  140  is connected to a drive power line  153 , while the drive power line  153  is connected to a drive power supply  150  that supplies current to the organic EL element  160 . 
   Near the TFT, a storage capacitor electrode  154  is disposed in parallel with the gate signal line  151 . The storage capacitor electrode  154  is made of a material such as chromium. A capacitor for storing charges is formed between the storage capacitor electrode  154  and the capacitor electrode  155  connected to source  131   s  of the first TFT  130  via a gate insulating film  112 . This storage capacitor  170  is provided for retaining voltage applied to the gate  142  of the second TFT  140 . 
   The first TFT  130 , or the switching TFT, will next be described. As shown in  FIG. 9A , a gate signal line  151  made of refractory metal such as chromium (Cr) or molybdenum (Mo), which also serves as gate electrodes  132 , is formed on an insulator substrate  110  made of quartz glass, non-alkali glass, or a similar material. 
   Above these layers, a gate insulating film  112  and an active layer  131  composed of poly-silicon film are sequentially formed. The active layer  131  comprises the so-called LDD (Lightly Doped Drain) structure. Specifically, low-concentration regions  131 LD are formed on both sides of each gate  132 . The source  131   s  and the drain  131   d,  which are high-concentration regions, are further disposed on the outboard sides of the low-concentration regions  131 LD. 
   An interlayer insulating film  115  formed by a sequential lamination of a SiO 2  film, a SiN film, and a SiO 2  film is provided on the entire surface over the gate insulating film  112  and the active layer  131 . A contact hole formed in a position corresponding to the drain  141   d  is filled with metal such as Al, forming a drain electrode  116  constituting one continuous component with a drain signal line  152 . Further, a planarizing insulating film  117  made of an organic resin or a similar material is formed over the entire surface for planarization. 
   The second TFT  140 , or the TFT for driving the organic EL element, will next be described. 
   As shown in  FIG. 9B , gate electrodes  142  composed of refractory metal such as Cr or Mo are formed on an insulator substrate  110  made of silica glass, non-alkali glass, or a similar material. On top of these, a gate insulating film  112  and an active layer  141  composed of poly-silicon film are sequentially formed. The active layer  141  comprises intrinsic or substantially intrinsic channels  141   c  formed above the gate electrodes  142 , and the source  141   s  and drain  141   d  formed on respective sides of these channels  141   c  by ion doping. 
   An interlayer insulating film  115  formed by a sequential lamination of a SiO 2  film, a SiN film, and a SiO 2  film is provided on the entire surface over the gate insulating film  112  and the active layer  141 . A contact hole formed in a position corresponding to the drain  141   d  is filled with metal such as Al, thereby forming the drive power line  153  connecting to a drive power supply  150 . Further, a planarizing insulating film  117  made of an organic resin or a similar material is formed over the entire surface for planarization. A contact hole is formed in the planarizing insulating film  117  in a position corresponding to the source  141   s.  A transparent electrode made of ITO that contacts the source  141   s  through this contact hole, namely, the anode  161  of the organic EL element, is formed on the planarizing insulating film  117 . 
   The organic EL element  160  is formed by first laminating the anode  161  constituted by a transparent electrode made of ITO or similar material. The emissive element layer  166  is then superimposed. The emissive element layer  166  comprises a first hole-transport layer  162  composed of a material such as MTDATA (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine), a second hole-transport layer  163  composed of a material such as TPD (N,N′-diphenyl-N,N′-di(3-mthylphenyl)-1,1′-biphenyl-4,4′-diamine), an emissive layer  164  composed of, for example, Bebq 2  (bis(10-hydroxybenzo[h]quinolinato)beryllium) including quinacridone derivatives, and an electron transport layer  165  composed of Bebq 2  or similar material. Subsequently, the cathode  167  is formed which may be composed of a magnesium-indium alloy. The above-mentioned layers of the organic EL element  160  are laminated in the described order. 
   In the present embodiment, the cathode  167  extends covering the entire display pixel region  200  as shown in  FIG. 7 , but does not reach the drive circuit region  251  disposed surrounding region  200 . The cathode  167  is formed in a contour corresponding to or slightly larger than the display pixel region  200 . 
   The structure of the peripheral drive circuit region  251  indicated by a single-dot broken line and located outside the display pixel region  200  in  FIG. 7  will now be explained. As described above, region  251  includes a horizontal drive circuit  120 , vertical drive circuits  101 , and an input wiring terminal  24  for supplying electric power (voltage, current) from an external power supply. 
   In the following, the TFT of the buffer shown in  FIG. 4  (inverter  500 ) is used as an example of a circuit constituting the peripheral drive circuit region  251  to explain a configuration according to the present embodiment.  FIG. 10  shows the cross-sectional configuration of the inverter  500  of  FIG. 4  according to the present embodiment. 
   As seen in the  FIG. 10 , the structure of the inverter corresponds with that shown in  FIG. 5  concerning the layers from the gate electrodes  511  to the planarizing insulating film formed on the insulator substrate  110 . 
   However, differing from the structure shown in  FIG. 5 , the cathode  167  of the organic EL element  160  formed in the display pixel region  200  is not present over the planarizing insulating film  526 . 
   When forming the cathode  167 , a metal mask or a similar component that can cover the peripheral drive circuit region  251  excluding the display pixel region  200  is placed on the planarizing insulating film  526 . Subsequently, a magnesium-indium alloy, which is the material constituting the cathode  167 , is deposited on the planarizing insulating film  526  using an evaporation method. The cathode  167  can thereby be formed only in the display pixel region  200  without extending in the peripheral drive circuit region  251 . 
   By forming the cathode  167  only in the display pixel region, characteristic changes after turning on the power can be prevented in the inverter and clocked inverter with a CMOS configuration using the n-type and p-type channel TFT employed in the peripheral drive circuit region  251 . 
   As changes in threshold voltages of the inverters can be minimized, generation of penetration current can be suppressed, thereby preventing increase of power consumption. 
   Although the above embodiment was described concerning an example using the so-called bottom gate type TFT having gate electrodes disposed beneath the active layer close to the substrate, the present invention is not limited to such a structure. The present invention may be implemented using a top gate type TFT having gate electrodes disposed above the active layer, and similar effects as that of the example using a bottom gate type TFT can be achieved. 
   It is noted that the peripheral drive circuit region  251  is defined as a region comprising third TFT constituting vertical drive circuits  101  and horizontal drive circuit  120  for supplying signals to drive the first and second TFT  130 , 140  located within the display pixel region  200 . 
   The cathode  167  of the organic EL element need only be formed in at least the display pixel region  200 . The cathode  167  may also be formed, for example, in a position between the horizontal drive circuit  120  and a vertical drive circuit  101  in the plan view, as long as the cathode  167  is not formed in a region where the peripheral drive circuits are present. Preferably, the cathode  167  is formed only within the display pixel region  200  as described above. 
   The cathode  167  of the organic EL element may be present over the signal wiring region  24  that supplies signals to the substrate  100  on which the organic EL element is formed. However, to minimize negative influences such as generation of parasitic capacitance in signal wires, it is preferable that the cathode  167  be absent over region  24 . 
   Further, although the above embodiment was explained using an organic EL display device as an example, the present invention is not limited to organic EL displays. Similar effects can be obtained by implementing the present invention in an inorganic EL display device which uses, as an emissive element instead of organic EL elements, inorganic EL elements comprising inorganic emissive materials. Alternatively, the present invention may be applied in a vacuum fluorescent display (VFD) having a fluorescent layer as the emissive layer between two electrodes. 
   Concerning the first and second TFT  130 , 140  formed in the display pixel region  200  and the third TFT constituting the peripheral drive circuit  251  in the above-described embodiment, corresponding structures such as gate electrodes, gate insulating films, and active layers can be formed in the same manufacturing processes. For example, the active layers of those TFT composed using poly-silicon can be formed in one process. An amorphous silicon film may be formed on the entire substrate and then polycrystallized by a method such as laser annealing. The poly-silicon film created in this way may be used as an active layer in each TFT.