Patent Document

CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority of Japanese Patent Application No. 2006-44584 filed on Feb. 21, 2006, which is incorporated herein by reference in its entirety. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an active matrix organic EL display apparatus having circuitry used for driving an electroluminescence (EL) element. 
       BACKGROUND OF THE INVENTION 
       [0003]    Because, unlike liquid crystal display apparatuses, electroluminescence (EL) elements require no backlight, making them suitable for thinner displays, and their viewing angle is not limited, there has been a growing demand for practical organic EL display apparatuses employing self-emissive organic electroluminescence (EL) elements. Organic EL display apparatuses differ from liquid crystal display apparatuses employing liquid crystal cells in which display is controlled by a voltage, in that brightness of light emitted by the organic EL element used therein is controlled by the value of electric current flowing through the EL element. 
         [0004]    In general, an active matrix organic EL display apparatus is formed by a set of pixels each composed of three or more sub-pixels, and each sub-pixel has a function of displaying a color of red, blue, green, and so on. These sub-pixels emit light when an electric current in accordance with a predetermined voltage or a greater voltage applied between an anode electrode and a cathode electrode flows therein. 
         [0005]      FIG. 5  shows a structure of a conventional active matrix organic EL display apparatus  100 . A positive power source voltage supplied from a positive power source supply circuit  105  is applied to each sub-pixel  101  in each pixel  102  by a positive power source line  109 . Further, a negative power source voltage supplied from a negative power source supply circuit  106  is applied to each sub-pixel  101  in each pixel  102  by a negative power source line  110 . Also, a signal line  107  is provided corresponding to each column of sub-pixels  101  for supplying an electrical signal (a data signal) for display which is supplied from a signal line driving circuit  104  to each sub-pixel  101 . In addition, a scan line  108  extending from a scan line driving circuit  103  is provided for each row of sub-pixels  101 . In this example, each pixel  102  is composed of three sub-pixels  101  arranged in the row direction. 
         [0006]    Each sub-pixel  101  includes a current value control section  115  formed by a switching element  114 , an electrostatic capacitor  113 , and a driver element  112 , and a light emitting element  111 . 
         [0007]    By setting the scan line  108  to a selection level, the switching element  114  is turned on. The electrostatic capacitor  113  is charged with an electrical signal of the signal line  107  to determine a gate voltage of the driver element  112 , so that an electric current in accordance with the gate voltage flows from the positive power source line  109  to the negative power source line  110  via the driver element  112  and the light emitting element  111 . 
         [0008]    As described above, when each pixel includes three sub-pixels, one pixel includes a total of nine elements. However, disposition of a large number of elements increases the probability of defects occurring. 
         [0009]    In order to deal with the above disadvantage, a structure shown in  FIG. 6  is proposed. (See. U.S. Patent Application Publication Nos. 2005-0104875A1 and 2005-0140604A1 and Japanese Patent Laid-Open Publication No. 2003-122306, and FIG. 5 of “A 1.8 in. QVGA AMOLED Display with New Driving Method and Ultra Slim Technology” by W-K. Kwak et al, SID 05 Digest pgs. 1448-14-51 and “A 14.1 inch Full Color AMOLED Display with Top Emission Structure and a-Si TFT Backplane” by J. H. Jung et al, SID 05 Digetst, pgs. 1538-1541. 
         [0010]    In the structure shown in  FIG. 6 , a positive power source voltage supplied from a positive power source supply circuit  205  is applied to each pixel  202  by a positive power source line  209 . Further, a negative power source voltage supplied from a negative power source supply circuit  206  is applied to each sub-pixel  201  in each pixel  202  by a negative power source line  210 . Also, a signal line  207  is provided corresponding to each column of pixels  202  for supplying an electrical signal for display which is supplied from a signal line driving circuit  204  to each pixel  202 . In addition, a scan line  208  extending from a scan line driving circuit  203  is provided for each row of pixels  202 . In this example, each pixel  202  is formed by including three sub-pixels  201  arranged in the row direction. 
         [0011]    Each pixel  202  includes a current value control section  215  formed by a switching element  214 , an electrostatic capacitor  213 , and a driver element  212 , and three sub-pixels  201  connected to the current value control section  215 . 
         [0012]    Each sub-pixel  201  includes a sub-pixel selection element  216  which functions as a light emission selection element connected to the single driver element  212 , and a light emitting element  211 . The sub-pixel selection elements  216 R,  216 G, and  216 B of three sub-pixels are sequentially turned on by selection control lines  206 R,  206 G, and  206 B, respectively, extending from the scan line driving circuit  203 . Thus, one pixel  202  includes three sub-pixels  201 , in each of which a driving current is supplied to the light emitting element  211 R,  211 G, or  211 B via the sub-pixel selection element  216 R,  216 G, or  216 B. 
         [0013]    With regard to the circuit shown in  FIG. 6 , one frame is divided into three sub-frames, as shown in the driving waveform of  FIG. 7 . In each sub-frame, when the scan line  208  rises to an H level, a signal of the signal line  207  at this time is stored in the electrostatic capacitor  213 , and the corresponding electric current flows in the driver element  212  and is then supplied to the sub-pixel  201  selected by the selection control line  206 , thereby achieving light emission. 
         [0014]    With this structure in which one current value control section  215  is disposed for each pixel  202  and is commonly used by sub-pixels  201  within the pixel, the number of TFTs can be reduced by one and the number of electrostatic capacitors can be reduced by two per one pixel compared to the structure shown in  FIG. 5 . As such, the number of elements can be reduced and therefore the probability of defects occurring can be lowered. 
         [0015]    Here, according to the related technology described above, one frame which is a minimum unit for display of one video image is composed of at least two sub-frames, as shown by the driving waveform of  FIG. 7 . Each sub-frame displays a single color, and a plurality of sub-frames are sequentially displayed at a high speed for superimposing colors, whereby desired colors and tones are displayed in accordance with the average brightness per unit time. 
         [0016]    Further, when the threshold voltage, mobility, or the like of the driver element  212  differ among the pixels  202 , the driving current also differs for each pixel even if the data signal is identical. Accordingly, a circuit in which a compensation circuit for compensating for a variation among the driver elements is incorporated, as shown in  FIG. 8 , has also been proposed (See Japanese Patent Laid Open Publication 2003-122306 and FIG. 5(b) of Kwak et al). 
         [0017]    Here, it is important that the sub-pixel selection element  216  be disposed between the current value control section  215  and the light emitting element  211 , regardless of whether or not a compensation circuit is provided. Especially when a compensation circuit is incorporated, it is necessary to provide the sub-pixel selection element  216  with not only the function of selecting the sub-pixel  201  but also the function of preventing an electric current from flowing to the light emitting element  211  during detection of the characteristics of the driver element  212  by the compensation circuit. In this regard, by providing a single TFT having these two functions as the sub-pixel selection element, rather than disposing two separate TFTs in order to achieve these two functions, the number of TFTs can be further reduced. 
         [0018]    In the circuit shown in  FIG. 8 , a positive voltage VDD is supplied via a positive power source line  311  to each pixel  300 . Further, a negative power source voltage VEE is supplied via a negative power source line  312  to each sub pixel  301  ( 301   a ,  301   b ) of each pixel  300 . Also, a signal line Di (i indicates a column number) is provided corresponding to each column of pixels  300  to supply a data signal to each pixel  300 . A scan line Sj is also provided for each row of pixels  300 . In the shown example, each pixel  300  is formed by including two sub-pixels  301  ( 301   a ,  30   1   b ) arranged in the row direction. 
         [0019]    Each pixel  300  includes a switching element  304  and a driver element (p-channel)  305 , and a compensation circuit  310 , and two sub-pixels  301   a  and  301  are connected to the driver element  305 . 
         [0020]    Each of the sub-pixels  301   a  and  301   b  includes a sub-pixel selection element  302  ( 302   a ,  302   b ) connected to the single driver element  305 , and a light emitting element  303  ( 303   a ,  303   b ). The sub-pixel selection elements  302   a  and  302   b  of the two sub pixels are sequentially turned on by the selection control lines E j,1  and E j,2  (refers to a row number). 
         [0021]    The compensation circuit  310  is provided between the switching element  304  and a gate of the driver element  305 . One end of the switching element  304  is connected with the signal line Di and the other end of the switching element  304  is connected with the compensation circuit  310 . Here, the other end of the switching element  304  is connected with the gate of the driver element  305  via a driver characteristics storage capacitor  308  and is also connected with the positive power source line  311  by means of a switching TFT  307  and a brightness signal storage capacitor  309 . Further, the gate of the driver element  305  is connected to a connection point between the driver element  305  and the sub-pixels  301   a  and  301   b  via a switching TFT  306 . In addition, a reset line Rj is connected to the gate of the switching TFT  307  and the gate of the switching TFT  306 . 
         [0022]    In order to explain the role of the sub-pixel selection element  302  in a pixel circuit in which the compensation circuit  310  is incorporated as described above, the operation of the pixel circuit shown in  FIG. 8  will be described with reference to timing charts shown in  FIG. 9 . At a time before time A in  FIG. 9 , the potential of the reset signal line R is set such that the two switching TFTs  306  and  307  are in a conducting state. Consequently, electrodes at both ends of the brightness signal storage capacitor  309  are reset, and as the sub-pixel selection element  302   a  is in a conducting state, the potential of the gate of the driver element  305  becomes sufficiently lower than the potential of the positive power source line  311  and the potential difference at this time is stored at both ends of the driver characteristics storage capacitor  308 . Then, at the time A, while the both sub-pixel selection TFTs  302   a  and  302   b  are placed in a non-conducting state, the driver element  305  remains in a conducting state because a sufficiently high voltage is applied between the source and the gate of the driver element  305  by the driver characteristics storage capacitor  308 . However, an electric current flowing from the positive power source line  311  via the driver element  305  is supplied to the driver characteristics storage capacitor  308  via the switching TFT  306  and thus raises the gate potential of the driver element  305 . Then, when the potential difference at the source-gate of the driver element  305  equals the threshold voltage Vth of the driver element  305 , the driver element  305  is placed to a non-conducting state, and consequently the threshold voltage of the driver element  305  is recorded in the driver characteristics storage capacitor  308 . Then, after placing the switching TFTs  306  and  307  in a non-conducting state, the switching TFT  304  is placed in a conducting state at time B and a brightness voltage (data) signal Vdata corresponding to the light emitting element  303   a  to be selected in that sub-frame is recorded in the brightness signal storage capacitor  309 . As a result, the potential of the gate of the driver element  305  becomes (Vdata−Vth), and therefore the value of the electric current flowing in the light emitting element  303   a  in the period between C and D is I=β(VDD−Vdata) 2 , which does not depend on Vth. Here, the value of β depends on the mobility, shape, and the like of the driver TFT. 
         [0023]    Again, what is important is the fact that the sub-pixel selection TFT  302  has two functions: a function of preventing an electric current from flowing in the light emitting element  303  at the time of detecting the threshold voltage and a function of selecting either the light emitting element  303   a  or the light emitting element  303   b  to which an electric current controlled by the current value control section  300  is applied during the light emission period. In particular, in order to achieve the selection control, it is necessary to dispose the sub-pixel selection TFT  302  between the current value control section  300  and the light emitting element  303 . 
         [0024]    Circuit structures forming active matrix organic EL display apparatuses can be classified into two types: a cathode common structure in which cathode electrodes of all the light emitting elements are connected to a negative power source line and anode electrodes are connected to a pixel circuit and an anode common structure in which anode electrodes of all the light emitting elements are connected to a positive power source line and cathode electrodes are connected to a pixel circuit. Which of these types should be selected may depend on the process steps for manufacturing a display apparatus and the device structure of the light emitting element. 
         [0025]    Here, a pixel circuit in which a plurality of sub-pixels are connected to a single driver element as shown in  FIG. 8  is assumed using a circuit having a cathode common structure which is formed only of N-channel amorphous silicon TFTs shown in Reference 5. In this case, as a current blocking TFT  313  is required when detecting a threshold voltage as shown in  FIG. 10 , the number of necessary TFTs is increased by one compared to the structure shown in  FIG. 8 , which makes it difficult to achieve the advantage of increased yield. Further, the structure of three TFTs connected in series with respect to the light emitting element causes a problem of increase in power consumption. 
       SUMMARY OF THE INVENTION 
       [0026]    It is an object of the present invention to provide an active matrix apparatus comprising: 
         [0027]    a driver element having a drain electrode connected to a positive power source line; 
         [0028]    a light emission selection element having a drain electrode connected to a source electrode of the driver element and a gate electrode connected to a light emission control line; 
         [0029]    a light emitting element having an anode electrode connected to a source electrode of the light emission selection element and a cathode electrode connected to a negative power source line, the light emitting element capable of emitting light when an electric current is directed therethrough; 
         [0030]    a signal selection element having a drain electrode or a source electrode connected to a signal line which transmits a brightness signal and a gate electrode connected to a scan line; 
         [0031]    a driver characteristics storage capacitor having a first electrode connected to a gate electrode of the driver element and a second electrode connected to the source electrode or the drain electrode of the signal selection element; and 
         [0032]    a first switching element having one of a drain electrode or a source electrode thereof connected to the drain electrode of the driver element, the other of the source electrode or the drain electrode thereof connected to the gate electrode of the drive element, and a gate electrode connected to a reset line. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Preferred embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0034]      FIG. 1  is a diagram showing a structure of a first embodiment of the present invention; 
           [0035]      FIG. 2  is a timing chart of the first embodiment; 
           [0036]      FIG. 3  is a diagram showing a structure of a second embodiment of the present invention; 
           [0037]      FIG. 4  is a timing chart of the second embodiment; 
           [0038]      FIG. 5  is a diagram showing a structure of a first related art example; 
           [0039]      FIG. 6  is a diagram showing a structure of a second related art example; 
           [0040]      FIG. 7  is a timing chart of the second related art example; 
           [0041]      FIG. 8  is a diagram showing a structure of a third related art example; 
           [0042]      FIG. 9  is a timing chart of the second related art example; and 
           [0043]      FIG. 10  is a diagram showing a structure of a fourth related art example. 
       
    
    
     DETAILED DESCRIPTION 
       [0044]    Preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted, however, the scope of the invention is not limited to the illustrated examples. 
       Embodiment 1 
       [0045]      FIG. 1  shows a structure of a first embodiment according to the present invention, and  FIG. 2  is a timing chart of the circuit shown in  FIG. 1 . In this embodiment, a current value control section  1  has an effect of reducing the sensitivity of a driver element  5  to a threshold voltage variation among pixels. Further,  FIG. 1  shows a pixel circuit located in the j-th row and the i-th column, and N-channel TFTs are used for all the transistors in this circuit structure. 
         [0046]    One of a source electrode and a drain electrode of a signal selection element  7  having a gate electrode connected to a scan line Sj is connected to a signal line Di. The other of the source and drain electrodes of the signal selection element  7  is connected with a second electrode  9  of a TFT characteristics storage capacitor  8 , a first electrode  10  of which is connected with a gate electrode of the driver element  5 . 
         [0047]    Further, one of a drain electrode and a source electrode of a first switching element  6  is connected to a positive power source line  11 , and the other of the drain and source electrodes of the first switching element  6  is connected to the gate electrode of the driver element  5 . A reset line Rj is connected to a gate electrode of the first switching element  6 . One of a drain electrode and a source electrode of a second switching element  16  having a gate electrode connected with the reset line Rj is connected with a connection point between the signal selection element  7  and the second electrode  9  of the TFT characteristics storage capacitor  8 , and the other of the drain and source electrodes of the second switching element  16  is connected with a connection point between the driver element  5  and a sub pixel  2  ( 2   a ,  2   b ). Further, a second electrode  15  of a brightness signal storage capacitor  13  is connected to a connection point between the signal selection element  7  and the TFT characteristics storage capacitor  8 , and a first electrode  14  of the brightness signal storage capacitor  13  is connected with the positive power source line  11  (the drain electrode of the driver element  5 ). 
         [0048]    Prior to writing a brightness voltage signal to the current value control section  1  by means of the signal line Di, at a time before time A in  FIG. 2 , a signal of the reset line Rj is set to a potential (in this case, H level) which places the first switching element  6  and the second switching element  16  in a conducting state. 
         [0049]    Thus, the first and second switching elements  6  and  16  are turned on, and a voltage of the positive power source line  11  is set at the first electrode  10  of the TFT characteristics storage capacitor  8  and a voltage connected to the sub-pixel  2  is set to the second electrode  9  of the TFT characteristics storage capacitor  8 . Consequently, a potential difference which is larger than the threshold voltage between the gate and the source of the drive element  5  is stored between the electrodes  9  and  10  of the TFT characteristics storage capacitor  8 . 
         [0050]    Subsequently, the potential of the selection control lines E (j, k) is set to a potential which turns all the sub-pixel selection elements  3  ( 3   a ,  3   b ) in the j-th row off. In this state, because the potentials of the drain and the gate of the driver element  5  are identical and the potential difference which is larger than the gate-source threshold voltage of the driver element is generated at the TFT characteristics storage capacitor  8 , the driver element  5  is in a conducting state. However, because all the sub-pixel selection elements  3  are turned off, no electric current flows through the light emitting elements  4 . As a result, the source potential of the driver element  5  increases, and when the gate-source potential of the driver element  5  equals the threshold voltage of the driver element  5 , the driver element  5  is turned off. Namely, the threshold voltage of the driver element  5  is recorded in the TFT characteristics storage capacitor  8 . 
         [0051]    Then, with the reset line Rj being set to a potential (L level) which places the first switching element  6  and the second switching element  16  in a non-conducting state, and the scan line Sj being set to a potential which places the signal selection element  7  in a conducting state, a brightness signal (electrical signal) voltage supplied from the signal line Di is recorded in the brightness signal storage capacitor  13  via the signal selection element  7 . At this time, because the threshold voltage of the driver element is held at both ends of the TFT characteristics storage capacitor due to the threshold voltage detection process described above, the gate voltage of the driver element  5  has a value obtained by adding the threshold voltage of the driver element  5  to the brightness signal voltage which is recorded. 
         [0052]    Subsequently, with the scan line Sj being set to a potential (L level) which puts the signal selection element  7  in a non-conducting sate and one or more (typically one) of the selection control lines E (j, k) being selected, one or more (typically one) of the light emitting elements  4  ( 4   a ,  4   b ) is placed in a light emitting state. 
         [0053]    The value of the current id flowing in the driver element  5  at this time can be represented by the following expression: 
         [0000]        id =(β/2) ( Vgs−Vth ) 2    
         [0000]    In the above expression, β is a value which is determined by the mobility, the shape, and the material of the driver element  5 , Vgs is a potential between the gate and the source of the driver element  5 , and Vth is the threshold voltage of the driver element  5 . 
         [0054]    As described above, the value of the gate potential of the drive element  5  is a value obtained by adding the threshold voltage of the driver element  5  to the brightness signal voltage. Therefore, when the brightness signal voltage is represented by Vdata, the following equation 
         [0000]        Vg=V data+ Vth    
         [0000]    can be obtained. Consequently, the current id can be represented by the following expression. 
         [0000]        Id =(β/2)( V data− Vo ) 2    
         [0000]    Thus, the current value id does not depend on the threshold voltage of the driver element  5 , so that the display quality can be increased. Here, Vo is a source potential of the driver element  5  when the light emitting element  4  emits light. 
         [0055]    With the above structure, by turning the sub-pixel selection element  3  off, the electric current of the driver element  5  can be turned off and the threshold voltage of the driver element  5  can be stored in the TFT characteristics storage capacitor  8 . Consequently, even when the signal selection element  7 , the first and second switching elements  6  and  16 , the driver element  5 , and the sub-pixel selection elements  3  are formed by N-channel TFTs, it is not necessary to provide a switching element for turning the electric current of the driver element off. It is therefore possible to form a pixel circuit using amorphous silicon TFTS with the same number of elements as the number of elements in the pixel circuit using P-channel TFTs shown in  FIG. 8 . 
       Embodiment 2 
       [0056]      FIG. 3  shows another embodiment to which the present invention is applied. This pixel circuit has an effect of reducing the sensitivity of the driver element  5  to a variation of β, in addition to a variation of the threshold voltages, among the pixels. 
         [0057]    As shown in  FIG. 3 , the end of the signal selection element  7  opposite to the end connected to the signal line Di is connected with the end (source electrode) of the driver element  5  which is connected to the sub pixel  2 . This end of the signal selection element  7  opposite to the signal line Di is also connected with the second electrode  9  of the TFT characteristics storage capacitor  8 , the first electrode  10  of which is connected to the gate electrode of the driver element  5 . Further, the positive power source line  11  and the gate of the driver element  5  are connected via the first switching element  6 , and the gate of the first switching element  6 , similar to the gate of the signal selection element  7 , is connected to the scan line Sj or the reset lint Rj (to the reset line Rj in the example shown in  FIG. 3 ). 
         [0058]    Prior to the timing A in  FIG. 4 , the potential of the selection control lines E (j,k) is set to a potential which places all the sub-pixel selection elements  3  in the j-th row in a non-conducting state. Then, with the scan line Sj being set to a potential (H level) which puts the signal selection element  7  and the first switching element  6  in a conducting state, a brightness signal current is caused to flow via the signal line Di. At this time, because the first switching element  6  is turned on, the gate potential and the drain potential of the driver element  5  are identical, to allow the brightness signal current to flow in the driver element  5 . Consequently, when the brightness signal current is represented by i data , the voltage represented by the following expression is generated between the gate and the source of the driver element  5 : 
         [0000]      Vgs=Vth+√{square root over ( )}(2i data /β) 
         [0059]    Subsequently, the scan line Sj is set to a potential (L level) which puts the signal selection element  7  and the first switching element  6  in a non-conducting state, and one or more (typically one) of the selection control lines E (j,k) is selected to place one or more of the light emitting elements  4  in a light emitting state. At this time, the value of current id flowing in the driver element  5  can be represented by the following expression: 
         [0000]      Id=(β/2)(Vgs−Vth) 2 =i data    
         [0060]    As such, with the use of the pixel circuit shown in  FIG. 3 , as in the above example, the current value id does not depend on the threshold value Vth and β of the driver element  5 , so that the display quality can be increased. 
         [0061]    While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims. 
       PARTS LIST 
       [0000]    
       
         current value control section 
           2  sub-pixel 
           2   a  sub-pixel 
           2   b  sub pixel 
           3  sub-pixel selection element 
           3   a  sub-pixel selection element 
           3   b  sub-pixel selection element 
           4  light emitting elements 
           4   a  light emitting elements 
           4   b  light emitting elements 
           5  driver element 
           6  switching element 
           7  signal selection element 
           8  storage capacitor 
           9  second electrode 
           10  first electrode 
           11  positive power source line 
           13  storage capacitor 
           14  first electrode 
           15  second electrode 
           16  second switching element 
           100  organic EL display 
           101  sub-pixel 
           102  pixel 
           105  power source supply circuit 
           106  negative power source supply circuit 
           107  signal line 
           108  scan line 
           109  positive power source line 
           110  negative power source line 
           111  light emitting element 
           112  driver element 
           113  electrostatic capacitor 
           114  switching element 
           115  current value control section 
           201  sub-pixel 
           202  pixels 
           203  scan line driving circuit 
           204  signal driving circuit 
           205  positive power source supply circuit 
           206  negative power source supply circuit 
           206 R selection control line 
           206 G selection control line 
           206 B selection control line 
           207  signal line 
           208  scan line 
           209  positive power source line 
           210  negative power source line 
           211  light emitting element 
           211 R light emitting element 
           211 G light emitting element 
           211 B light emitting element 
           212  driver element 
           213  electrostatic capacitor 
           214  switching element 
           215  current value control section 
           216  sub-pixel selection element 
           216 G sub-pixel selection element 
           216 R sub-pixel selection element 
           216 B sub-pixel selection element 
           300  pixel 
           301  sub-pixel 
           301   a  sub-pixel 
           301   b  sub-pixel 
           302   a  sub-pixel selection element 
           302   b  sub-pixel selection element 
           302  sub-pixel TFT 
           303  light emitting element 
           303   a  light emitting element 
           303   b  light emitting element 
           304  switching element 
           305  driver element 
           306  switching TFT 
           307  switching TFT 
           308  storage capacitor 
           309  storage capacitor 
           310  compensation circuit 
           311  positive power source line 
           312  negative power source line 
           313  current blocking TFT

Technology Category: g