Abstract:
A display device includes (a) a plurality of pixels arranged in a matrix, each of the pixels including a light-emitting device, a switch and a transistor, (b) a scanning line extending in a first direction, (c) a data line extending in a second direction perpendicular to the first direction, (d) a first bias voltage line extending in the second direction, (e) a bias voltage generating circuit which applies a bias voltage to the bias voltage line, (f) a second bias voltage line which surrounds the pixels and is a closed line, and (g) a third bias voltage line which electrically connects the bias voltage generating circuit to the second bias voltage line. The first bias voltage line is electrically connected at opposite ends thereof to the second bias voltage line. The switch is turned on when the scanning line is activated, to thereby allow image signals to be transmitted to the gate of the transistor therethrough from the data line. The second and third bias voltage lines are designed to have such a wire resistance that a constant current is supplied to the light-emitting device from the bias voltage generating circuit through the first, second and third bias voltage lines.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates to a display device including an active device, and more particularly to an active matrix type display device including a spontaneous light-emitting device such as an organic electroluminescence (EL).  
           [0003]    2. Description of the Related Art  
           [0004]    A portable communication terminal such as a cellular phone has been widely used recently. As a display unit of such a portable communication terminal, a liquid crystal display device is widely used.  
           [0005]    A liquid crystal display device including a back light unit is accompanied with a problem of much power consumption for enhancing a brightness in a display screen. To solve this problem, a display device including an organic electroluminescence (hereinafter, referred to simply as “organic EL display device”) attracts an attention as a display device suitable to a portable communication terminal, as having been suggested in Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.  
           [0006]    Hereinbelow is set forth a summary of Nikkei Electronics, No. 765, Mar. 13, 2000, pp. 55-62.  
           [0007]    As a display device including a spontaneous light-emitting device which emits a light when a current runs therethrough, there are known a plasma display (PDP) device and an electroluminescence (EL) device. An electroluminescence device is grouped into an inorganic one and an organic one with respect to a material of which an electroluminescence device is composed, and is further grouped into a simple matrix type device and an active matrix type device with respect to a structure.  
           [0008]    [0008]FIG. 1 is a block diagram of a simple matrix type organic EL display device.  
           [0009]    As illustrated in FIG. 1, the simple matrix type organic EL display device includes a data line driver circuit  55  to which a plurality of data lines  53  are electrically connected, a scanning line driver circuit  56  to which a plurality of scanning lines  54  are electrically connected, and a plurality of pixels arranged in a matrix.  
           [0010]    Each of the pixels is comprised of an electroluminescence device  51 , a capacitor  52  electrically connected between an anode and a cathode of the electroluminescence device  51 , one of the data lines  53  to which the anode of the electroluminescence device  51  is electrically connected, and one of the scanning lines  54  to which the cathode of the electroluminescence device  51  is electrically connected.  
           [0011]    The data line driver circuit  55  activates one of the data lines  53  and the scanning line driver circuit  56  activates one of the scanning lines  54  to thereby supply the electroluminescence device  51  electrically connected to the thus activated data and scanning lines  53  and  54 , with a current from the activated data line  53  towards the activated scanning line  54 . As a result, the electroluminescence device  51  emits a light with a brightness determined in accordance with the current running through the electroluminescence device  51 .  
           [0012]    Since a simple matrix type organic EL display device has a relatively simple structure as mentioned above, it can be fabricated with low costs. However, it is difficult for a simple matrix type organic EL display device to increase the number of pixels for accomplishing a higher density in pixels.  
           [0013]    Since a scanning line is selected one by one, and then, a light-emitting diode in an associated pixel is made to emit a light in a simple matrix type organic EL display device, a period of time during which a light-emitting diode in a pixel emits a light is equal to A/B wherein A indicates a frame period and B indicates the number of scanning lines. In order to keep a brightness constant in such a limited period of time, it would be necessary to instantaneously flow a much current through a pixel.  
           [0014]    If the number of pixels is increased, the data line  53  would have an increased wire length. The data lines  53  are generally composed of a transparent material such as ITO (Indium Tin Oxide), and hence, has a high wire resistivity. As a result, as the data lines  53  have an increased wire length, the data lines  53  would have an increased wire resistance.  
           [0015]    Thus, there occurs a significant voltage drop in the data lines  53 , because the data lines  53  have an increased wire resistance, and further because a much current runs through the data lines  53 .  
           [0016]    Such a significant voltage drop results in that a voltage on the data line  53  located farther away from the data line driver circuit  55  becomes smaller than a voltage on the data line  53  located closer to the data line driver circuit  55 . This causes that a smaller current runs through the electroluminescence device  51  electrically connected to the data line  53  located farther away from the data line driver circuit  55 .  
           [0017]    That is, since a smaller current runs through the electroluminescence device  51  electrically connected to the data line  53  located farther away from the data line driver circuit  55 , because of an increased wire resistance of the data lines  53 , the electroluminescence device  51  would emit a light in a smaller amount, resulting in non-uniformity in a brightness in a display screen. Specifically, a pixel located farther away from the data line driver circuit  55  would have a smaller brightness.  
           [0018]    [0018]FIG. 2 is a block diagram of a conventional active matrix type organic electroluminescence display device.  
           [0019]    As illustrated in FIG. 2, the conventional active matrix type organic EL display device includes a data line driver circuit  68  to which a plurality of data lines  65  are electrically connected, a scanning line driver circuit  69  to which a plurality of scanning lines  66  are electrically connected, a bias voltage source  610 , a common bias voltage line  611  through which a bias voltage is applied from the bias voltage source  610 , a plurality of bias voltage lines  67  electrically connected to the bias voltage line  611 , and a plurality of pixels arranged in a matrix.  
           [0020]    Each of the pixels is comprised of an electroluminescence device  61 , a first thin film transistor (TFT)  62  electrically connected between an anode of the electroluminescence device  61  and one of the bias voltage lines  67 , a second thin film transistor (TFT)  63  electrically connected between one of the data lines  65  and a gate of the first thin film transistor  62 , and a capacitor  64  electrically connected between a gate of the first thin film transistor  62  and one of the bias voltage lines  67 .  
           [0021]    When the scanning line driver circuit  69  activates one of the scanning lines  66 , the second thin film transistor  63  electrically connected to the thus activated scanning line  66  is turned on, and hence, a current runs to the capacitor  64  through the data line  65  and the second thin film transistor  63  from the data line driver circuit  68 , resulting in that the capacitor  64  is electrically charged.  
           [0022]    Thus, a gate voltage of the first thin film transistor  62  becomes high. When the gate voltage of the first thin film transistor  62  becomes higher than a threshold voltage, the first thin film transistor  62  is turned on, resulting in that a current is supplied to the electroluminescence device  61  through the common bias voltage line  611  and the bias voltage line  67  from the bias voltage source  610 . Thus, the electroluminescence device  610  emits a light at a brightness in accordance with the current supplied thereto.  
           [0023]    As is obvious in view of the above, the active matrix type organic EL display device and is characterized in that even if the number of scanning lines were increased, it is ensured to have a period of time during which a light is emitted, equal to a frame period of time, differently from the simple matrix type organic EL display device.  
           [0024]    Herein, an active matrix type liquid crystal display device is compared to the above-mentioned active matrix type organic EL display device.  
           [0025]    In an active matrix type liquid crystal display device, a transmissivity, which corresponds to a brightness in an active matrix type organic EL display device, is in proportion to a voltage to be applied to liquid crystal. In contrast, a brightness in an active matrix type organic EL display device is in proportion to a current, and a voltage supplied to the bias voltage lines  67  from the bias voltage source  610  is fixed at a constant voltage.  
           [0026]    Since an organic EL display device is driven by a current, a thin film transistor simply conducting an on/off operation cannot be used in an organic EL display device unlike an active matrix type liquid crystal display device. An organic EL display device has to use a thin film transistor having an on-resistance small enough for a current to run therethrough.  
           [0027]    Such a thin film transistor having a small on-resistance cannot be fabricated by a process for fabricating an amorphous silicon thin film transistor, and hence, has to be fabricated by a process for fabricating a low-temperature polysilicon thin film transistor which process is usually used for fabricating a display device capable of displaying images with high accuracy.  
           [0028]    In a low-temperature polysilicon thin film transistor, a thin film transistor and a driver circuit can be fabricated on a glass substrate. When multi gradation display is to be accomplished, almost all circuits associated with scanning lines and a part of circuits (selection switches) associated with data lines are fabricated on a glass substrate, and a complex circuit for controlling gradation is comprised of a semiconductor integrated circuit formed on a singly crystal substrate.  
           [0029]    An active matrix type liquid crystal display device uses red, green and blue color filters for displaying colored images.  
           [0030]    In contrast, an active matrix type organic EL display device uses organic EL devices emitting red, green and blue lights, for displaying colored images.  
           [0031]    However, the active matrix type organic EL display device is accompanied with problems that an organic EL device emitting a red light has a shorter lifetime than those of organic EL devices emitting green and blue lights, and that the organic EL device does not emit a pure red light, but emit an orange light.  
           [0032]    In the active matrix type organic EL display device, red, green and blue lights may be mixed to one another to thereby produce a white light, and pixels associated with red, green and blue may be fabricated through the use of color filters like a liquid crystal display device.  
           [0033]    In the above-mentioned simple matrix type organic EL display device, as the number of pixels is increased, a data line would have a longer wire length, and hence, have a greater wire resistance.  
           [0034]    Thus, a voltage drop would occur in a data line because of an increase in a wire resistance thereof and further because of a much current running through a data line. This causes a problem that since a current running through an electroluminescence device electrically connected to a data line located remote from the data line driver circuit is reduced, the electroluminescence device would emit a light in a smaller amount, resulting in non-uniformity in a brightness in a display screen.  
           [0035]    On the other hand, though the active matrix type organic EL display device has a merit that a period of time during which the display device can emit a light which period is equal to a frame period of time can be ensured, the active matrix type organic EL display device is accompanied with the following problem like the above-mentioned simple matrix type organic EL display device.  
           [0036]    Bias voltage lines or transparent electrodes would have an increased wire resistivity and an increased wire resistance, as the number of pixels is increased. This results in that the bias voltage lines would have an increased wire resistance, a pixel located far away from the bias voltage source would have a reduced brightness, and hence, non-uniformity occurs in a display screen of the active matrix type organic EL display device.  
           [0037]    A further problem common to the conventional simple matrix type organic EL display device and the conventional active matrix type organic EL display device is that extra power has to be supplied to the bias voltage lines from the bias voltage source in order to compensate for reduction in a brightness in a pixel which reduction is caused by an increase in a wire resistance in the bias voltage lines. This problem is quite serious to a display device required to accomplish reduction in power consumption.  
           [0038]    Japanese Unexamined Patent Publication No. 7-326311 has suggested an electron source including M wires extending in a row direction, formed on an electrically insulating substrate, N wires extending in a column direction, formed on said row-direction wires with an insulating layer sandwiched therebetween, and a surface conductive type electron emitting device including a thin film having at least one pair of electrodes and an electron emitter. Each of the electrodes is electrically connected to both the row-direction wires and the column-direction wires. A plurality of the surface conductive type electron emitting devices are arranged in a matrix. The row- and column-direction wires are designed to include terminals through which a voltage is applied thereto, at opposite ends.  
           [0039]    Japanese Unexamined Patent Publication No. 10-112391 has suggested a X-Y matrix type organic thin film electroluminescence display device including a light-emitting layer composed at least of organic material. In the display device, an electrode for a high resistance is electrically connected to a data electrode wire, and an electrode for a low resistance is electrically connected to a scanning electrode wire, to thereby reduce a voltage drop caused by a wire resistance.  
           [0040]    Japanese Unexamined Patent Publication No. 10-239655 has suggested a liquid crystal display device including an upper signal line driver circuit and a lower signal line driver circuit between which signal lines extend. The upper and lower signal line driver circuits are electrically connected to each other through first and second power lines. Branch lines extend from the first and second power lines, and are electrically connected to a liquid crystal driver circuit. The first power line is designed such that a half of the first power line extending from a point at which the branch line extend to the liquid crystal driver circuit, to an end of the first power line has a wire resistance equal to a wire resistance of the other half extending from the point to the other end of the first power line. Similarly, the second power line is designed such that a half of the second power line extending from a point at which the branch line extend to the liquid crystal driver circuit, to an end of the second power line has a wire resistance equal to a wire resistance of the other half extending from the point to the other end of the second power line.  
         SUMMARY OF THE INVENTION  
         [0041]    In view of the above-mentioned problems in the conventional display devices, it is an object of the present invention to provide a display device which is capable of, even if a bias voltage line would have an increased wire length because of an increase in the number of pixels, reducing and uniformizing a wire resistance of a bias voltage line extending from a bias voltage generating circuit to each of pixels, avoiding reduction in a brightness caused by reduction in a current running through a light-emitting device, resulted from an increase in a wire resistance in a bias voltage line, and avoiding non-uniformity in a brightness in a display screen, caused by non-uniformity in a wire resistance in bias voltage lines extending from a bias voltage generating circuit to each of pixels.  
           [0042]    It is also an object of the present invention to provide a display device which is capable of reducing a wire resistance in bias voltage lines to thereby reduce power consumption in the bias voltage lines.  
           [0043]    There is provided a display device including (a) a plurality of pixels arranged in a matrix, each of the pixels including a light-emitting device, a switch and a transistor, (b) at least one scanning line extending in a first direction, (c) at least one data line extending in a second direction perpendicular to the first direction, (d) at least one first bias voltage line extending in the second direction, (e) a bias voltage generating circuit which applies a bias voltage to the bias voltage line, (f) a second bias voltage line which surrounds the pixels, and (g) a third bias voltage line which electrically connects the bias voltage generating circuit to the second bias voltage line. The light-emitting device is electrically connected to one of a source and a drain of the transistor. The first bias voltage line is electrically connected to the other of a source and a drain of the transistor. The transistor has a gate electrically connected to the data line through the switch. The first bias voltage line is electrically connected at opposite ends thereof to the second bias voltage line. The switch is turned on when the scanning line is activated, to thereby allow image signals to be transmitted to the gate of the transistor therethrough from the data line. The second and third bias voltage lines are designed to have such a wire resistance that a constant current is supplied to the light-emitting device from the bias voltage generating circuit through the first, second and third bias voltage lines.  
           [0044]    It is preferable that the second bias voltage line is rectangular in shape.  
           [0045]    The display device may further include a first driver which drives the scanning line a second driver which drives the data line.  
           [0046]    The display device may further include a capacitor electrically connected between the gate and the source or drain of the transistor.  
           [0047]    It is preferable that the light-emitting device is comprised of an electroluminescence (EL) device.  
           [0048]    It is preferable that the second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a smaller wire resistance per a unit length.  
           [0049]    It is preferable that the second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a broader width.  
           [0050]    It is preferable that the bias voltage line segment is tapered in width.  
           [0051]    It is preferable that the second bias voltage line is comprised of a first wiring layer having a resistivity smaller than a predetermined resistivity, and a wiring layer of the scanning or data line, the first wiring layer and the wiring layer being vertically layered one on another, the first wiring layer and the wiring layer being connected to each other through a through-hole.  
           [0052]    It is preferable that the second bias voltage line has an inner area defined as an area surrounded by itself, the inner area being greater than a predetermined area such that the second bias voltage line acts as a capacitor for removal of noises.  
           [0053]    The display device may further include at least one bias bus line extending in the first direction between two portions of the second bias voltage line opposing to each other.  
           [0054]    The display device may further include bias bus lines extending in the first direction between two portions of the second bias voltage line opposing to each other, the bias bus lines being arranged by every M pixel rows wherein M is an integer equal to or greater than 1.  
           [0055]    The display device may further include bias bus lines extending in the first direction between two portions of the second bias voltage line opposing to each other, the bias bus lines being arranged by every non-constant number of pixel rows.  
           [0056]    It is preferable that the second bias voltage line is configured to be a closed loop.  
           [0057]    It is preferable that the third bias voltage line has a width greater than a width of said second bias voltage line.  
           [0058]    There is further provided a display device including (a) a plurality of pixels arranged in a matrix, each of the pixels including a light-emitting device, a switch and a transistor, (b) at least one scanning line extending in a column direction, (c) at least one data line extending in a row direction, (d) first to N-th first bias voltage lines extending in the column direction wherein N is an integer equal to or greater than 2, (e) a bias voltage generating circuit having first to N-th output terminals through which a bias voltage is applied to the first to N-th first bias voltage lines, (f) first to N-th second bias voltage lines which surround the pixels, and (g) first to N-th third bias voltage lines which electrically connects the first to N-th output terminals of the bias voltage generating circuit to the first to N-th second bias voltage lines, respectively, the light-emitting device being electrically connected to one of a source and a drain of the transistor, the first to N-th first bias voltage lines being electrically connected to the other of a source and a drain of the transistor in the first to N-th rows, the transistor having a gate electrically connected to the data line through the switch, each of the first to N-th first bias voltage lines being electrically connected at opposite ends thereof to an associated second bias voltage line among the first to N-th second bias voltage lines, the switch being turned on when the scanning line is activated, to thereby allow image signals to be transmitted to the gate of the transistor therethrough from the data line, the first to N-th second and third bias voltage lines being designed to have such a wire resistance that a constant current is supplied to the light-emitting device from the bias voltage generating circuit through the first to N-th first, second and third bias voltage lines.  
           [0059]    It is preferable that each of the first to N-th second bias voltage lines is rectangular in shape.  
           [0060]    The display device may further include a first driver which drives the scanning line a second driver which drives the data line.  
           [0061]    It is preferable that each of the first to N-th second bias voltage line is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a smaller wire resistance per a unit length.  
           [0062]    It is preferable that each of the first to N-th second bias voltage lines is comprised of a plurality of bias voltage line segments, and that a bias voltage line segment located closer to the bias voltage generating circuit is designed to have a broader width.  
           [0063]    It is preferable that the bias voltage line segment is tapered in width.  
           [0064]    It is preferable that each of the first to N-th second bias voltage lines is comprised of a first wiring layer having a resistivity smaller than a predetermined resistivity, and a wiring layer of the scanning or data line, the first wiring layer and the wiring layer being vertically layered one on another, the first wiring layer and the wiring layer being connected to each other through a through-hole.  
           [0065]    It is preferable that an innermost second bias voltage line among the first to N-th second bias voltage lines has an inner area defined as an area surrounded by itself, the inner area being greater than a predetermined area such that the innermost second bias voltage line acts as a capacitor for removal of noises.  
           [0066]    The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other.  
           [0067]    The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other, each of the first to N-th bias bus lines being arranged by every M pixel rows wherein M is an integer equal to or greater than 1.  
           [0068]    The display device may further include first to N-th bias bus lines each extending in the column direction between two portions of each of the first to N-th second bias voltage lines opposing to each other, each of the first to N-th bias bus lines being arranged by every non-constant number of pixel rows.  
           [0069]    It is preferable that each of the first to N-th second bias voltage lines is configured to be a closed loop.  
           [0070]    It is preferable that each of the first to N-th third bias voltage lines has a width greater than a width of the associated second bias voltage line.  
           [0071]    The advantages obtained by the aforementioned present invention will be described hereinbelow.  
           [0072]    The display device in accordance with the present invention makes it possible to reduce and uniformize a wire resistance of a bias voltage line extending from a bias voltage generating circuit to each of pixels, even if a bias voltage line would have an increased wire length because of an increase in the number of pixels, avoid reduction in a brightness caused by reduction in a current running through a light-emitting device, resulted from an increase in a wire resistance in a bias voltage line, and avoids non-uniformity in a brightness in a display screen, caused by non-uniformity in a wire resistance in bias voltage lines extending from a bias voltage generating circuit to each of pixels.  
           [0073]    The display device in accordance with the present invention also makes it possible to reduce a wire resistance in bias voltage lines to thereby reduce power consumption in the bias voltage lines.  
           [0074]    In addition, reduction in power consumption in bias voltage lines ensures an extension in lifetime of bias voltage lines.  
           [0075]    Since the second bias voltage line has a large area surrounded by itself, it would be possible to uniformize a bias voltage to be applied through a bias voltage line, and improve an image quality in the display device, by forming a capacitor by means of the second bias voltage line to thereby remove spike noises entering the second bias voltage line.  
           [0076]    In addition, it would be possible to optimally compensate for color balance by controlling a bias voltage to thereby control a current running through a light-emitting device, even if a light emission efficiency of the light-emitting device is lowered with an increase in a total period of time during which the light-emitting device emits a light, and resultingly, the light-emitting device is degraded.  
           [0077]    The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0078]    [0078]FIG. 1 is a block diagram of a conventional simple matrix type organic electroluminescence display device.  
         [0079]    [0079]FIG. 2 is a block diagram of a conventional active matrix type organic electroluminescence display device.  
         [0080]    [0080]FIG. 3 is a block diagram of a display device in accordance with the first embodiment of the present invention.  
         [0081]    [0081]FIG. 4A is a block diagram of a display device in accordance with the second embodiment of the present invention.  
         [0082]    [0082]FIG. 4B is an enlarged view of a pixel in the display device illustrated in FIG. 4A.  
         [0083]    [0083]FIG. 5 is a block diagram of a display device in accordance with the third embodiment of the present invention.  
         [0084]    [0084]FIG. 6 is a block diagram of a display device in accordance with the fourth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0085]    Preferred embodiments in accordance with the present invention will be explained hereinbelow with reference to drawings.  
         [0086]    [0086]FIG. 3 is a block diagram of a display device in accordance with the first embodiment of the present invention.  
         [0087]    The display device in accordance with the first embodiment is comprised of a data line driver circuit  68  to which a plurality of data lines  65  are electrically connected and which drives the data lines  65 , a scanning line driver circuit  69  to which a plurality of scanning lines  66  are electrically connected and which drives the scanning lines  66 , a bias voltage generating circuit  11  which drives later mentioned first bias voltage lines  67 , a plurality of first bias voltage lines  67 , a second bias voltage line  13 , a third bias voltage line  13 C electrically connecting the bias voltage generating circuit  11  to the second bias voltage line  13 , and a plurality of pixels arranged in a matrix.  
         [0088]    Each of the pixels is comprised of an electroluminescence device  61  having an anode  61 A and a cathode  61 C to which a constant bias voltage is applied, a first thin film transistor (TFT)  62  electrically connected between the anode  61 A of the electroluminescence device  61  and one of the first bias voltage lines  67 , a second thin film transistor (TFT)  63  electrically connected between one of the data lines  65  and a gate of the first thin film transistor  62 , and a capacitor  64  electrically connected between a gate of the first thin film transistor  62  and one of the first bias voltage lines  67 .  
         [0089]    The second bias voltage line  13  is rectangular in shape, and surrounds a pixel area  12  in which the pixels are arranged in a matrix.  
         [0090]    The second bias voltage line  13  is electrically connected to an output terminal of the bias voltage generating circuit  11  through the third bias voltage line  13 C. Each of the first bias voltage lines  67  is connected to the second bias voltage line  13  at nodes  131 A and  131 B,  132 A and  132 B, - - - , respectively.  
         [0091]    In the conventional active matrix type organic electroluminescence display device illustrated in FIG. 2, each of the bias voltage lines  67  is connected to the common bias voltage line  611  at one node, and the common bias voltage line  611  is electrically connected to an output terminal of the bias voltage source  610 . In contrast, in the display device in accordance with the first embodiment, illustrated in FIG. 3, each of the first bias voltage lines  67  is connected to the second bias voltage line  13  at both upper nodes  131 A,  132 A, - - - , and lower nodes  131 B,  132 B, - - - .  
         [0092]    When the scanning line driver circuit  69  activates one of the scanning lines  66 , the second thin film transistor  63  electrically connected to the thus activated scanning line  66  is turned on, and hence, a current runs into the capacitor  64  through the data line  65  and the second thin film transistor  63  from the data line driver circuit  68 , resulting in that the capacitor  64  is electrically charged.  
         [0093]    When the scanning line driver circuit  69  inactivates one of the scanning lines  66 , the second thin film transistor  63  electrically connected to the thus inactivated scanning line  66  is turned off, and hence, electric charges accumulated in the capacitor  64  are kept accumulated as they are, and the capacitor  64  electrically connected to a gate of the first thin film transistor  62  has a constant terminal voltage. The terminal voltage is biased to a gate of the first thin film transistor  62 . When a gate voltage in the first thin film transistor  62  becomes higher than a threshold voltage, the first thin film transistor  62  is turned on. As a result, a current is supplied to the electroluminescence device  61  from the bias voltage generating circuit  11  through the third bias voltage line  13 C, the second bias voltage line  13 , and the first bias voltage line  67 , and the electroluminescence device  61  emits a light with a brightness defined in accordance with the supplied current.  
         [0094]    A current Iel to be supplied to the electroluminescence device  61  is defined by a gate voltage and a voltage between a source and a drain in the first thin film transistor  62 . When a width of a pulse to be applied to a gate is varied to thereby accomplish multi gradation, for instance, in accordance with the processes suggested in Japanese Patent No. 2784615 or Japanese Unexamined Patent Publication No. 11-231835, a voltage between a source and a drain in the first thin film transistor  62  is in the range of 0.1 to 0.2 voltage at greatest. A voltage at the anode  61 A of the electroluminescence device  61  is calculated by subtracting a voltage between a source and a drain in the first thin film transistor  62  from an output voltage Vb output from the bias voltage generating circuit  11 . Accordingly, when gradation is controlled by varying a width of a pulse to be applied to a gate, the current Iel is dependent on the output voltage Vb output from the bias voltage generating circuit  11 .  
         [0095]    In other words, in the display device in accordance with the first embodiment, gradation associated with an image signal to be input through the data line  65  is controlled by a width of a pulse to be applied to a gate of the first thin film transistor  62 , and a brightness based on which gradation is determined is controlled by the voltage Vb output from the bias voltage generating circuit  11 .  
         [0096]    In FIG. 3, the second bias voltage line  13  is composed of a material having a low resistivity in order to reduce a wire resistance, and is designed to have a wire width greater than width of the scanning line  66  and the first bias line  67  both located in the pixel area  12 .  
         [0097]    Accordingly, a wire resistance from each of the pixels to the output terminal of the bias voltage generating circuit  11  through the first, second and third bias voltage lines  67 ,  13  and  13 C is remarkably smaller than a wire resistance from each of the pixels to the bias voltage source  610  in the conventional active matrix type organic EL display device illustrated in FIG. 2, because the second bias voltage line  13  has a small wire resistance, and further because each of the first bias voltage lines  67  is connected to the second bias voltage line  13  at the upper and lower nodes  131 A,  132 A, - - - and  131 B,  132 B, - - - .  
         [0098]    An example of the above-mentioned case is explained hereinbelow.  
         [0099]    The bias voltage generating circuit  11  output a current to the first bias voltage line  67  through the third bias voltage line  13 C having a low resistance, the second bias voltage line  13 , and the nodes  131 A and  131 B. The current is supplied to the light-emitting device  61  constituting the pixel which is electrically connected to the first bias voltage line  67  and activated.  
         [0100]    Accordingly, a voltage gradient in the first bias voltage line  67  is significantly relaxed, and non-uniformity in a brightness caused by non-uniformity in a current running through the light-emitting device  61  can be remarkably improved.  
         [0101]    In other words, if a voltage output from the bias voltage generating circuit  11 , and a brightness in a pixel located in the pixel area  12 , namely, a current associated with a brightness of the light-emitting device constituting the pixel are given, wire resistances of the second bias voltage line  13  and the third bias voltage line  13 C are calculated so as to satisfy the thus given voltage and brightness (or current), and the second bias voltage line  13  and the third bias voltage line  13 C are designed to have the thus calculated wire resistance.  
         [0102]    The second bias line  13  may be formed so as to have a low resistance by layering the second bias voltage line  13  and the first bias voltage lines  67  or the scanning lines  66  one on another, and electrically connecting them through via-holes.  
         [0103]    The second bias voltage line  13  and the scanning lines  66  are formed so as not to short-circuit with each other when the second bias voltage line  13  and the scanning lines  66  overlap each other, except the scanning lines extending in parallel with a vertically extending portion of the second bias voltage line  13 .  
         [0104]    The second bias voltage line  13  may be designed to have an inner area defined as an area surrounded by the second bias voltage line  13  which inner area is larger than a predetermined area. The second bias voltage line  13  having such an inner area can define a capacity which is capable of removing spike-like noises entering the second bias voltage line  13 . This ensures that a bias voltage applied from the bias voltage generating circuit  11  is stabilized, and resultingly, image quality in a display screen can be enhanced.  
         [0105]    As mentioned earlier, the conventional simple matrix type organic EL display device and the conventional active matrix type organic EL display device have a common problem that extra power has to be supplied to the bias voltage lines from the bias voltage source in order to compensate for reduction in a brightness in a pixel which reduction is caused by an increase in a wire resistance in the bias voltage lines. This problem is quite serious to a display device required to accomplish reduction in power consumption.  
         [0106]    In accordance with the display device in the first embodiment, since the first, second and third bias voltage lines  67 ,  13  and  13 C connecting the bias voltage generating circuit  11  to each of the pixels would have a reduced wire resistance, even if the first and second bias voltage lines had an increased wire length because of an increase in the number of pixels, power consumption in the pixels can be reduced, ensuring reduction in power consumption in the overall display device.  
         [0107]    [0107]FIG. 4A is a block diagram of a display device in accordance with the second embodiment of the present invention, and FIG. 4B is an enlarged view of a pixel in the display device illustrated in FIG. 4A.  
         [0108]    The display device in accordance with the second embodiment is comprised of a data line driver circuit  68  to which a plurality of data lines  65  are electrically connected and which drives the data lines  65 , a scanning line driver circuit  69  to which a plurality of scanning lines  66  are electrically connected and which drives the scanning lines  66 , a bias voltage generating circuit  11  which drives later mentioned first bias voltage lines  67 , a plurality of first bias voltage lines  67 , a second bias voltage line  13 , a third bias voltage line  13 C electrically connecting the bias voltage generating circuit  11  to the second bias voltage line  13 , a plurality of bias bus lines  14 , and a plurality of pixels  21  arranged in a matrix in a pixel area  12 .  
         [0109]    Each of the pixels has the same structure as the structure of the pixel in the first embodiment.  
         [0110]    As illustrated in FIG. 4B, each of the pixel  21  is designed to have a first connection port  21 A at which the scanning line  66  is connected to the pixel, a second connection port  21 B at which the data line  65  is connected to the pixel, and a third connection port  21 C at which the first bias voltage line  67  is connected to the pixel.  
         [0111]    The second bias voltage line  13  is rectangular in shape, and surrounds the pixel area  12  in which the pixels  21  are arranged in a matrix.  
         [0112]    The second bias voltage line  13  is electrically connected to an output terminal of the bias voltage generating circuit  11  through the third bias voltage line  13 C. Each of the first bias voltage lines  67  is connected to the second bias voltage line  13  at upper and lower nodes  131 A,  132 A, - - - and  131 B,  132 B, - - - .  
         [0113]    The bias bus lines  14  have a small wire resistance, and extend in parallel with the scanning lines  66  between vertically extending portions of the second bias voltage line  13 . The bias bus lines  14  are electrically connected to the second bias voltage line  13  at nodes  141 A and  141 B,  142 A and  142 B, - - - .  
         [0114]    The first bias voltage lines  67  and the bias bus lines  14  are electrically connected to each other at intersections of them. That is, by arranging the bias bus lines  14  by every M rows of the pixels  21  wherein M is an integer equal to or greater than 1, it would be possible to shorten a wire length of the first bias voltage lines  67  having a high resistivity which wire length contributes to a wire resistance of the first bias voltage lines  67 . As a result, it would be possible to significantly reduce a wire resistance in a path from each of the pixels  21  to the bias voltage generating circuit  11  through the bias bus lines  14 , the second bias voltage line  13  and the third bias voltage line  13 C.  
         [0115]    Though the bias bus lines  14  are arranged by every M rows of the pixels in the second embodiment, the bias bus lines  14  may be arranged by every non-constant number of pixel rows. For instance, a first bias bus line may be arranged between second and third rows of the pixels  21 , a second bias bus line may be arranged between fifth and sixth rows of the pixels  21 , and a third bias bus line may be arranged between tenth and eleventh rows of the pixels  21 . That is, the first bias bus line is spaced away from the second bias voltage line  13  by two rows of the pixels, the second bias bus line is spaced away from the first bias bus line by three rows of the pixels, and the third bias bus line is spaced away from the second bias bus line by five rows of the pixels.  
         [0116]    By arranging a plurality of bias bus lines by every non-constant number of pixel rows, it would be possible to substantially equalize wire resistances in wires from each of the pixels  21  to the bias voltage generating circuit  11 , taking into consideration that a current density is different from one another in each of the wires.  
         [0117]    [0117]FIG. 5 is a block diagram of a display device in accordance with the third embodiment of the present invention.  
         [0118]    The display device in accordance with the third embodiment has the same structure as the structure of the display device in accordance with the first embodiment, illustrated in FIG. 3, except a structure of the second bias voltage line  13 .  
         [0119]    In the display device in accordance with the third embodiment, the second bias voltage line  13  is designed to have a greater width at a location closer to the bias voltage generating circuit  11 , and have a smaller width at a location farther away from the bias voltage generating circuit  11 .  
         [0120]    Specifically, the second bias voltage line  13  is comprised of a first bias voltage line segment  31  connected to the third bias voltage line  13 C and horizontally extending, a second bias voltage line segment  32  connected to the third bias voltage line  13 C and vertically extending, a third bias voltage line segment  33  connected to the first bias voltage line segment  31  and vertically extending, and a fourth bias voltage line segment  34  connected to the second bias voltage line segment  32  and horizontally extending.  
         [0121]    The first bias voltage line segment  31  has a width equal to that of the second bias voltage line segment  32 . The third bias voltage line segment  33  has a width equal to that of the fourth bias voltage line segment  34 .  
         [0122]    The third bias voltage line  13 C is designed to have a greater width than a width of the first and second bias voltage line segments  31  and  32 , and the first and second bias voltage line segments  31  and  32  are designed to have a greater width than a width of the third and fourth bias voltage line segments  33  and  34 .  
         [0123]    Though the third bias voltage line  13 C and the first to fourth bias voltage lines  31  to  34  are designed to have a fixed width in the third embodiment, they may be designed to be tapered. Specifically, the third bias voltage line  13 C may be tapered such that a portion closer to the bias voltage generating circuit  11  has a greater width and a portion farther away from the bias voltage generating circuit  11  has a smaller width. Similarly, the first and fourth bias voltage line segments  31  and  34  may be tapered such that a portion closer to the left end has a greater width and a portion closer to the right end has a smaller width. Similarly, the second and third bias voltage line segments  32  and  33  may be tapered such that a portion closer to the upper end has a greater width and a portion closer to the lower end has a smaller width.  
         [0124]    By designing the third and second bias voltage lines  13 C and  13  such that a portion located closer to the bias voltage generating circuit  11  has a greater width and a portion located farther away from the bias voltage generating circuit  11 , it would be possible to substantially equalize wire resistances in wires from each of the pixels  21  to the bias voltage generating circuit  11 , taking into consideration that a current density is different from one another in each of the wires.  
         [0125]    [0125]FIG. 6 is a block diagram of a display device in accordance with the fourth embodiment of the present invention.  
         [0126]    The display device in accordance with the fourth embodiment is structurally different from the display device in accordance with the first embodiment, illustrated in FIG. 3, with respect to the number of the second bias voltage lines  13 . Specifically, whereas the display device in accordance with the first embodiment is designed to include one second bias voltage line  13 , the display device in accordance with the fourth embodiment is designed to include three second bias voltage lines  43 A,  43 B and  43 C and three associated third bias voltage lines  44 A,  44 B and  44 C.  
         [0127]    The second bias voltage line  43 A surrounds the pixel area  12 , and is electrically connected to a first output port (not illustrated) of a bias voltage generating circuit  41  through the third bias voltage line  44 A. A plurality of first bias voltage lines  42 A are electrically connected to the second bias voltage line  43 A at upper and lower nodes.  
         [0128]    The second bias voltage line  43 B surrounds the second bias voltage line  43 A, and is electrically connected to a second output port (not illustrated) of the bias voltage generating circuit  41  through the third bias voltage line  44 B. A plurality of first bias voltage lines  42 B are electrically connected to the second bias voltage line  43 B at upper and lower nodes.  
         [0129]    The second bias voltage line  43 C surrounds the second bias voltage line  43 B, and is electrically connected to a third output port (not illustrated) of the bias voltage generating circuit  41  through the third bias voltage line  44 C. A plurality of first bias voltage lines  42 C are electrically connected to the second bias voltage line  43 C at upper and lower nodes.  
         [0130]    The second bias voltage lines  43 A,  43 B and  43 C are electrically independent of one another.  
         [0131]    Though not illustrated in FIG. 6, the first bias voltage lines in fourth to sixth columns are electrically connected to the second bias voltage lines  43 A to  43 C at upper and lower nodes, respectively. The first bias voltage lines in seventh or greater columns are electrically connected to the second bias voltage lines  43 A to  43 C at upper and lower nodes in the same way.  
         [0132]    The structure of the display device in accordance with the fourth embodiment makes it possible to control a current running through the electroluminescence devices  61  independently column by column, and hence, it would be possible to control a brightness of the electroluminescence devices  61  in each of rows.  
         [0133]    Hereinbelow is explained an example.  
         [0134]    For instance, first electroluminescence devices each emitting a red light are arranged in the leftmost column, second electroluminescence devices each emitting a green light are arranged in a column adjacent to the leftmost column, and third electroluminescence devices each emitting a blue light are arranged in a column adjacent to the previous column. The first, second and third electroluminescence devices are repeatedly arranged in this order in the example display device.  
         [0135]    In accordance with the example display device, even if light emission efficiencies of the first to third electroluminescence devices are deteriorated as a total period of time during which the first to third electroluminescence devices emit red, green and blue lights increases, and hence, the first to third electroluminescence devices are degraded, it would be possible to control a brightness of each of the first to third electroluminescence devices, and hence, color balance could be compensated for to be kept optimal.  
         [0136]    The display device in accordance with the fourth embodiment makes it possible to control a current running through the electroluminescence devices  61  independently column by column, and hence, it would be possible to control a brightness of the electroluminescence devices  61  in each of rows. In addition, a plurality of the second bias voltage lines  43 A to  43 C significantly relaxes a voltage gradient in the first bias voltage lines  42 A to  42 C, and resultingly, it would be possible to improve non-uniformity in a brightness, caused by the voltage gradient, and the non-uniformity in a current running through the electroluminescence devices, associated with the voltage gradient.  
         [0137]    In accordance with the display device in the fourth embodiment, since the first, second and third bias voltage lines connecting the bias voltage generating circuit  41  to each of the pixels would have a reduced wire resistance, even if the first and second bias voltage lines had an increased wire length because of an increase in the number of pixels, power consumption in the pixels can be reduced, ensuring reduction in power consumption in the overall display device.  
         [0138]    Though the display device in accordance with the fourth embodiment is designed to include the three second bias voltage lines  43 A to  43 C and the associated three third bias voltage lines  44 A to  44 C, the number of the second and third bias voltage lines is not to be limited to three. The display device may be designed to include two, four or more second and third bias voltage lines.  
         [0139]    Though not illustrated, the display device in accordance with the fourth embodiment may be designed to include the bias bus lines  14  illustrated in FIG. 4A, in which case, the bias bus lines  14  are formed in association with each of the second bias voltage lines.  
         [0140]    By designing the display device to include the bias bus lines  14 , it would be possible to shorten a wire length of the first bias voltage lines  42 A to  42 C having a high resistivity which wire length contributes to a wire resistance of the first bias voltage lines  42 A to  42 C. As a result, it would be possible to significantly reduce a wire resistance in a path from each of the pixels to the bias voltage generating circuit  41  through the bias bus lines  14 , the second bias voltage lines  43 A to  43 C, and the third bias voltage lines  44 A to  44 C.  
         [0141]    In the above-mentioned first to fourth embodiments, the electroluminescence device  61  is used as a light-emitting device. However, it should be noted that a light-emitting device other than an electroluminescence device may be applied to the display device in accordance with the present invention.  
         [0142]    In the display device illustrated in FIG. 6, sources or drains of the first thin film transistors  62  arranged in the same column are electrically connected to the first bias voltage line  42 A,  42 B or  42 C. However, it should be noted that sources or drains of the first thin film transistors  62  arranged in the same row may be electrically connected to the first bias voltage line  42 A,  42 B or  42 C.  
         [0143]    In the above-mentioned first to fourth embodiments, the second bias voltage lines  13 ,  31  to  34  and  43 A to  43 C are configured to be a closed loop. However, it should be noted that it is not always necessary for the second bias voltage lines to a closed loop. The second bias voltage lines are merely required to surround the pixel area  12  in a loop.  
         [0144]    While the present invention has been described in connection with certain preferred embodiments, it is to be understood that the subject matter encompassed by way of the present invention is not to be limited to those specific embodiments. On the contrary, it is intended for the subject matter of the invention to include all alternatives, modifications and equivalents as can be included within the spirit and scope of the following claims.  
         [0145]    The entire disclosure of Japanese Patent Application No. 2000-228405 filed on Jul. 28, 2000 including specification, claims, drawings and summary is incorporated herein by reference in its entirety.