Patent Publication Number: US-2015084940-A1

Title: Display apparatus and fabrication method for display apparatus

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This is a Continuation application of U.S. patent application Ser. No. 13/598,677, filed Aug. 30, 2012, which is a Continuation application of U.S. patent application Ser. No. 11/984,715, filed Nov. 21, 2007, which in turn claims priority from Japanese Application 2006-342132 filed in the Japan Patent Office on Dec. 20, 2006, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an active matrix type display apparatus wherein pixel circuits each including light emitting elements such as organic EL (Electroluminescence) elements are arranged in a matrix and a fabrication method for the display apparatus. 
     2. Description of the Related Art 
     In an image display apparatus such as a liquid crystal display unit, a great number of pixels are arrayed in a matrix, and in order to display an image, the light intensity is controlled for each of the pixels in response to information of an image to be displayed. 
     While this similarly applies to an organic EL display unit or a like display unit, an organic EL display unit is a self-luminous type display unit which has a light emitting element for each of pixel circuits. The organic EL display unit is advantageous in that, in comparison with a liquid crystal display unit, the visibility of an image is high and a backlight need not be provided, and the speed of response is high. 
     Further, the organic EL display unit is much different from the liquid crystal display unit in that the gradation of color development is obtained by controlling the luminance of each light emitting element through the current value applied to the light emitting element. In short, the light emitting element is of the current controlled type. 
     While, in an organic EL display unit, a simple matrix system and an active matrix system can be applied as a driving system therefor similarly to a liquid crystal display unit, the former system has a problem in that, while the structure is simple, it is difficult to implement a large display unit having a high definition. Therefore, development of the active matrix system is carried out energetically wherein current applied to a light emitting element in the inside of each pixel circuit is controlled by an active element, usually by a TFT (Thin Film Transistor), provided in the pixel circuit. 
       FIG. 1  is shows a configuration of a general organic EL display apparatus. 
     Referring to  FIG. 1 , the display apparatus  1  shown includes a pixel array section  2  wherein pixel circuits (PXLC)  2   a  are arranged in an m×n matrix, a horizontal selector (HSEL)  3 , and a writing scanner (WSCN)  4 . The display apparatus further includes signal lines or data lines SGL 1  to SGLn selected by the horizontal selector  3  such that a data signal corresponding to luminance information is supplied thereto, and scanning lines WSL 1  to WSLm selectively driven by the writing scanner  4 . 
     It is to be noted that the horizontal selector  3  and the writing scanner  4  may be formed from a MOSIC or the like on polycrystalline silicon or around the pixels MOSIC. 
       FIG. 2  shows an example of a configuration of a pixel circuit  2   a  shown in  FIG. 1 . It is to be noted that the configuration is disclosed, for example, in U.S. Pat. No. 5,684,365 or Japanese Patent Laid-Open No. Hei 8-234683. 
     Referring to  FIG. 2 , the pixel circuit shown has the simplest circuit configuration from among a great number of proposed circuit configurations and is a two-transistor driving type circuit. 
     The pixel circuit  2   a  includes a p-channel thin film field-effect transistor (hereinafter referred to as TFT)  11  and another TFT  12 , a capacitor C 11 , and an organic EL light emitting element (OLED)  13  which serves as a light emitting element. Further, in  FIG. 2 , reference characters SGL and WSL denote a signal line and a scanning line, respectively. 
     Since an organic EL light emitting element in most cases has a rectification characteristic, it is sometimes called OLED (Organic Light Emitting Diode). While a symbol of a diode is used to indicate a light emitting element in  FIG. 2  and so forth, the rectification characteristic is not necessarily required for the OLED in the following description. 
     The TFT  11  is connected at the source thereof to a power supply potential Vcc, and the light emitting element  13  is connected at the cathode thereof to the ground potential GND. Operation of the pixel circuit  2   a  shown in  FIG. 2  is described below. 
     Step ST 1 : 
     The scanning line WSL is placed into a selected state (here, into a low-level state) and a writing potential Vdata is applied to the signal line SGL. Consequently, the TFT  12  is rendered conducting to allow the capacitor C 11  to be charged or discharged and the gate potential of the TFT  11  is changed to the potential Vdata. 
     Step ST 2 : 
     The scanning line WSL is placed into a non-selected state (here, into a high-level state). Consequently, the signal line SGL and the TFT  11  are electrically isolated from each other. However, the gate potential of the TFT  11  is retained in stability by the capacitor C 11 . 
     Step ST 3 : 
     The current to be supplied to the TFT  11  and the light emitting element  13  is changed to current which has a value corresponding to a gate-source voltage Vgs of the TFT  11 , and the light emitting element  13  continues to emit light with the luminance corresponding to the current value. 
     The operation for selecting the scanning line WSL to transmit the luminance information applied to a data line to the inside of a pixel as at the step ST 1  is hereinafter referred to as “writing.” 
     As described above, in the pixel circuit  2   a  shown in  FIG. 2 , once writing of the potential Vdata is performed, then the light emitting element  13  continues to emit light with the fixed luminance until next rewriting of a potential is performed. 
     As described above, in the pixel circuit  2   a  of  FIG. 2 , the gate application voltage of the TFT  11  which is a driving transistor is changed to control the value of the current to be supplied to the EL light emitting element  13 . 
     At this time, the p-channel driving transistor is connected at the source thereof to the power supply potential Vcc, and the TFT  11  always operates in a saturation region. Therefore, the source of the driving transistor serves as a constant current source having a current value calculated in accordance with the following expression 1: 
         Ids= ½·μ( W/L )Cox( Vgs−|Vth| ) 2   (1)
 
     where μ, Cox, W, L, Vgs and Vth indicate the mobility of a carrier, the gate capacitance per unit area, the gate width, the gate length, the gate-source voltage of the TFT  11  and the threshold value of the TFT  11 , respectively. 
     In the simple matrix type image display apparatus, each of light emitting elements emits light only at a selected moment, but in the active matrix type image display apparatus, each light emitting element continues to emit light also after the writing comes to an end as described hereinabove. Therefore, the active matrix type image display apparatus is advantageous particularly for a large display unit having a high definition in that the peak luminance and the peak current of the light emitting elements can be decreased in comparison with the simple matrix type image display apparatus. 
       FIG. 3  illustrates aged deterioration of a current-voltage (I-V) characteristic of an organic EL light emitting element. Referring to  FIG. 3 , a curved line indicated by a solid line indicates a characteristic in an initial state, and a curved line indicated by a broken line indicates a characteristic after aged deterioration. 
     Generally, as times passes, the I-V characteristic of an organic EL light emitting element deteriorates as seen in  FIG. 3 . 
     However, in the two-transistor driving in  FIG. 2 , constant current is continuously supplied to the organic EL light emitting element in order to perform the constant current driving as described above. Therefore, even if the I-V characteristic of the organic EL light emitting element deteriorates, the light emission luminance of the EL device does not deteriorate with time. 
     Incidentally, while the pixel circuit  2   a  of  FIG. 2  is formed from the p-channel TFTs, if the pixel circuit  2   a  can be formed from n-channel TFTs, then an existing amorphous silicon (a-Si) process can be used for TFT production. Consequently, reduction of the cost of a TFT substrate can be anticipated. 
     Now, a basic pixel circuit where each transistor is replaced with an n-channel TFT is described. 
       FIG. 4  shows a pixel circuit wherein the p-channel TFTs in the circuit shown in  FIG. 2  are replaced with re-channel transistors. 
     The pixel circuit  2   b  shown in  FIG. 4  includes an re-channel TFT  21  and another n-channel TFT  22 , a capacitor C 21 , and an organic EL light emitting element (OLED)  23  serving as a light emitting element. Further, in  FIG. 4 , reference characters SGL and WSL denote a data line and a scanning line, respectively. 
     In the pixel circuit  2   b , the TFT  21  serving as a driving transistor is connected at the drain side thereof to the power supply potential Vcc and at the source thereof to the anode of the EL light emitting element  23  in such a manner as to form a source follower circuit. 
       FIG. 5  illustrates an operation point of the TFT  21  as the driving transistor and the EL light emitting element  23  in an initial state. In  FIG. 5 , the axis of abscissa indicates the drain-source voltage Vds of the TFT  21 , and the axis of ordinate indicates the drain-source current Ids of the TFT  21 . 
     As seen in  FIG. 5 , the source voltage depends upon an operation point between the TFT  21  serving as the driving transistor and the EL light emitting element  23 , and has a value which differs depending upon the gate voltage. 
     Since the TFT  21  is driven in a saturation region, the current Ids is supplied which has the current value calculated in accordance with the equation represented by the expression 1 regarding the voltage Vgs corresponding to the source voltage at the operation point. 
     SUMMARY OF THE INVENTION 
     The pixel circuit described above is the simplest circuit including the TFT  21  as the driving transistor, the TFT  22  as the switching transistor and the OLED  23 . However, a configuration may be applied wherein the power signal to be applied to a power supply line is changed over between two signals and also the video signal to be supplied to a signal line is changed over between two signals so as to correct the threshold value and the mobility. 
     Or, a different configuration may be applied wherein TFTs for cancellation of the mobility and the threshold value as well as a driving transistor and a switching transistor connected in series to an OLED are provided. 
     In an active matrix type organic EL display panel which includes pixel circuits arranged in a matrix and including a TFT as a switching transistor or a TFT for the threshold value or for the mobility which is provided separately, a gate pulse is applied to the gate of a desired one of the TFTs through a wiring line. The gate pulse is produced by a vertical scanner such as a writing scanner disposed on one or both sides of the active matrix type organic EL display panel. 
     Where a pixel circuit includes two or more TFTs to each of which the pulse signal is to be applied, the timings for applying the pulse signals are significant. 
     However, as increase in size and definition of a panel advances, defects such as short-circuiting between wiring lines and short-circuiting between layers apparently increase. 
     Particularly, if short-circuiting of signal lines or gate lines occur between layers or in the same layer, then this gives rise to appearance of a line defect and causes a low yield. 
     This problem has an increasing influence as increase in size and definition of a panel advances. 
     Therefore, it is demanded to provide a display apparatus which can achieve improvement of the yield of a panel and a fabrication method for the display apparatus. 
     According to an embodiment of the present invention, there is provided a display apparatus including a plurality of pixel circuits arrayed in a matrix, a driving wiring line to which the pixel circuits are connected, and a plurality of signal lines wired so as to cross with the driving wiring line and having the pixel circuits connected thereto, the signal lines being wired in parallel to each other. 
     Preferably, the signal lines include a main signal line and a sub signal line wired in parallel to each other, and the main signal line and the sub signal line are connected to each other at two predetermined first and second positions across each of the pixel circuits in a wiring direction of the signal lines individually by connection wiring lines. 
     Preferably, where one of the pixel circuits which is to be connected is defective, the main signal line is cut at a predetermined portion thereof between a first position and the defect position and at another predetermined portion thereof between a second position and the defect position, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     Preferably, the signal lines include a main signal line and a sub signal line wired in parallel to each other, and, where one of the pixel circuits which is to be connected is defective, the main signal line is cut at a predetermined portion thereof between, from between two predetermined first and second positions across the defective pixel circuit in a wiring direction of the signal lines, the first position and the defect position and at another predetermined portion thereof between the second position and the defect position and the main signal line and the sub signal line are connected to each other at the first position by a connection wiring line and also at the second position by another connection wiring line while the sub signal line is cut on the outside of a processing region with respect to the first position and also on the outside of the processing region with respect to the second position, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     Preferably, the signal lines include a main signal line and a sub signal line wired in parallel to each other and the sub signal line is wired for each of the pixel circuits, and, where one of the pixel circuits which is to be connected is defective, the main signal line is cut at a predetermined portion thereof between, from between two predetermined first and second positions across the defective pixel circuit in a wiring direction of the signal lines, the first position and the defect position and at another predetermined portion thereof between the second position and the defect position and the main signal line and the sub signal line are connected to each other at the first position by a connection wiring line and also at the second position by another connection wiring line, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     According to a second embodiment of the present invention, there is provided a fabrication method for a display apparatus which includes a plurality of pixel circuits arrayed in a matrix, a driving wiring line to which the pixel circuits are connected, and a plurality of signal lines wired so as to cross with the driving wiring line and having the pixel circuits connected thereto, the signal lines being wired in parallel to each other, including the steps of wiring the signal lines which include a main signal line and a sub signal line which extend in parallel to each other, connecting the main signal line and the sub signal line to each other at two predetermined first and second positions across each of the pixel circuits in a wiring direction of the signal lines individually by connection wiring lines, cutting, where one of the pixel circuits which is to be connected is defective, the main signal line at a predetermined portion thereof between the first position and the defect position, and cutting the main signal line at another predetermined portion thereof between the second position and the defect position, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     According to a further embodiment of the present invention, there is provided a fabrication method for a display apparatus which includes a plurality of pixel circuits arrayed in a matrix, a driving wiring line to which the pixel circuits are connected, and a plurality of signal lines wired so as to cross with the driving wiring line and having the pixel circuits connected thereto, the signal lines being wired in parallel to each other, including the steps of wiring the signal lines which include a main signal line and a sub signal line which extend in parallel to each other, cutting, where one of the pixel circuits which is to be connected is defective, the main signal line at a predetermined portion thereof between, from between two predetermined first and second positions across the defective pixel circuit in a wiring direction of the signal lines, the first position and the defect position, cutting the main signal line at another predetermined portion thereof between the second position and the defect position, connecting the main signal line and the sub signal line to each other at the first position by a connection wiring line, connecting the main signal line and the sub signal line to each other at the second position by another connection wiring line, cutting the sub signal line on the outside of a processing region with respect to the first position, and cutting the sub signal line on the outside of the processing region with respect to the second position, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     According to a still further embodiment of the present invention, there is provided a fabrication method for a display apparatus which includes a plurality of pixel circuits arrayed in a matrix, a driving wiring line to which the pixel circuits are connected, and a plurality of signal lines wired so as to cross with the driving wiring line and having the pixel circuits connected thereto, the signal lines being wired in parallel to each other, including the steps of wiring the signal lines which include a main signal line and a sub signal line which extend in parallel to each other, the sub signal line being wired for each of the pixel circuits, cutting, where one of the pixel circuits which is to be connected is defective, the main signal line at a predetermined portion thereof between, from between two predetermined first and second positions across the defective pixel circuit in a wiring direction of the signal lines, the first position and the defect position, cutting the main signal line at another predetermined portion thereof between the second position and the defect position, connecting the main signal line and the sub signal line to each other at the first position by a connection wiring line, and connecting the main signal line and the sub signal line to each other at the second position by another connection wiring line, whereby the main signal line between the first and second positions is replaced with the sub signal line. 
     With the display apparatus and the fabrication methods therefor, improvement of the yield of a panel can be anticipated. 
     The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a general organic EL display apparatus; 
         FIG. 2  is a circuit diagram showing an example of a configuration of a pixel circuit shown in  FIG. 1 ; 
         FIG. 3  is a diagram illustrating aged deterioration of a current-voltage (I-V) characteristic of an organic EL light emitting element; 
         FIG. 4  is a circuit diagram showing a pixel circuit wherein p-channel TFTs in the circuit shown in  FIG. 2  are replaced with n-channel TFTs; 
         FIG. 5  is a diagram illustrating an operation point between a TFT serving as a driving transistor and an EL light emitting element in an initial state; 
         FIG. 6  is a block diagram showing a configuration of an organic EL display apparatus to which a pixel circuit according to a first embodiment of the present invention is applied; 
         FIG. 7  is a circuit diagram showing a particular configuration of a pixel circuit shown in  FIG. 6 ; 
         FIGS. 8A to 8C  are timing charts illustrating basic operation of the pixel circuit shown in  FIG. 7 ; 
         FIG. 9  is a schematic top plan view showing part of the pixel circuit shown in  FIG. 7  and illustrating an example of a first countermeasure for achieving improvement of the yield of a panel; 
         FIG. 10  is a circuit diagram schematically showing an equivalent circuit of the pixel circuit shown in  FIG. 7  and illustrating the example of the first countermeasure shown in  FIG. 9 ; 
         FIG. 11  is a schematic view illustrating a method (fabrication method) of bypassing a defective pixel circuit in the example of the first countermeasure shown in  FIG. 9 ; 
         FIG. 12  is a schematic top plan view showing part of the pixel circuit shown in  FIG. 7  and illustrating an example of a second countermeasure for achieving improvement of the yield of a panel; 
         FIG. 13  is a circuit diagram schematically showing an equivalent circuit of the pixel circuit shown in  FIG. 7  and illustrating the example of the second countermeasure shown in  FIG. 12 ; 
         FIG. 14  is a schematic view illustrating a method (fabrication method) of bypassing a defective pixel circuit in the example of the second countermeasure shown in  FIG. 12 ; 
         FIG. 15  is a schematic top plan view showing part of the pixel circuit shown in  FIG. 7  and illustrating an example of a third countermeasure for achieving improvement of the yield of a panel; 
         FIG. 16  is a circuit diagram schematically showing an equivalent circuit of the pixel circuit shown in  FIG. 7  and illustrating the example of the third countermeasure shown in  FIG. 15 ; 
         FIG. 17  is a schematic view illustrating a method (fabrication method) of bypassing a defective pixel circuit in the example of the third countermeasure shown in  FIG. 15 ; 
         FIG. 18  is a schematic plan view and sectional view showing part of the pixel circuit shown in  FIG. 7  and illustrating an example of a countermeasure for improving the picture quality and so forth; 
         FIG. 19  is a schematic plan view and sectional view showing a configuration wherein a capacitor is disposed at a position overlapping with a scanning line or gate line in a layering direction of layers for comparison with the configuration shown  FIG. 18 ; 
         FIG. 20  is a sectional view schematically showing part of the pixel circuit shown in  FIG. 7  and illustrating another example of the countermeasure for improving the picture quality and so forth; 
         FIGS. 21A to 21E  are timing charts illustrating particular operation of the pixel circuit shown in  FIG. 7 ; 
         FIG. 22  is a circuit diagram showing a state of the pixel circuit shown in  FIG. 7  within a light emission period in operation of the pixel circuit; 
         FIG. 23  is a similar view but showing the pixel circuit shown in  FIG. 7  in a state wherein the voltage is set to a certain voltage within a non-light emission period in operation of the pixel circuit; 
         FIG. 24  is a similar view but showing the pixel circuit shown in  FIG. 7  in a state wherein an offset signal is inputted in operation of the pixel circuit; 
         FIG. 25  is a similar view but showing the pixel circuit shown in  FIG. 7  in a state wherein the voltage is set to a power supply voltage in operation of the pixel circuit; 
         FIG. 26  is a diagram illustrating a transition of a source voltage of a driving transistor when the voltage is set to the power supply voltage in operation of the pixel circuit in  FIG. 7 ; 
         FIG. 27  is a circuit diagram showing the pixel circuit in  FIG. 7  in a state wherein a data signal is written in operation of the pixel circuit; 
         FIG. 28  is a diagram illustrating a transition of the source voltage of the driving transistor in response to the mobility in operation of the pixel circuit in  FIG. 7 ; 
         FIG. 29  is a circuit diagram showing the pixel circuit in  FIG. 7  in a light emission state in operation of the pixel circuit; 
         FIG. 30  is a block diagram showing a configuration of an organic EL display apparatus to which a pixel circuit according to a second embodiment of the present invention is applied; 
         FIG. 31  is a circuit diagram showing a particular configuration of a pixel circuit shown in  FIG. 30 ; and 
         FIGS. 32A to 32F  are timing charts illustrating basic operation of the pixel circuit shown in  FIG. 31 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIGS. 6 and 7 , there is shown a configuration of an organic EL display apparatus which uses a pixel circuit according to a first embodiment of the present invention. The display apparatus  100  shown includes a pixel array section  102  including pixel circuits  101  arranged in an m×n matrix therein, a horizontal selector (HSEL)  103 , a writing scanner (WSCN)  104 , and a power driving scanner (PDSCN)  105 . The display apparatus  100  further includes signal lines SGL 101  to SGL 10   n  to which an input signal SIN such as a data signal Vsig and an offset signal Vofs according to luminance information selected by the horizontal selector  103  is supplied. The display apparatus  100  further includes scanning lines WSL 101  to WSL 10   m  as driving wiring lines selectively driven by a gate pulse (scanning pulse) GP from the writing scanner  104 . The display apparatus  100  further includes power driving lines PSL 101  to PSL 10   m  as driving wiring lines receiving and being driven by a power signal PSG which is set to a voltage Vcc (for example, a power supply voltage) or another voltage Vss (for example, a negative side voltage) by the power driving scanner  105 . 
     It is to be noted that, while the pixel circuits  101  are arranged in an m×n matrix in the pixel array section  102 , in order to simplify the illustration, an example wherein the pixel circuits are arranged in 2 (=m)×3 (=n) matrix is shown in  FIG. 6 . 
     Also in  FIG. 7 , a particular configuration of one of the pixel circuits is shown in order to simplify the illustration. 
     Referring to  FIG. 7 , the pixel circuit  101  according to the present embodiment includes an n-channel TFT  111  serving as a driving transistor, another n-channel TFT  112  serving as a switching transistor, a capacitor C 111 , a light emitting element  113  formed from an organic EL light emitting element (OLED: opto-electric device), a first node ND 111 , and a second node ND 112 . 
     In the pixel circuit  101 , the TFT  111  serving as a driving transistor, the node ND 111  and the light emitting element (OLED)  113  are connected in series between a power driving line or power supply line PSL ( 101  to  10   m ) and a predetermined reference potential Vcat such as, for example, a ground potential. 
     In particular, the light emitting element  113  is connected at the cathode and the anode thereof to the reference potential Vcat and the first node ND 111 , respectively, and the TFT  112  is connected at the source thereof to the first node ND 111  and the TFT  111  is connected at the drain thereof to the power driving line PSL. 
     Further, the TFT  111  is connected at the gate thereof to the second node ND 112 . 
     Further, the first and second electrodes of the capacitor C 111  are connected to the first and second nodes ND 111  and ND 112 , respectively. 
     The source and the drain of the TFT  112  are connected to a signal line SGL and the second node ND 112 , respectively. Further, the gate of the TFT  112  is connected to the scanning line WSL. 
     In this manner, in the pixel circuit  101  according to the present embodiment, the capacitor C 111  as a pixel capacitor is connected between the gate and the source of the TFT  111  serving as a driving transistor. 
       FIGS. 8A to 8C  are timing charts illustrating basic operation of the pixel circuit shown in  FIG. 7 . 
     More particularly,  FIG. 8A  illustrates a gate pulse or scanning pulse GP to be applied to the scanning line WSL;  FIG. 8B  illustrates a power signal PSG to be applied to the power driving line PSL; and  FIG. 8C  illustrates an input signal SIN to be applied to the signal line SGL. 
     Referring to  FIGS. 8A to 8C , in order to cause the light emitting element  113  of the pixel circuit  101  to emit light, a power signal Vss, which may be, for example, a negative voltage, is applied to the power driving line PSL within a non-light emission period. Further, an offset signal Vofs is propagated to the signal line SGL so that it is inputted to the second node ND 112  through the TFT  112 . Thereafter, a power signal Vcc which corresponds to a power supply voltage is applied to the power driving line PSL to correct the threshold value of the TFT  111 . 
     Thereafter, a data signal Vsig according to luminance information is applied to the signal line SGL and written into the second node ND 112  through the TFT  112 . At this time, since the writing is performed while current is applied to the TFT  111 , mobility correction is performed concurrently. 
     Then, the TFT  112  is placed into a non-conductive state and the light emitting element  113  emits light in response to the luminance information. 
     Incidentally, as increase in size and definition of a panel advances, defects such as short-circuiting between wiring lines or short-circuiting between layers increase. 
     Particularly, if short-circuiting between layers or short-circuiting in the same layer occurs with signal lines or gate lines, then a line defect appears on a display image, resulting in the yield. 
     Therefore, though not shown in  FIGS. 6 and 7 , in the present embodiment, the following countermeasure is taken in order to achieve increase of that yield of a panel. In particular, not one original signal line but a plurality of signal lines (in the present embodiment, two signal lines) are wired in parallel to each other for each pixel column. Consequently, even if a signal line SGL and a scanning line (or gate line WSL) are short-circuited or opened to make a pixel circuit defective, for example, after fabrication, the two signal lines are connected to each other and cut at desired portions such that the defective pixel circuit is bypassed. 
     First to third examples of the countermeasure are described below. 
       FIG. 9  schematically shows part of a pixel circuit and illustrates a first example of the countermeasure which achieves increase of the yield of a panel. 
       FIG. 10  shows a schematic equivalent circuit to the pixel circuit and illustrates the first countermeasure example. 
     Referring to  FIGS. 9 and 10 , in the first countermeasure example, a plurality of signal lines, in the present embodiment, an original main signal line SGLM and a sub signal line SGLS, are wired in parallel to each other. The main signal line SGLM and the sub signal line SGLS are connected like a ladder through connection wiring lines CNL 1  and CNL 2  at two predetermined first and second positions P 1  and P 2  across the pixel circuit in a wiring direction of the signal lines. 
     It is to be noted that, in  FIGS. 9 and 10 , the second position P 2  is selected as a position which does not overlap with the power signal line PSL which serves as a power supply line. 
       FIG. 11  illustrates a method (fabrication method) wherein a defective pixel circuit in the example of the first countermeasure is bypassed. 
     In the method illustrated in  FIG. 11 , short-circuiting between the scanning line (or gate line) to which the gate of the TFT  112  of the pixel circuit  101  is connected and the main signal line SGLM occurs at a position P 3 . 
     In this instance, in order to achieve bypassing of the defective pixel circuit, the main signal line SGLM is cut at a predetermined portion thereof between the positions P 1  and P 3  and also at another predetermined portion between the positions P 2  and P 3 . Consequently, the main signal line SGLM between the positions P 1  and P 2  is replaced with the sub signal line SGLS. 
     In other words, in the first countermeasure example, a method is applied wherein a portion at which short-circuiting or opening occurs is cut such that a line defect is converted into a point defect. 
       FIG. 12  schematically shows part of a pixel circuit and illustrates a second example of the countermeasure which achieves increase of the yield of a panel. 
       FIG. 13  shows a schematic equivalent circuit to the pixel circuit and illustrates the second countermeasure example. 
     The second countermeasure example is different from the first countermeasure example in that, while a plurality of signal lines, in the present embodiment, an original main signal line SGLM and a sub signal line SGLS, are wired in parallel to each other as seen in  FIGS. 12 and 13 , the signal lines are not connected like a ladder through the connection wiring lines CNL 1  and CNL 2  at two predetermined positions P 1  and P 2  across the pixel circuit in the wiring direction of the signal line. 
     Then, where the pixel circuit is defective, a cutting process of the main signal line SGLM and a connection process of the connection wiring lines CNL 1  and CNL 2  at the positions P 1  and P 2  are performed. 
     The reason why two signal lines including an original main signal line SGLM and a sub signal line SGLS are wired in parallel to each other is described below. 
     While the pixel circuit of  FIG. 7  is a 2Tr+1C pixel circuit including two transistors and one capacitor, in the 2Tr+1C pixel circuit, a signal line has to use two potentials including the offset potential Vofs and the data potential Vsig within a period  1 H as described hereinabove. 
     Therefore, the signal line may require a frequency equal to twice an ordinarily necessary frequency. If, in this state, a plurality of signal lines are connected like a ladder within one pixel as in the case of the first countermeasure example, then this increases the capacitance, which may sometimes be disadvantageous in reduction in power consumption. 
     Therefore, in the present second countermeasure example, two signal lines including an original main signal line SGLM and a sub signal line SGLS are merely wired in parallel to each other. 
       FIG. 14  illustrating a method, that is, a fabrication method, wherein a defect pixel circuit in the second countermeasure example is bypassed. 
     Also in the example of  FIG. 14 , a scanning line (or gate line) WSL to which the gate of the TFT  112  of the pixel circuit  101  and the main signal line SGLM are short-circuited at a position P 3  similarly as in the example of  FIG. 11 . 
     In this instance, in order to bypass the defective pixel circuit, the main signal line SGLM is cut at a predetermined point thereof between a position P 1  and the position P 3  and further cut at another predetermined point between a further position P 2  and the position P 3 . Then, the main signal line SGLM and the sub signal line SGLS are connected to each other at the position P 1  by a connection wiring line CNL 11  using a metal CVD method or the like. Besides, the main signal line SGLM and the sub signal line SGLS are connected to each other at the second position P 2  by a connection wiring line CNL 12  using a CVD method or the like. Then, the sub signal line SGLS is cut outside the processing region with respect to the position P 1  and further cut outside the processing region with respect to the second position P 2  thereby to replace the original main signal line SGLM between the first and second positions P 1  and P 2  with the sub signal line SGLS. 
     With the second countermeasure example, not only the yield of a panel can be improved, but also it is possible to suppress the capacitance of the signal lines low and achieve reduction in power consumption of the driver. 
       FIG. 15  schematically shows part of a pixel circuit and illustrates a third example of the countermeasure which achieves increase of the yield of a panel. 
       FIG. 16  shows a schematic equivalent circuit to the pixel circuit and illustrates the third countermeasure example. 
     The present third countermeasure example is different from the second countermeasure example described above in that, while a plurality of signal lines, in the present embodiment, two signal lines including an original main signal line SGLM and a sub signal line SGLS, are wired in parallel to each other as seen in  FIGS. 15 and 16 , the sub signal line SGLS is provided for each unit of one pixel in the wiring direction. 
       FIG. 17  illustrates a countermeasure method or fabrication method wherein a defective pixel circuit is bypassed in the third countermeasure example. 
     Also in the example of  FIG. 17 , a scanning line (or gate line) WSL to which the gate of the TFT  112  of the pixel circuit  101  and the main signal line SGLM are short-circuited at a position P 3  similarly as in the examples of  FIGS. 11 and 14 . 
     In this instance, in order to bypass the defective pixel circuit, the main signal line SGLM is cut at a predetermined point between a position P 1  and the position P 3  and further cut at another predetermined position between a further position P 2  and the position P 3 . Then, the main signal line SGLM and the sub signal line SGLS are connected to each other at the position P 1  at one end of a sub signal line SGLSS by a connection wiring line CNL 11  using a metal CVD method or the like. Besides, the main signal line SGLM and the sub signal line SGLS are connected to each other at the second position P 2  at the other end of the sub signal line SGLSS by a connection wiring line CNL 12  using a CVD method or the like. Consequently, the main signal line SGLM between the first and second positions P 1  and P 2  is replaced with the sub signal line SGLS. 
     With the third countermeasure example, since there is no necessity to cut the sub signal line, the processing time for repair can be reduced. Therefore, the third countermeasure example is advantageous in that not only reduction in power consumption of the driver, reduction in cost and enhancement of the yield can be achieved, but also it is possible to achieve reduction of the cost by reduction of the tact time. 
     Further, in the display apparatus  100  of the present embodiment, in order to achieve improvement against shading and irregular striped patterns arising from a pulse delay caused by wiring line resistance and wiring line capacitance of a scanning line WSL which is a wiring line for applying a driving pulse (or gate pulse) to the gate of a TFT (or transistor) in the pixel circuit  101  and/or in order to achieve improvement against irregularities or roughness of an image caused by irregularities such as shading when the voltage of a power supply line drops, that is, in order to achieve improvement of the picture quality, the following countermeasures are taken. 
       FIG. 18  shows part of a pixel circuit and illustrates an example of the countermeasure for improving the picture quality and so forth. 
     Referring to  FIG. 18 , in the first countermeasure illustrated, a scanning line (or gate line) WSL to which the gate GT of the TFT  112  serving as a switching transistor in each pixel circuit  101  is connected is a wiring line of the same material and in the same layer as those of a power supply line (or power signal line) PSL formed from metal of low resistance such as, for example, aluminum (Al). Meanwhile, a signal line SGL formed from metal of low resistance such as, for example, aluminum (Al) is formed in a lower layer, that is, a substrate side layer not shown, than that of the scanning line WSL and the power supply line PSL. 
     Then, the scanning line (or gate line) WSL in the upper layer and a low-resistance wiring line layer  114  which is in the same layer and is made of the same material as those of the signal line SGL which is in the lower layer than that of the scanning line WSL are connected to each other through a contact hole  116  formed in an interlayer insulating film  115  made of SIN or SiO 2  thereby to form a two-stage wiring structure. 
     Further, in the present first countermeasure example, a capacitor C 111  is disposed at a position displaced so as not to overlap with the scanning line WSL in the layering direction of the layers. 
     It is to be noted that the TFT  112  in each pixel circuit is of the bottom gate type and the gate electrode (control terminal) thereof is pulled up with respect to a contact formed on the insulating film not shown so as to be connected to the scanning line WSL. 
     Generally, the gate electrode of a TFT is formed as a film by high-resistance wiring, for example, by such a method as sputtering of metal or alloy of molybdenum (Mo), tantalum (Ta) or the like. 
     As described above, in the present countermeasure example, the scanning line (or gate line) WSL is laid out in two-stage wiring including a layer same as that of the power supply line of low resistance and the layer of the interlayer insulating film  115  same as that of the signal line. 
     With the countermeasure example having such a characteristic as just described above, the resistance and the capacitance of the scanning line (or gate line) WSL can be reduced. In particular, the wiring line layer which forms power supply lines is formed from low-resistance metal and also the wiring line layer which form signal lines SGL is formed from low-resistance metal. Therefore, by the employment of the two-stage wiring lines, the resistance of the scanning lines WSL can be reduced to approximately a half. Consequently, the transient of the gate line for the TFTs  112  as switching transistors can be accelerated. 
     Further, the difference in pulse width between gate pulses GP at the output end of the gate pulse (or control signal) GP of the writing scanner  104  to the scanning lines WSL and a position spaced from the output end can be reduced. Consequently, it is possible to obtain uniform picture quality free from insufficient writing, irregularities and shading. 
     Thus, there is an advantage that the transient of the gate line can be accelerated and higher definition can be anticipated. 
       FIG. 19  shows a configuration as a comparative example with the configuration of  FIG. 18  wherein a capacitor is disposed at a position overlapping with a scanning line (or gate line) in a layering direction of layers. 
     Where the configuration wherein a capacitor or a signal line is disposed at a position overlapping with a scanning line WSL in the layering direction of layers as seen in  FIG. 19 , there is a tendency that the parasitic capacitance of the scanning lines (or gate line) WSL increases. 
     In contrast, where the capacitor C 111  is disposed at a position displaced so as not to overlap with the scanning line WSL in the layering direction of the layers as in the present countermeasure example, only the signal lines are disposed in an overlapping relationship under the scanning lines WSL. Consequently, increase of the parasitic capacitance can be prevented, and further acceleration of the propagation speed of a gate pulse can be implemented. 
       FIG. 20  shows part of a pixel circuit and illustrates another countermeasure example for improvement in the picture quality and so forth. 
     In the present countermeasure example, power supply lines (or power driving lines) PSL are disposed in multiple layers in order to achieve improvement against appearance of irregularities or roughness on an image caused by irregularities such as shading when the voltage of the power supply lines drops. 
     As described hereinabove, the original power supply line PSL is formed from a low-resistance wiring line of the same material (or Al or the like) and in the same layer as those of the scanning line (or gate line) WSL at a predetermined position of a gate insulating film  118 . 
     Further, a contact hole  121  is formed in the interlayer insulating film  115  formed on the power supply line PSL. Further, a low-resistance wiring line layer  122  of Al or the like formed on the interlayer insulating film  115  is connected to the power supply line PSL through the contact hole  121  to form a multilayer structure. Further, the power supply lines are formed in a two-stage wiring line structure to achieve reduction in resistance thereby to achieve improvement against appearance of irregularities or roughness on an image caused by irregularities such as shading when the voltage of the power supply lines drops. 
     For example, a flattening film  123  is formed on the low-resistance wiring line layer  122  of an upper layer and a contact hole  124  is formed in the flattening film  123 . The low-resistance wiring line layer  122  is connected to the anode electrode  125  formed on the flattening film  123  through the contact hole  124 . 
     With the present countermeasure example, it is possible to prevent appearance of irregularities or roughness on an image caused by irregularities such as shading when the voltage of the power supply lines drops. 
     Now, more particular operation of the configuration described above, principally of a pixel circuit, is described with reference to  FIGS. 21A to 21E  and  22  to  29 . 
     It is to be noted that  FIG. 21A  illustrates a gate pulse (or scanning pulse) GP applied to the scanning line WSL;  FIG. 21B  illustrates a power signal PSG applied to the power driving line PSL;  FIG. 21C  illustrates an input signal SIN applied to the signal line SGL;  FIG. 21D  illustrates a potential VND 112  at the second node ND 112 ; and  FIG. 21E  illustrates a potential VND 111  at the first node ND 111 . 
     First, when the EL light emitting element  113  is in a light emitting state, the power supply voltage Vcc is applied to the power driving line PSL and the TFT  112  is in an off state as seen in  FIGS. 21B and 22 . 
     At this time, since the TFT  111  serving as a driving transistor is set so as to operate in a saturation region, the current Ids flowing through the EL light emitting element  113  assumes a value indicated by the expression 1 in response to the gate-source voltage Vgs of the TFT  111 . 
     Then, within a non-light emission period, a voltage Vss is applied to a power driving line PSL serving as a power supply line as seen in  FIGS. 21B and 23 . At this time, if the voltage Vss is lower than the sum of a threshold value Vthe 1  of the EL light emitting element  113  and the cathode voltage Vcat, that is, if Vss&lt;Vthe 1 +Vcat is satisfied, then the EL light emitting element  113  emits no light and the power driving line PSL serving as a power supply line acts as the source of the TFT  111  serving as a driving transistor. At this time, the anode of the EL light emitting element  113 , that is, the first node ND 111 , is charged to the voltage Vss as seen in  FIG. 21E . 
     Further, as seen in  FIGS. 21A ,  21 C,  21 D,  21 E and  24 , when the potential at the signal line SGL becomes equal to the offset voltage Vofs, the gate pulse GP is set to the high level to turn on the TFT  112  to set the gate potential of the TFT  111  to the offset voltage Vofs. 
     At this time, the gate-source voltage of the TFT  111  assumes a value of Vofs−Vss. Since the threshold value correction operation may not be performed if the gate-source voltage (Vofs−Vss) of the TFT  111  is not equal to or higher than (is lower than) the threshold voltage Vth of the TFT  111 , it is necessary to make the gate-source voltage (Vofs−Vss) of the TFT  111  higher than the threshold voltage Vth of the TFT  111 , that is, so as to satisfy Vofs−Vss&gt;Vth. 
     Then, in the threshold value correction operation, the power signal PSG to be applied to the power driving line PSL is set to the power supply voltage Vcc again. 
     Where the power driving line PSL is set to the power supply voltage Vcc, the anode (node ND 111 ) of the EL light emitting element  113  functions as the source of the TFT  111 , and current flows as seen in  FIG. 25 . 
     Since the equivalent circuit to the EL light emitting element  113  can be represented by diodes and capacitors as seen in  FIG. 25 , as far as the relationship of Ve 1 ≦Vcat+Vthe 1  (the leak current of the EL light emitting element  113  is considerably lower than the current flowing through the TFT  111 ) is satisfied, the current of the TFT  111  is used to charge the capacitors C 111  and Ce 1 . 
     At this time, the voltage Ve 1  between the terminals of the capacitor Ce 1  rises as time passes as seen in  FIG. 26 . After lapse of a fixed interval of time, the gate-source voltage of the TFT  111  assumes the value of Vth. At this time, Ve 1 =Vofs−Vth≦Vcat+Vthe 1  is satisfied. 
     After the threshold value cancellation operation comes to an end, the potential at the signal line SGL is set to the voltage of the data signal Vsig while the TFT  112  is in an on sate as seen in  FIGS. 21A ,  21   c  and  27 . The data signal Vsig has a value corresponding to a gradation. At this time, since the TFT  112  is on, the gate potential at the TFT  111  is equal to the potential of the data signal Vsig as seen in  FIG. 21D . However, since current Ids flows from the power driving line PSL serving as a power supply line to the TFT  111 , the source potential of the TFT  111  rises as time passes. 
     At this time, if the source voltage of the TFT  111  does not exceed the sum of the threshold voltage Vthe 1  of the EL light emitting element  113  and the cathode voltage Vcat, that is, if the leak current of the EL light emitting element  113  is considerably lower than the current flowing through the TFT  111 , then the current flowing through the TFT  111  is used to charge the capacitors C 111  and Ce 1 . 
     At this time, since the threshold value correction operation of the TFT  111  has completed, the current supplied from the TFT  111  reflects the mobility μ. 
     More particularly, as seen in  FIG. 28 , where the mobility μ is high, also the current amount is great and also rise of the source voltage is rapid. Consequently, the gate-source voltage of the TFT  111  decreases reflecting the mobility μ, and after lapse of a fixed interval of time, the gate-source voltage becomes equal to the gate-source voltage Vgs with which the mobility is corrected fully. 
     Finally, as seen in  FIGS. 21A to 21C  and  29 , the gate pulse GP is changed over to the low level to turn off the TFT  112  to end the writing and cause the EL light emitting element  113  to emit light. 
     Since the gate-source voltage of the TFT  111  is fixed, the TFT  111  supplies fixed current Ids′ to the EL light emitting element  113 , and the voltage Ve 1  rises to a voltage Vx with which the fixed current Ids′ flows through the EL light emitting element  113  thereby to cause the EL light emitting element  113  to emit light. 
     Also in the present pixel circuit  101 , as the light emission time becomes long, the I-V characteristic of the EL light emitting element  113  varies. Therefore, also the potential at a point B (first node ND 111 ) in  FIG. 29  varies. However, since the gate-source voltage of the TFT  111  is kept to the fixed value, the current flowing through the EL light emitting element  113  does not vary. Therefore, even if the I-V characteristic of the EL light emitting element  113  deteriorates, the fixed current Ids continues to flow and the luminance of the EL light emitting element  113  is kept fixed. 
     In the pixel circuit driven in such a manner as described above, since it has such a configuration according to any of the first to third countermeasure examples as described above, improvement of the yield of a panel can be achieved. Further, it becomes possible to suppress the capacitance of each signal line low and to achieve reduction in power consumption of the driver. 
     Further, an image of high picture with appearance of shading, striped irregularities and so forth suppressed can be obtained. 
     As described above, in the description of the present first embodiment, the first to third countermeasure examples are described as countermeasures which can achieve improvement of the yield of a panel for the display apparatus  100  which includes the circuit of  FIG. 7 , that is, a 2Tr+1C pixel circuit including two transistors and one capacitor. 
     However, while the first to third countermeasure examples are effective for the display apparatus  100  which includes a 2Tr+1C pixel circuit, it is possible to apply the countermeasures also to display apparatus which include a pixel circuit configured such that TFTs for the mobility and for the threshold value cancellation are provided separately in addition to a driving transistor or a switching transistor connected in series to an OLED. 
     In the following, from among such display apparatus, a display apparatus which includes a 5Tr+1C pixel circuit including five transistors and one capacitor is described as a second embodiment of the present invention. 
       FIG. 30  shows a configuration of an organic EL display apparatus which adopts the pixel circuit according to the second embodiment of the present invention. 
       FIG. 31  shows a particular configuration of the pixel circuit according to the present embodiment. 
     Referring to  FIGS. 30 and 31 , the display apparatus  200  shown includes a pixel array section  202  in which pixel circuits  201  are arrayed in an m×n matrix, a horizontal selector  203  (HSEL), a writing scanner  204  (WSCN), a driving scanner  205  (DSCN), a first auto zero circuit  206  (AZRD 1 ), and a second auto zero circuit  207  (AZRD 2 ). The display apparatus  200  further includes signal lines SGL selected by the horizontal selector  203  and supplied with a data signal corresponding to luminance information, scanning lines WSL as second driving wiring lines selectively driven by the writing scanner  204 , and driving lines DSL serving as first driving wiring lines selectively driven by the driving scanner  205 . The display apparatus  200  further includes first auto zero lines AZL 1  serving as fourth driving wiring lines selectively driven by the first auto zero circuit  206 , and second auto zero lines AZL 2  serving as third driving wiring lines selectively driven by the second auto zero circuit  207 . 
     The pixel circuit  201  according to the present embodiment includes, as seen in  FIGS. 30 and 31 , a p-channel TFT  211 , n-channel TFTs  212  to  215 , a capacitor C 211 , a light emitting element  216  formed from an organic EL light emitting element (OLED: electro-optical element), a first node ND 211 , and a second node ND 212 . 
     A first switching transistor is formed from the p-channel TFT  211 ; a second switching transistor is formed from the TFT  213 ; a third switching transistor is formed from the TFT  215 ; and a fourth switching transistor is formed form the TFT  214 . 
     It is to be noted that a supply line (power supply potential) for the power supply voltage Vcc corresponds to a first reference potential, and the ground potential GND corresponds to a second reference potential. Further, the voltage Vss 1  corresponds to a fourth reference voltage, and the voltage Vss 2  corresponds to a third reference voltage. 
     In the pixel circuit  201 , the p-channel TFT  211 , the TFT  212  serving as a driving transistor, the first node ND 211  and the light emitting element (OLED)  216  are connected in series between the first reference potential (in the present embodiment, the power supply voltage Vcc) and the second reference potential (in the present embodiment, the ground potential GND). In particular, the light emitting element  216  is connected at the cathode thereof to the ground potential GND and at the anode thereof to the p-channel TFT  211 . The TFT  212  is connected at the source thereof to the first node ND 211  and at the drain thereof to the drain of the p-channel TFT  211 . The p-channel TFT  211  is connected at the source thereof to the power supply voltage Vcc. 
     The TFT  212  is connected at the gate thereof to the second node ND 212 , and the p-channel TFT  211  is connected at the gate thereof to the driving line DSL. 
     The TFT  213  is connected at the drain thereof to the first node ND 211  and a first electrode of the capacitor C 211 , at the source thereof to the fixed potential Vss 2 , and at the gate thereof to the second auto zero line AZL 2 . Further, the capacitor C 211  is connected at a second electrode thereof to the second node ND 212 . 
     The source and the drain of the TFT  214  are connected between the signal line SGL and the second node ND 212 , respectively. The TFT  214  is connected at the gate thereof to the scanning line WSL. 
     Further, the source and the drain of the TFT  215  are connected between the second node ND 212  and the voltage Vss 1 , respectively. The TFT  215  is connected at the gate thereof to the first auto zero line AZL 1 . 
     In this manner, the pixel circuit  201  according to the present embodiment is configured such that the capacitor C 211  serving as a pixel capacitor is connected between the gate and the source of the TFT  212  serving as a driving transistor and the source potential of the TFT  212  is connected to a fixed potential within the non-light emission period through the TFT  213  serving as a switching transistor while the gate and the drain of the TFT  212  are connected to each other so that correction of the threshold voltage Vth is performed. 
     Then, in the present second embodiment, the first to third countermeasures for the picture quality improvement described hereinabove in connection with the first embodiment are taken for at least one of the scanning line WSL and driving line DSL, or two or more or all of the scanning line WSL, driving line DSL and auto zero lines AZL 1  and AZL 2 . 
     By applying a desired one or ones of the countermeasures, a countermeasure against shading, striped irregularities and so forth arising from a delay of a driving signal (pulse) in wiring line resistance or wiring line capacitance over the overall panel is taken. Consequently, it is possible to obtain an image of good picture quality with occurrence of shading, irregularities and so forth suppressed. 
     Now, operation of the configuration described above, particularly operation of a pixel circuit, is described with reference to  FIGS. 32A to 32F . 
     It is to be noted that  FIG. 32A  illustrates a driving signal DS applied to the driving line DSL;  FIG. 32B  a driving signal WS (corresponding to the gate pulse GP in the first embodiment) applied to the scanning line WSL;  FIG. 32C  a driving signal AZ 1  applied to the first auto zero line AZL 1 ;  FIG. 32D  a driving signal AZ 2  applied to the second auto zero line AZL 2 ;  FIG. 32E  the potential at the second node ND 112 ; and  FIG. 32F  the potential at the first node ND 111 . 
     The driving signal DS of the driving line DSL by the driving scanner  205  is kept at the high level, and the driving signal WS to the scanning line WSL by the writing scanner  204  is kept at the low level. Further, the driving signal AZ 1  to the first auto zero line AZL 1  by the first auto zero circuit  206  is kept at the low level, and the driving signal AZ 2  to the second auto zero line AZL 2  by the second auto zero circuit  207  is kept at the high level. 
     As a result, the TFT  213  is turned on, and thereupon, current flows through the TFT  213  and the source potential Vs of the TFT  212  (potential of the node ND  211 ) drops to the voltage Vss 2 . Therefore, also the voltage applied to the EL light emitting element  216  drops to 0 V, and the EL light emitting element  216  is placed into a non-light emission state. 
     In this instance, even if the TFT  214  is turned on, the voltage held in the capacitor C 211 , that is, the gate potential of the second node ND 212 , does not vary. 
     Then, within the non-light emission period of the EL light emitting element  216 , the driving signal AZ 1  to the first auto zero line AZL 1  is set to the high level while the driving signal AZ 2  to the second auto zero line AZL 2  is kept to the high level as seen in  FIGS. 32C and 32D . Consequently, the potential at the second node ND 212  changes to the voltage Vss 1 . 
     Then, after the driving signal AZ 2  to the second auto zero line AZL 2  is changed over to the low level, the driving signal DS of the driving line DSL by the driving scanner  205  is changed over to the low level for a predetermined period of time. 
     Consequently, the TFT  213  is turned off and the TFTs  215  and  212  are turned on. As a result, current flows along the route of the TFTs  212  and  211  and the potential at the first node rises. 
     Then, the driving signal DS of the driving line DSL by the driving scanner  205  is changed over to the high level and the driving signal AZ 1  is changed over to the low level. 
     As a result, correction of the threshold voltage Vth of the driving transistor TFT  212  is performed, and the potential difference between the second node ND 212  and the first node ND 211  becomes equal to the threshold voltage Vth. 
     In this state, after lapse of a predetermined period of time, the driving signal WS to the scanning line WSL by the writing scanner  204  is kept to the high level for a predetermined period of time, the data is written into the node ND 212  from the data line, the driving signal DS to the driving line DSL by the driving scanner  205  is changed over to the high level while the driving signal WS is kept to the high level, and soon, the driving signal WS is changed over to the low level. 
     At this time, the TFT  212  is turned on and the TFT  214  is turned off, and correction of the mobility is performed. 
     In this instance, since the TFT  214  is in an off state and the gate-source voltage of the TFT  212  is fixed, the TFT  212  supplies fixed current Ids to the EL light emitting element  216 . Consequently, the potential at the first node ND 211  rises to the source potential Vx at which the current Ids flows to the EL light emitting element  216 . Consequently, the EL light emitting element  216  emits light. 
     Here, also in the present circuit, if the light emitting time period becomes long, then the current-voltage (I-V) characteristic varies. Therefore, also the potential at the first node ND 211  varies. However, since the gate-source voltage Vgs of the TFT  212  is kept to a fixed value, the current flowing through the EL light emitting element  216  does not vary. Therefore, even if the I-V characteristic of the EL light emitting element  216  varies, the current Ids continues to flow and the luminance of the EL light emitting element  216  does not vary. 
     In the pixel circuit driven in this manner, since a countermeasure against shading, striped irregularities and so forth arising from a delay of a driving signal (pulse) by wiring line resistance over the overall panel is taken, it is possible to obtain an image of good picture quality with occurrence of shading, irregularities and so forth suppressed. 
     While 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 following claims.