Abstract:
It is the object of the present invention to provide a simple and accurate testing circuit and a testing method while occupying as small space as possible in an image display device. By partly changing dummy pixels arranged in the periphery of a display region into a testing circuit, tests for detecting broken wires in data signal lines and scanning lines and whether pixels are controlled adequately can be conducted easily and accurately, occupying as small space as possible without a need of an additional complicated circuit. Accordingly, a display panel can be produced at a low cost.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of U.S. patent application Ser. No. 10/724,835, filed Dec. 2, 2003, which issued on May 30, 2006 as U.S. Pat. No. 7,053,649, which claims priority under 35 U.S.C. § 119 from Japanese Application No. 2002-354804, filed Dec. 6, 2002. The contents of the prior applications are incorporated herein by reference in their entirety. 

   FIELD OF THE INVENTION 
   The present invention relates to an image display device in which a plurality of pixels are arranged in matrix and a method of testing the image display device. 
   DESCRIPTION OF THE RELATED ART 
   In recent years, image display devices such as a liquid crystal display (LCD), an to electroluminescence (EL) display and the like have been advanced in precision and thus the degree of integration of elements has been remarkably improved as well. 
   It is an essential part of the production line of image display device to test if a circuit mounted on a substrate operates normally before shipment as a finished panel. The test process itself has been becoming more complicated in accordance with the higher precision. 
     FIG. 3  shows a configuration of a substrate on which a testing circuit is implemented in a conventional image display device. Such a configuration is shown, for example, in Japanese Patent Laid-Open No. 14-116423. The image display device comprises a substrate  301 , a source driver circuit  302 , a gate driver circuit  303 , a pixel  304 , a data signal line  305 , a scanning line  306 , a testing circuit  311 , a switch driver circuit  312 , an analog switch  313 , a testing line  314 , and testing terminals  315   a  and  315   b . The substrate  301  mounts the testing circuit  311  and the pixel  304  which are arranged in matrix, and the data signal line (source bus line)  305  and the scanning line (gate bus line)  306  are arranged so as to be orthogonal to each other. The scanning line  306  is connected to the gate driver circuit  303  and the data signal line  305  is connected to the source driver circuit  302 . 
   A pixel portion of the display device comprises a first pixel, which is provided in a display region and contribute to an image display and a second pixel, which is provided in outer edges of the display region (hereinafter referred to as a non-display region) and have no influence on the image display.  FIG. 7A  shows an overview thereof. The image display device comprises a substrate  701 , a source driver circuit  702 , a gate driver circuit  703 , a pixel portion  704 , a data signal line  707 , and a scanning line  706 . A non-display region  704   b  is provided at one side of outer edges of a display region  704   a  for simplicity, however, it is often the case that the non-display region  704   b  is provided so as to surround the outer edges of the display region  704   a.    
   In the display device, each pixels connected to the scanning line  306  is controlled by the scanning line  306 . A video signal is sequentially supplied to the source driver circuit  302  and the video signal is outputted simultaneously to the data signal line  305  when a latch signal is inputted, and afterwards inputted to each pixel. 
   The configuration of a pixel in the display device is explained in details with reference to  FIG. 7B . Each of the first and second pixels comprises a data signal line  711 , a scanning line  712 , a current supply line  717 , a switching thin film transistor  713  (thin film transistor is hereinafter referred to as TFT), a driver TFT  714 , a capacitor  715 , a light emitting element  716 , and a power source line  718 . 
   The switching TFT  713  is provided between a gate electrode of the driver TFT  714  and the data signal line  711 , and the scanning line  712  is connected to a gate electrode of the switching TFT  713 . Therefore, the driver TFT  714  can be controlled by a signal outputted to the data signal line  711  when the switching TFT  713  is ON. The capacitor  715  is provided between the gate electrode of the driver TFT  714  and the current supply line  717  and holds a voltage between gate and source of the driver TFT  714 . 
   The difference between the first and second pixels is explained hereafter. In the first pixel, a light emitting element is provided between a driver TFT and a power source, a light is emitted when a current is supplied to the light emitting element by supplying a current from the current supply line through the driver TFT. The light emitting element emits a light according to the potential of video signals inputted from a source signal line to a gate electrode of the driver TFT. In the second pixel, a light emitting element is not connected to a driver TFT, hence the second pixel is a dummy pixel which does not emit light. 
   One reason why such a non-display region having a dummy pixel is provided in peripheral portions of a display region is that the peripheral portion of a pixel portion tends to be inhomogeneous compared with the center part thereof in the step for forming a liquid crystal element and a light emitting element in the pixel portion in a flat panel display such as a liquid crystal display device or a light emitting device. See, for example, Japanese Patent Laid-Open No. 5-241153. 
   A short circuit between wirings and broken wires of the display device can be detected by a method of testing an output by bringing a probe pin into contact with the detecting pad  315   a , or by a method of using a testing circuit  311 . The testing circuit  311  supplies a potential level of each data signal line to a testing line  314  sequentially while driving a switch control circuit  312 , then a short circuit between wirings and broken wires of the display device can be detected by bringing a probe pin into contact with the testing terminal  315   b.    
   The method of testing by bringing a probe pin into contact with the testing terminal  315   a  requires a lot of time since every testing terminal of every testing line has to be tested. Instead, when a number of probe pins is increased for saving time, expensive testing apparatus is required. As another idea, it is suggested that the testing circuit  311  is formed over a substrate, however, the testing circuit requires a large space since the switching driver circuit  312  is required in the testing circuit, although the testing circuit  311  has nothing to do with the image display. 
   An image display device having a testing circuit without a switching driver circuit is suggested (for example, U.S. Pat. No. 2,618,042).  FIG. 4  is a configuration diagram of the image display device which comprises a substrate  401 , a source driver circuit  402 , a gate driver circuit  403 , a pixel  404 , a data signal line  405 , a scanning line  406 , a video signal line  407 , analog switches  408  and  412 , a testing circuit  411 , a testing line  413 , and testing terminals  414   a  and  414   b . The data signal line  405  is connected to a gate of the analog switch  412 . 
   Faults such as broken wires of the data signal lines  405  can be detected by inputting a testing pulse to the video signal line  407  and observing an output waveform from the analog switch  412  at a testing terminal  414   b.    
   SUMMARY OF THE INVENTION 
   The above-mentioned testing method only tests the operations of the gate driver circuit  403  and the source driver circuit  402  and the qualities of the scanning lines  406  and data signal lines  405 , therefore it is not sufficient unless the method can test whether a thin film transistors in a pixel  404  are controlled adequately. 
   It is an object of the invention to provide an image display device with the minimum testing area which can be tested easily in a short time without using many probe pins, and which can not only test a driver circuit, scanning lines, and data signal lines, but also test whether TFTs inside the pixels are controlled adequately. It is a further object of the invention to provide a method for testing the image display device. 
   The invention provides a testing circuit or a dummy circuit by changing a part of a pixel circuit in a display device so that circuits in pixels and whether pixels are controlled adequately are easily and accurately tested, while making the space for testing circuit as small as possible. 
   An image display device comprises a pixel portion comprising a display region and a testing region wherein the display region and the testing region comprise a first pixel circuit and a second pixel circuit, respectively. Each of the first and second pixel circuits comprises a data signal line, a scanning line, a driver transistor, and a switching transistor provided between the data signal line and a gate electrode of the driver transistor wherein a gate electrode of the switching transistor is electrically connected to the scanning line, the driver transistor of the first pixel circuit is provided between a current supply line and a light emitting element, and the driver transistor of the second pixel circuit is provided between the current supply line and a testing terminal. 
   An image display device comprises a pixel portion comprising a display region and a testing region, wherein the display region and the testing region comprises a first pixel circuit and a second pixel circuit, respectively. The first pixel circuit comprises a data signal line, a scanning line, a driver transistor provided between a current supply line and a light emitting element, and a switching transistor provided between the data signal line and a gate electrode of the driver transistor. The second pixel circuit comprises the data signal line, the scanning line, and the switching transistor provided between the data signal line and a testing output terminal wherein a gate electrode of the switching transistor of the first pixel circuit is electrically connected to the scanning line. 
   An image display device of the invention is characterized by providing the testing region in a dummy pixel region in the outer edge portion of the display region. 
   An image display device comprises a pixel portion comprising a display region and a testing region wherein the display region and the testing region comprise a first pixel circuit and a second pixel circuit, respectively. Each of the first and second pixel circuits comprises a data signal line, a scanning line, a driver transistor, and a switching transistor provided between the data signal line and a gate electrode of the driver transistor. A gate electrode of the switching transistor is electrically connected to the scanning line, the driver transistor of the first pixel circuit is provided between a current supply line and a light emitting element, the driver transistor of the second pixel circuit is provided between the current supply line and a testing terminal, and the scanning line is placed in a selected state to provide a conduction to the switching transistor in the second pixel circuit and a drain current of the driver transistor is outputted to the testing output terminal according to the signal outputted to the data signal line. 
   A method for testing an image display device of the invention is characterized in that a signal outputted to the data signal line is a video signal. 
   A method for testing an image display device of the invention is characterized in that a signal outputted to the data signal line is a testing pulse. 
   An image display device comprises a pixel portion comprising a display region and a testing region, wherein the display region and the testing region comprise a first pixel circuit and a second pixel circuit, respectively. The first pixel circuit comprises a data signal line, a scanning line, a driver transistor provided between a current supply line and a light emitting element, and a switching transistor provided between the data signal line and a gate electrode of the driver transistor. The second pixel circuit comprises the data signal line, the scanning line, and the switching transistor provided between the data signal line and a testing output terminal. A gate electrode of the switching transistor of the first pixel circuit is electrically connected to the scanning line, and the scanning line is placed in a selected state to provide a conduction to the switching transistor in the second pixel circuit and output the signal which is outputted to the data signal line to the testing output terminal. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1A and 1B  are diagrams showing an embodiment mode of the invention. 
       FIGS. 2A and 2B  are diagrams showing an embodiment mode of the invention. 
       FIG. 3  is a configuration diagram showing a general image display device and a conventional testing circuit as an example. 
       FIG. 4  is a configuration diagram showing a general testing circuit as an example. 
       FIGS. 5A to 5C  are diagrams showing an embodiment of the invention. 
       FIGS. 6A and 6B  are diagrams showing an embodiment of the invention. 
       FIGS. 7A and 7B  are configuration diagrams showing a general image display device. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The invention is explained in more details in the following embodiment modes. 
   Embodiment Mode 1 
     FIG. 1A  shows a testing circuit according to the embodiment mode 1 of the invention. The testing circuit comprises a data signal line  101 , a scanning line  102 , a switching TFT  103 , a driver TFT  104 , a capacitor  105 , a current supply line  106 , a testing cell  107 , and a testing line  108 . The circuit of  FIG. 1A  is used for testing data signal lines. The testing circuit comprises a pixel circuit as shown in  FIG. 1B  and the testing cell  107  corresponds to the pixel circuit. The pixel circuit of  FIG. 1B  comprises a data signal line  111 , a scanning line  112 , a switching TFT  103 , a driver TFT  104 , a capacitor  105 , an EL element  116 , a current supply line  117 , and a power source line  118 . 
   In the pixel circuit, the driver TFT  104  supplies a current to the EL element  116  to emit light. The EL element  116  emits light in accordance with a potential of a video signal which is inputted to a gate of the driver TFT  104  from the source signal line  111  through the switching TFT  103 . 
   The configuration of the testing cell  107  is explained by comparison with the pixel circuit of  FIG. 1B . First, the EL element  116  to which the driver TFT  104  supplies a current is removed from the pixel circuit. Secondly, the driver TFT  104  is detached from the power source line  118  and connected to the testing line  108  instead. The testing line  108  is connected to a testing terminal. The scanning line  112  shared by pixel circuits is detached from the gate driver circuit so that an independent switch signal line  102  of the testing circuit is obtained. The testing cell  107  is obtained in this manner. The driver TFT  104  operates as an analog switch which is controlled by video signals when a signal is inputted to the switch signal line  102  and the switching TFT  103  is ON. 
   A method of testing the data signal line is explained hereafter. A testing circuit as shown in  FIG. 1A  is formed over a substrate. In the testing circuit, n data signal lines which are connected to the pixel portion are connected to S 1 , S 2 , . . . , Sn one to one. A potential of the video signal is outputted to each data signal line, and switching signals are inputted at this time. The driver TFT  104  is controlled in accordance with the video signal when the switching TFT  103  is ON, and the test can be carried out by comparing the video signal with the measured output value OUT. By inputting the testing pulse into the video signal, faults can be located easily. 
     FIG. 5B  shows a timing chart at the time of testing. A substrate  501  as shown in  FIG. 5A  has a configuration that video signals are sequentially supplied to a first latch circuit from a shift register, and outputted simultaneously from a second latch circuit to each data signal line when a latch signal is high. Because of this, n data signal lines S 1 , S 2 , . . . , Sn are selected respectively as shown in  FIG. 5C  to provide n times video signal input periods V. The output signal of this time is a High output as a driver TFT in one testing cell is in the state of ON when a latch signal is selected. 
   Embodiment Mode 2 
     FIGS. 2A and 2B  each shows a testing circuit according to the embodiment mode 2 of the invention. This circuit is used for the test of scanning lines. The testing circuit is prepared based on the pixel circuit of  FIG. 1B  as in embodiment mode 1, and a testing cell  205  in  FIG. 2A  corresponds to the pixel circuit. 
   The testing circuit in  FIG. 2A  comprises a scanning line  201 , a switching TFT  202 , a capacitor  203 , a current supply line  204 , a testing cell  205 , and a testing line  206 . 
   The configuration of the testing cell  205  is explained by comparison with a pixel circuit. An EL element and a driver TFT are removed from a pixel circuit. The power source line  118  connected to the EL element is detached from the drier TFT  104  and connected to the testing line  206 . Also, the data signal line  111  shared by pixel circuits is detached from the source driver circuit and connected to the power voltage VDD, thereby the testing cell  205  is obtained. The switching TFT operates as an analog switch which is controlled by the gate driver circuit. 
   Driver TFTs do not have to be removed when the pixels need to have the same configurations so the film thickness of EL is homogenous in the display region with dummy pixels in the periphery thereof. In that case, a node of the switching TFT, the driver TFT, and the capacitor is connected to the testing line which is connected to the testing terminal. 
   A method of testing the scanning signal line is explained hereafter. A testing circuit as shown in  FIG. 2A  is formed over a substrate. In the testing circuit, n scanning lines which are connected to the pixel portion are connected to G 1 , G 2 , . . . , Gn one to one. In accordance with the output from the gate driver circuit to each scanning line  201 , the switching TFTs  202  are sequentially turned ON, and test can be conducted by measuring the output value OUT thereof. 
   The scanning line cannot be inputted a testing pulse unlike the data signal line, and an output portion should be devised. In  FIG. 2A , two outputs are provided as OUT 1  and OUT 2  to which the scanning lines are connected alternately. The shift register selects the switching TFTs  202  sequentially so that a square wave can be detected, which makes the judgment of the test clearer.  FIG. 6B  shows a timing chart. Each scanning line G 1 , G 2 , . . . , Gn is sequentially turned ON and High output is alternately outputted. A merit of outputting such a square wave is that the location of fault can be specified by counting the number thereof. 
   As a countermeasure against the fact that the testing pulse cannot be inputted, a delay Buffer  211  and a NAND  212  can be added as shown in  FIG. 2B  so that the Buffer makes the delay and square waves can be detected by the output thereof and NAND calculation. 
   Embodiment 
   Embodiments of the invention are explained hereafter. 
   Embodiment 1 
     FIG. 5A  shows embodiment 1 of the invention. A display device shown here comprises a substrate  501 , a source driver circuit  502 , a gate driver circuit  503 , a pixel  504 , a data signal line  505 , and a scanning line  506 . Among the pixels arranged in matrix, the last row of pixels opposed to the source driver circuit  502  is selected. In the case where dummy pixels are arranged in the periphery of a display region, they may be selected. The selected pixels are changed as follows. The EL element is removed from the pixel and the TFT in the pixel is detached from the power source line which is connected to the EL element, and connected to a testing terminal instead. One testing terminal is employed in the embodiment of  FIG. 5A  by connecting all the pixels in the row in common, although testing terminals may be plural if needed for the testing reason. Further, the scanning line  506  shared by pixels is detached from the gate driver circuit  503  so that an independent switch signal line is obtained. The switch signal line is independent for simplicity of the test in this embodiment, however, it may remain connected as far as it is controlled by the gate driver circuit. The test of the data signal line is carried out by inputting a testing pulse S 1 , S 2 , . . . , Sn in a state that the switch signal line is inputted, and by observing the output signals when latch signals are inputted. 
   In this embodiment, the TFT in the pixel is used as it is, however, the size of the TFT may be changed analogously in order to increase the sensitivity for confirming the operation of TFT in the pixel. 
   Embodiment 2 
     FIG. 6A  shows the second embodiment of the invention. The display device shown here comprises a substrate  601 , a source driver circuit  602 , a gate driver circuit  603 , a pixel  604 , a data signal line  605 , and a scanning line  606 . Among the pixels arranged in matrix, the last row of pixels opposed to the gate driver circuit  603  is selected. In the case where dummy pixels are arranged in the periphery of a display region, they may be selected. The selected pixels are changed as follows. An EL element and a driver TFT are removed from the pixel, and the TFT in the pixel is detached from the power source line connected to the EL element, and connected to a testing line instead. In the embodiment of  FIG. 6A , two output terminals are employed to which the scanning line  606  is connected alternately. Further, the data signal line  605  shared by pixels is detached from the source driver circuit  602  and connected to a power voltage instead. The data signal line  605  is connected to the power voltage for the simplicity of the test in this Embodiment, however, they may remain connected as far as a detectable potential level by the source driver circuit  602  is applied. The test of the scanning lines is conducted as shown in  FIG. 6B , by operating the gate driver circuit and observing the output signals thereof. 
   In this embodiment, the TFT in the pixel is used as it is, however, the size of the TFT may be changed analogously in order to increase the sensitivity for confirming the operation of TFT in the pixel. 
   The invention is made in view to provide an image display device and a testing method which provide a testing circuit by changing a part of a pixel circuit in a display device or using a dummy pixel so that the test for detecting broken wirings and the like of the data signal line and the scanning lines and whether pixels are controlled adequately is easily and accurately conducted, and even the location of faults can be detected if any, while making the space for testing circuit as small as possible.