Patent Publication Number: US-6909243-B2

Title: Light-emitting device and method of driving the same

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to techniques for a semiconductor integrated circuit and its driving method. The invention also relates to a light-emitting device that has a semiconductor integrated circuit of the present invention in its driver circuit portion and a pixel portion. In particular, the present invention relates to an active matrix type light-emitting device in which the semiconductor integrated circuit of the present invention is applied to a signal line driver circuit of the driver circuit portion. 
   2. Description of the Related Art 
   In recent years, research and development of light-emitting devices using self-light-emitting elements such as organic light-emitting diodes (OLEDs) have progressed. An OLED has an anode and a cathode, and has a structure in which an organic compound layer is sandwiched between the aforementioned anode and cathode. Light-emitting devices using OLEDs have characteristics in that they have suitably fast response speed for animated displays, low voltage, low power consumption driving, or the like. Thus, light-emitting devices using light-emitting elements are expected to be widely used for various purposes, including new-generation mobile telephones and personal digital assistants (PDAs) and are attracting attention as the next-generation displays. 
   When displaying a multi-gray scale image using a light-emitting device with a self-light-emitting element, a current input method can be given as a driving method thereof. In the current input method, the luminance of the relevant light-emitting element is controlled by writing the current value form data onto the pixel as the image signal. It is possible that the image signal of the current input method is either an analog value (analog driving method) or a digital value (digital driving method). 
   As a signal line driver circuit with the above-mentioned current input system, for example, a circuit shown in  FIG. 10A  is proposed (refer to A. Yumoto et al., Proc. Asia Display/IDW &#39;01 pp.1395-1398 (2001)). In  FIG. 10A , a pair of current source circuits is provided to each of signal lines. In the structure of the circuit in  FIG. 10A , pairs of current source circuits A 1  and B 1 , A 2  and B 2 , . . . are respectively connected with the signal lines. The pair of current source circuits A and B alternately conduct an operation of reading and storing an image signal in a form of a current value (image signal current) and an operation of writing a signal to a pixel through a signal line. That is, while the current source circuit A conducts the operation of reading and setting a signal current, the current source circuit B conducts the operation of writing a signal to a light-emitting element provided in a pixel region through a signal line. Conversely, while the current source circuit A conducts the operation of writing a signal to a light-emitting element provided in a pixel region through a signal line, the current source circuit B conducts the operation of reading and setting a signal current. 
   Operation timings of the current source circuits A and B are shown in FIG.  10 B.  FIG. 10B  is a schematic block diagram of the following operation. In a k-th row selection period (horizontal period), while the circuit A 1  conducts the operation of reading and storing a signal (R 1 ), the circuit B 1  conducts the operation of writing a signal to a signal line (W 1 ). Further, in the next (k+1)-th row selection period, while the circuit A 1  conducts the operation of writing a signal to a signal line (W 1 ), the circuit B 1  conducts the operation of reading and storing a signal (R 1 ). Moreover,  FIG. 10C  is a schematic diagram of the entire light-emitting device provided with the current source circuit. 
   However, in the above-mentioned driver circuit, a pair of current source circuits is provided to each signal line. Thus, the area of the current source circuit shown in  FIG. 10C  is large, and miniaturization of the signal line driver circuit is difficult to be realized. As a result, in the light-emitting device, the proportion of the signal line driver circuit is large, which obstructs reduction in size of a frame and leads to reduction in area of the pixel region. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in view of the above, and therefore has an object to provide a novel driving method for conducting gradation display with a circuit structure in which a current source circuit is provided to each signal line. Further, another object of the present invention is to attain miniaturization and reduction in size of a frame of a light-emitting device with the use of a signal line driver circuit that includes a current source circuit having a small area. 
   In order to solve the above-mentioned problems, according to the present invention, there is provided a driving method in which a period for reading and setting a signal (reading period) and a period for writing a set signal to a pixel (writing period) are separately provided in a selection period (horizontal period) for one row. Further, according to the present invention, provided is a light-emitting device with a structure in which a current source circuit is provided to each signal line. 
   In the present invention, first, the selection period (horizontal period) for one row is divided into plural periods. Then, in one of the divided periods, a (writing) operation of writing an image signal to a pixel from a current source circuit in a signal line driver circuit is performed in a certain column, while a (reading) operation of reading a signal current into a current source circuit in a signal line driver circuit is performed in another certain column. In another one of the divided periods, the reading operation is performed in the former certain column while the writing operation is performed in the latter certain column. 
   For example, a first scanning line (Ga) and a second scanning line (Gb) are provided. It is assumed that all the pixels each are provided with a pixel switch transistor for taking in an image signal to a pixel from a signal line and a current storage transistor. In this case, as to part of pixels in an arbitrary row, a gate of the current storage transistor of each of the pixels is connected with the second scanning line (Gb). It is assumed that, as to the other pixels in the line, a gate of the current storage transistor of each of the pixels is connected with a third scanning line (Gc). Also, it is assumed that the pixel switch transistor of each pixel is connected with the first scanning line (Ga). According to the present invention, the horizontal period is divided into a period for selecting the second scanning line (Gb) and a period for selecting the third scanning line (Gc). In the period for selecting the second scanning line (Gb), a (writing) operation of writing a signal to the pixel having the current storage transistor connected with the second scanning line (Gb) and a (reading) operation of reading an image signal current to the current source circuit of the signal line to the pixel having the current storage transistor connected with the third scanning line (Gc) that is not selected are performed simultaneously. Similarly, in the period for selecting the third scanning line (Gc), a (writing) operation of writing a signal to the pixel having the transistor connected with the third scanning line (Gc) and a (reading) operation of reading a signal current to the current source circuit connected with the signal line to the pixel having the current storage transistor connected with the second scanning line (Gb) that is not selected are performed simultaneously. 
   According to the driving method of the present invention, the proportion of the signal line driver circuit to the light-emitting device can be reduced, and thus, the reduction in size of a frame can be attained with a relatively large area of the pixel region to the light-emitting device. 
   Further, according to the present invention, provided is a light-emitting device in which each input line for an image signal current is shared by plural current source circuits. Thus, as to the light-emitting device, the number of input terminals (wirings) for image signals can be significantly reduced, and therefore, mounting of a peripheral IC chip becomes easy to be performed. Also, degradation in yield due to connection failure in a connecting portion of an FPC can be avoided. 
   Note that an organic compound layer in an organic light-emitting diode (OLED) in this specification indicates a layer containing an organic compound. The layer may be one containing an inorganic material, and further metal, metal complex, or the like. The category of the organic compound layer includes a hole injecting layer, a hole transporting layer, a light-emitting layer, a blocking layer, an electron transporting layer, an electron injecting layer, and the like. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram of a structure of a light-emitting device according to the present invention; 
       FIGS. 2A and 2B  are diagrams of driving timings of the light-emitting device according to the present invention; 
       FIG. 3  is a diagram of a structure of the light-emitting device according to the present invention; 
       FIGS. 4A and 4B  are diagrams of driving timings of the light-emitting device according to the present invention; 
       FIG. 5  is a diagram of a structure of the light-emitting device according to the present invention; 
       FIGS. 6A and 6B  are diagrams of driving timings of the light-emitting device according to the present invention; 
       FIGS. 7A and 7B  are schematic diagrams of current source circuits; 
       FIGS. 8A and 8B  are schematic diagrams of pixel structures; 
       FIGS. 9A and 9B  are schematic diagrams of the light-emitting device according to the present invention; 
       FIGS. 10A  to  10 C are schematic diagrams of a conventional light-emitting device; and 
       FIGS. 11A  to  11 H are diagrams of electronic equipments each of which uses the light-emitting device according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, an embodiment mode of the present invention will be described based on the accompanying drawings. Note that, in all the figures for the description of the embodiment mode, identical parts are denoted by the same reference symbols, and repetition of explanation is omitted. 
   [Embodiment Mode 1] 
     FIG. 5  shows an example of a signal line driver circuit according to the present invention. Note that  FIG. 5  shows a peripheral portion of current source circuits A 1 , A 2 , . . . , A (n−1) , A n . 
   The signal line driver circuit has the current source circuits A 1 , A 2 , . . . , A (n−1) , A n  and an image signal input switches (Sw) on/off of which is controlled by control signals a 1 , a 2 , . . . , a (n−1) , a n . The current source circuits A 1 , A 2 , . . . , A (n−1) , A n  output an image signal current to signal lines S 1 , S 2 , . . . , S (n−1) , S n , respectively. In a pixel portion, a first scanning line (Ga) and second and third scanning lines (Gb, Gc) are provided so as to be substantially perpendicular to the signal lines S, and pixels are arranged in matrix. Each of the pixels is provided with a pixel switch transistor (Tr 1 ) and a current storage transistor (Tr 2 ). 
   The current source circuits are connected with the signal lines and the image signal input switches (Sw), respectively. In each row, a gate electrode of each pixel switch transistor (Tr 1 ) is connected with the first scanning line (Ga) of the row, and a gate electrode of each current storage transistor (Tr 2 ) is connected with the second scanning line (Gb) or the third scanning line (Gc) of the row. 
   Next, a driving method of the above example will be described with reference to  FIGS. 6A and 6B .  FIG. 6A  is a diagram showing timings of selection and non-selection (assumed that: High corresponds to selection and conduction; and Low corresponds to non-selection and insulation in this example) in a row selection period.  FIG. 6B  is a block diagram in which reading (R) to the current source circuits and writing (W) to light-emitting elements are shown. 
   As shown in  FIG. 6A , the row selection period is divided into plural (two) periods such as T 1  and T 2 . During one of the divided periods, for example, T 1 , a high signal is input to select the second scanning line (Gb). For example, in an m-th row selection period, the current storage transistors Tr 2   m1  and T 2   m2  connected to the second scanning line (Gb) are brought into an on state, and the image current is written into the pixels from the signal lines S 1  and S 2  connected with the transistors Tr 1   m1  and Tr 1   m2 . (regions of W 1  and W 2  in FIG.  6 B). At this time, the control signals a 1  and a 2  become signals that bring the image signal input switches (Sw) into an off state (Low), and the input signals are not read into the current source circuits A 1  and A 2 . During T1, the current storage transistors Tr 2   m(n−1)  and Tr 2   mn  connected to the third scanning line (Gc) that is not selected (Low) are in an off state, and the signals are not written into the pixels. At this time, the control signals a (n−1)  and a n  sequentially become high signals to bring the switches into an on state, and the current is read into the current source circuits A (n−1)  and A n  (regions of R (n−1)  and R n  in FIG.  6 B). 
   Further, during another period in the m-th row selection period, T2, a high signal is input to select the third scanning line (Gc). Then, the current storage transistors Tr 2   m(n−1)  and Tr 2   mn  connected to the third scanning line (Gc) are brought into an on state, and the image signal current is written into the pixels from the signal lines S (n−1)  and S n  connected to the transistors Tr 2   m(n−1)  and Tr 2   mn  (regions of W (n−1)  and W n  in FIG.  6 B). At this time, the control signals a (n−1)  and a n  become low signals, and the input signals are not read into the current source circuits A (n−1)  and A n . During T 2 , the transistors Tr 2   m1  and Tr 2   m2  connected to the second scanning line (Gb) that is not selected (Low) are in an off state, and the image signals are not written into the pixels. At this time, the control signals a 1  and a 2  sequentially become high signals, and the current is read into the current source circuits A 1  and A 2  (regions of R 1  and R 2  in FIG.  6 B). 
   Next, description will be made of structural examples of the current source circuits.  FIGS. 7A and 7B  show examples of constant current sources provided in the current source circuits A 1 , A 2 , . . . . The current source circuits shown in  FIGS. 7A and 7B  are ones used on a low voltage side. However, the present invention is not limited to this. Further, since a source electrode and a drain electrode may be replaced with each other due to the polarity of a transistor and the voltage level, the source electrode or drain electrode of the transistor is referred to as a first electrode or second electrode. 
   First, description will be made of the circuit in FIG.  7 A. The constant current source in  FIG. 7A  includes a first transistor  701 , a second transistor  702 , a third transistor  703 , a fourth transistor  704 , and a capacitor element  709  that holds a gate-source voltage of the third transistor  703 . The first transistor  701  corresponds to each of the switches Sw 1 , Sw 2 , . . . Sw (n−1) , and Sw n , in FIG.  5 . 
   A gate electrode of the first transistor  701  is connected with a gate electrode of the second transistor  702 , and a first electrode of the first transistor  701  is connected with a second electrode of the second transistor  702 , a first electrode of the third transistor  703 , and a first electrode of the fourth transistor  704 . A first electrode of the second transistor  702  is connected with a gate electrode of the third transistor  703 . A second electrode of the fourth transistor  704  is connected with a signal line. A capacitor element  709  is connected between the gate electrode and a second electrode of the third transistor  703 . 
   A signal current reading operation of the circuit is described. A control signal a n , which is input to the respective gate electrodes of the first transistor  701  and the second transistor  702 , brings the transistors into an on state. A signal current is made to flow to the third transistor  703  through the first transistor  701 . At this time, the gate-source voltage and a source-drain voltage of the third transistor  703  are equal to each other. Thereafter, the first transistor  701  and the second transistor  702  are brought into an off state. Then, a current value of an image signal is stored as charge accumulated in the capacitor element  709 , and thus, the third transistor  703  has an ability to make a signal current flow. Next, a signal current writing operation of the circuit is explained. A control signal b n  that is input brings the fourth transistor  704  into an on state, and the signal current, which has been stored through the reading operation, is written into a signal line S 1  from the third transistor  703  through the fourth transistor  704 . 
   Sequentially, description will be made of the circuit in FIG.  7 B. The current source circuit in  FIG. 7B  includes a first transistor  711 , a second transistor  712 , a third transistor  713  and a fourth transistor  714  that constitute a current mirror circuit, and a capacitor element  719  that holds a gate-source voltage of the third transistor. The first transistor  711  corresponds to the switch Sw 1  in FIG.  5 . Note that the third transistor  713  and the fourth transistor  714  may have the same size. 
   A gate electrode of the first transistor  711  is connected with a gate electrode of the second transistor  712 , and a first electrode of the first transistor  711  is connected with a second electrode of the second transistor  712  and a first electrode of the third transistor  713 . A first electrode of the second transistor  712  is connected with a gate electrode of the third transistor  713 . A first electrode of the fourth transistor  714  is connected with a signal line. 
   A signal current reading operation of the circuit is described. First, the control signal a n , which is input to the respective gate electrodes of the first transistor  711  and the second transistor  712 , brings the transistors into an on state. An image signal current is made to flow to the third transistor  713  through the first transistor. At this time, the gate-source voltage and a source-drain voltage of the third transistor  713  are equal to each other. Thereafter, the first transistor  711  and the second transistor  712  are brought into an off state. Then, a current value of an image signal is stored as charge accumulated in the capacitor element  719 , and thus, the third transistor  713  and the fourth transistor  714  each have an ability to make a signal current flow. Next, a signal current writing operation of the circuit is explained. The signal current is written into the signal line S 1  from the fourth transistor  714 . Note that a fifth transistor may be provided between the fourth transistor  714  and the signal line to control a timing, at which the signal current flows to the signal line, with the control signal b n . 
   The structural examples of the constant current source circuits of the present invention have been described above. However, the present invention is not limited to the structures, connections or operation methods of  FIGS. 7A and 7B , and any circuit may be adopted as long as it is a circuit through which a constant current can be made to flow. 
   Next, description will be made of pixels according to the present invention.  FIGS. 8A and 8B  each show a structural example of adjacent two pixels. A pixel circuit of the present invention may be any one as long as it is of a system with which a signal current corresponding to an image signal can be stored and generated (referred to as current input system). Since the connection between a source electrode and a drain electrode may be changed due to the polarity of a transistor, the source electrode or drain electrode of the transistor is referred to as a first electrode or second electrode. 
   First, description will be made with reference to  FIG. 8A. A  pixel has a signal line  830 , a first scanning line (Ga)  831 , a second scanning line (Gb)  832 , a third scanning line (Gc)  833 , a power source line  834 , a first transistor  801 , a second transistor  802 , a third transistor  803 , a fourth transistor  804 , a capacitor element  809 , and a self-light-emitting element  820 . The first transistor is a pixel switch transistor; the second transistor is a current storage transistor; and the fourth transistor is a transistor for driving a self-light-emitting element. 
   Gate electrodes of the first transistor  801  and the fourth transistor  804  are connected with the first scanning line (Ga)  831 , a first electrode of the first transistor  801  is connected with the signal line  830 , and a second electrode of the first transistor  801  is connected with a first electrode of the second transistor  802 , a first electrode of the third transistor  803 , and a first electrode of the fourth transistor  804 . A gate electrode of the second transistor  802  is connected with the second scanning line (Gb)  832 , and a second electrode of the second transistor  802  is connected with a gate electrode of the third transistor  803  and the capacitor element  809 . A second electrode of the third transistor  803  is connected with the power source line  834 . A second electrode of the fourth transistor  804  is connected with one of electrodes of the light-emitting element  820 . The capacitor element  809  is arranged between the gate electrode and the second electrode of the third transistor, and holds a gate-source voltage of the fourth transistor  804 . The power source line  834  and the other electrode of the light-emitting element  820  are set at predetermined potentials, respectively. 
   The adjacent pixel has a similar structure, but differs in the following point from the above pixel. That is, the point is that the gate electrode of the second transistor  802  is connected with the third scanning line (Gc)  833 . 
   Further, in  FIG. 8B , a pixel has the signal line  830 , the first scanning line (Ga)  831 , the second scanning line (Gb)  832 , the third scanning line (Gc)  833 , the power source line  834 , a first transistor  811 , a second transistor  812 , a third transistor  813 , a fourth transistor  814 , a capacitor element  819 , and the self-light-emitting element  820 . The first transistor is the pixel switch transistor; the second transistor is the current storage transistor; and the fourth transistor is the transistor for driving a self-light-emitting element. Note that the third transistor  813  and the fourth transistor  814  may have the same size. 
   A gate electrode of the first transistor  811  is connected with the first scanning line (Ga)  831 , a first electrode of the first transistor  811  is connected with the signal line  830 , and a second electrode of the first transistor  811  is connected with a first electrode of the second transistor and a first electrode of the third transistor  813 . A gate electrode of the second transistor  812  is connected with the second scanning line (Gb)  832 , and a second electrode of the second transistor  812  is connected with gate electrodes of the third transistor  813  and the fourth transistor  814 . A second electrode of the third transistor  813  and a first electrode of the fourth transistor are connected with the power source line  834 . A second electrode of the fourth transistor is connected with one of electrodes of the light-emitting element  820 . The capacitor element  819  is arranged between the gate electrode and the second electrode of the third transistor, and holds a gate-source voltage of the third transistor. The power source line  834  and the other electrode of the light-emitting element  820  are set at predetermined potentials, respectively. 
   The adjacent pixel has a similar structure, but differs in the following point from the above pixel. That is, the point is that the gate electrode of the second transistor  802  is connected with the third scanning line (Gc)  833 . 
   From the above, the pixels of the example in  FIGS. 8A  or  8 B have characteristics that the gate electrode of the second transistor is connected with either the second scanning line (Gb) or the third scanning line (Gc). 
   As described above, according to the present invention, it is characterized in that: a gate selection period is divided into plural periods, for example, T 1  and T 2 ; and both the (writing) operation of writing a signal to the pixel having the transistor connected with the scanning line that is selected and the (reading) operation of reading a signal current to the current source circuit connected with the signal line connected with the scanning line that is not selected are performed during T 1  or T 2  in the same row selection period. According to the driving method of the present invention, the area of the signal line driver circuit can be reduced, and thus, miniaturization of a light-emitting device can be realized. Moreover, in the light-emitting device, reduction in size of a frame can be attained, which means the proportion of the signal line driver circuit is small while the proportion of the pixel region is large. 
   Furthermore, in this embodiment mode, each input line for image signals is shared by the plural current source circuits, and thus, the number of terminals for taking in the image signals from the outside can be significantly reduced. As a result of the reduction in the number of connection terminals with respect to the outside, degradation in yield due to connection failure can also be avoided. 
   Embodiments 
   Hereinafter, the present invention will be specifically described based on embodiments. 
   [Embodiment 1] 
   In this embodiment, description will be made of a structure and a driving method in the case where each input line for an image signal current is shared by four current source circuits. Also, the circuits described with reference to  FIGS. 7A and 7B  and  FIGS. 8A and 8B  may be used for a pixel structure and a constant current source in this embodiment. However, the present invention is not limited to the circuits in  FIGS. 7A and 7B  and  FIGS. 8A and 8B . 
     FIG. 1  shows a structure in which each input line for image signals is shared by four current source circuits. In  FIG. 1 , current source circuits A 1 , A 2 , . . . , image signal input switches Sw 1 , Sw 2 , . . . on/off of which is controlled by control signals a 1 , a 2 , . . . , and signal lines S 1 , S 2 , . . . are provided. Then, the first scanning line (Ga) and the second and third scanning lines (Gb), (Gc) are provided so as to be substantially perpendicular to the respective signal lines, and each pixel is arranged at an intersecting point of the signal line and the first scanning line (Ga) or the second and third scanning lines (Gb), (Gc). In each pixel, pixel switch transistors Tr 1   11 , Tr 1   12 , . . . and current storage transistors Tr 2   11 , Tr 2   12 , . . . are provided. 
   Each of the current source circuits in the signal line driver circuit is connected with the signal line and the image signal input switch. Gate electrodes of the current storage transistors Tr 2   11  and Tr 2   12  are connected with the second scanning line (Gb), and gate electrodes of the current storage transistors Tr 2   13  and Tr 2   14  are connected with the third scanning line (Gc). First electrodes (source electrodes or drain electrodes) of the pixel switch transistors Tr 1   11 , Tr 1   12 , Tr 1   13 , and Tr 1   14  are connected with the respective signal lines S 1 , S 2 , S 3 , and S 4 , and gate electrodes thereof are connected with the first scanning line (Ga). In addition, the current source circuits A 1 , A 2 , A 3 , and A 4  are connected with one image signal current input line through the respective switches. 
   Next, the driving method of the present invention will be described with reference to  FIGS. 2A and 2B . The description is made for a first column through a fourth column in a first row, but the same goes for and the other rows.  FIG. 2A  is a diagram showing timings of selection and non-selection (assumed that: High corresponds to selection and conduction; and Low corresponds to non-selection and insulation in this example) in a row selection period.  FIG. 2B  is a block diagram in which reading (R) to the current source circuits in the signal line driver circuit and writing (W) to the pixels from the current source circuits are shown. 
   As shown in  FIG. 2A , the row selection period is divided into t 1  and t 2 . In the first-row selection period, the first scanning line (Ga) in the row is at High through t 1  and t 2 , and the pixel switch transistors Tr 1   11 , Tr 1   12 , Tr 1   13 , and Tr 1   14  are in an on state. Durin the period of t 1 , a high signal is input to the third scanning line (Gc) in the state in which a low signal is input to the second scanning line (Gb). Therefore, the transistors Tr 2   13  and Tr 2   14  connected to the third scanning line (Gc) are brought into an on state, and such a state is brought about in which the image signal current can be stored into the pixels from the signal lines S 3  and S 4  (regions of W 3  and W 4  in FIG.  2 B). At this time, the control signals a 3  and a 4  become signals that bring the image signal input switches into an off state (Low), and the image signals are not read into the current source circuits A 3  and A 4 . During t 1 , the transistors Tr 2   11 , and Tr 2   12  connected to the second scanning line (Gb) that is not selected (Low) are in an off state, and the image signal current is not stored into the pixels. At this time, the control signals a 1  and a 2  are at High, and bring the image signal input switches into an on state. The image signal current is read into the current source circuits A 1  and A 2  (regions of R 1  and R 2  in FIG.  2 B). 
   Further, during t 2 , a high signal is input to the second scanning line (Gb) in the state in which a low signal is input to the third scanning line (Gc). Therefore, the transistors Tr 2   11  and Tr 2   12  connected with the second scanning line (Gb) are brought into an on state, and such a state is brought about in which the image signal current can be stored into the pixels from the signal lines S 1  and S 2  (regions of W 1  and W 2  in FIG.  2 B). At this time, the control signals a 1  and a 2  become signals that bring the switches into an off state (Low), and the input signals are not read into the current source circuits A 1  and A 2 . During t 2 , the transistors Tr 2   13  and Tr 2   14  connected to the third scanning line (Gc) that is not selected (Low) are in an off state, and the image signal current is not stored into the pixels. At this time, the control signals a 3  and a 4  are at High, and bring the image signal input switches into an on state. The current is read into the current source circuits A 3  and A 4  (regions of R 3  and R 4  in FIG.  2 B). 
   As described above, according to the present invention, it is characterized in that: the row selection period is divided into plural periods (two of t 1  and t 2  in this embodiment); and the (writing) operation of writing the image signal current to the pixel and the (reading) operation of reading the signal current to the current source circuit in the signal line driver circuit are performed during the same row selection period. According to the driving method of the present invention, the area of the signal line driver circuit can be reduced, and thus, miniaturization of a light-emitting device can be realized. Moreover, in the light-emitting device, reduction in size of a frame can be attained, which means the proportion of the signal line driver circuit is small while the proportion of the pixel region is large. 
   Furthermore, in this embodiment, each input line for image signals is shared by the plural current source circuits, and thus, the number of terminals for taking in the image signals from the outside can be significantly reduced. As a result of the reduction in the number of connection terminals with respect to the outside, degradation in yield due to connection failure can also be avoided. 
   [Embodiment 2] 
   In this embodiment, description will be made of a structure and a driving method in the case where each input line for an image signal is shared by eight current source circuits. Also, the circuits described with reference to  FIGS. 7A and 7B  and  FIGS. 8A and 8B  are used for a pixel structure and a constant current source in this embodiment. However, the present invention is not limited to the circuits in  FIGS. 7A and 7B  and  FIGS. 8A and 8B . 
     FIG. 3  shows a structure in which each input line for image signals is shared by eight current source circuits. In  FIG. 3 , current source circuits A 1 , A 2 , . . . , image signal input switches on/off of which is controlled by control signals a 1 , a 2 , . . . , and signal lines S 1 , S 2 , . . . are provided. Then, the first scanning line (Ga) and the second and third scanning lines (Gb), (Gc) are provided so as to be substantially perpendicular to the respective signal lines, and each pixel is arranged at an intersecting point of the signal line and the first scanning line (Ga) or the second and third scanning lines (Gb), (Gc). In each pixel, pixel switch transistors Tr 1   11 , Tr 1   12 , . . . and current storage transistors Tr 2   11 , Tr 2   12 , . . . are provided. 
   Each of the current source circuits in the signal line driver circuit is connected with the signal line and the image signal input switch. Gate electrodes of the current storage transistors Tr 2   11 , Tr 2   12 , Tr 2   13 , Tr 2   14  are connected with the second scanning line (Gb), and gate electrodes of the current storage transistors Tr 2   15 , Tr 2   16 , Tr 2   17 , Tr 2   18  are connected with the third scanning line (Gc). First electrodes (source electrodes or drain electrodes) of the pixel switch transistors Tr 1   11 , Tr 1   12 , . . . , Tr 1   17 , Tr 1   18  are connected with the respective signal lines S 1 , S 2 , . . . , S 7 , S 8 , and gate electrodes thereof are connected with the first scanning line (Ga). In addition, the current source circuits A 1 , A 2 , . . . , A 7 , A 8  are connected with one image signal current input line through the respective switches. 
   Next, the driving method of the present invention will be described with reference to  FIGS. 4A and 4B . The description is made only for a first column through an eighth column in a first row, but the same goes for the other columns and the other rows.  FIG. 4A  is a diagram showing timings of selection and non-selection (assumed that: High corresponds to selection and conduction; and Low corresponds to non-selection and insulation in this example) in a row selection period.  FIG. 4B  is a block diagram in which reading (R) to the current source circuits in the signal line driver circuit and writing (W) to the pixels from the current source circuits are shown. 
   As shown in  FIG. 4A , the row selection period is divided into t 1  and t 2 . In the first-row selection period, the first scanning line (Ga) in the row is at High through t 1  and t 2 , and the pixel switch transistors Tr 1   11 , Tr 1   12 , . . . , Tr 1   17 , Tr 1   18  are in an on state. During the period of t 1 , a high signal is input to the third scanning line (Gc) in the state in which a low signal is input to the second scanning line (Gb). Therefore, the transistors Tr 2   15 , Tr 2   16 , Tr 2   17 , Tr 2   18  connected to the third scanning line (Gc) are brought into an on state, and such a state is brought about in which the image signal current can be stored into the pixels from the signal lines S 5 , S 6 , S 7 , S 8  (regions of W 5 , W 6 , W 7 , W 8  in FIG.  4 B). At this time, the control signals a 5 , a 6 , a 7 , a 8  become signals that bring the image signal input switches into an off state (Low), and the image signals are not read into the current source circuits A 5 , A 6 , A 7 , A 8 . During t 1 , the transistors Tr 2   11 , Tr 2   12 , Tr 2   13 , Tr 2   14  connected to the second scanning line (Gb) that is not selected (Low) are in an off state, and the image signal current is not stored into the pixels. At this time, the control signals a 1 , a 2 , a 3 , a 4  are at High, and bring the image signal input switches into an on state. The image signal current is read into the current source circuits A 1 , A 2 , A 3 , A 4  (regions of R 1 , R 2 , R 3 , R 4  in FIG.  4 B). 
   Further, during t 2 , a high signal is input to the second scanning line (Gb) in the state in which a low signal is input to the third scanning line (Gc). Therefore, the transistors Tr 2   11 , Tr 2   12 , Tr 2   13 , Tr 2   14  connected with the second scanning line (Gb) are brought into an on state, and such a state is brought about in which the image signal current can be stored into the pixels from the signal lines S 1 , S 2 , S 3 , S 4  (regions of W 1 , W 2 , W 3 , W 4  in FIG.  4 B). At this time, the control signals a 1 , a 2 , a 3 , a 4  become signals that bring the switches into an off state (Low), and the input signals are not read into the current source circuits A 1 , A 2 , A 3 , A 4 . During t 2 , the transistors Tr 2   15 , Tr 2   16 , Tr 2   17 , Tr 2   18  connected to the third scanning line (Gc) that is not selected (Low) are in an off state, and the image signal current is not stored into the pixels. At this time, the control signals a 5 , a 6 , a 7 , a 8  are at High, and bring the image signal input switches into an on state. The current is read into the current source circuits A 5 , A 6 , A 7 , A 8  (regions of R 5 , R 6 , R 7 , R 8  in FIG.  4 B). 
   As described above, according to the present invention, it is characterized in that: the row selection period is divided into plural periods (two of t 1  and t 2  in this embodiment); and the (writing) operation of writing the image signal current to the pixel and the (reading) operation of reading the signal current to the current source circuit in the signal line driver circuit are performed during the same row selection period. According to the driving method of the present invention, the area of the signal line driver circuit can be reduced, and thus, miniaturization of a light-emitting device can be realized. Moreover, in the light-emitting device, reduction in size of a frame can be attained, which means the proportion of the signal line driver circuit is small while the proportion of the pixel region is large. 
   Furthermore, in this embodiment, each input line for image signals is shared by the plural current source circuits, and thus, the number of terminals for taking in the image signals from the outside can be significantly reduced. As a result of the reduction in the number of connection terminals with respect to the outside, degradation in yield due to connection failure can also be avoided. 
   [Embodiment 3] 
     FIGS. 9A and 9B  are schematic diagrams of a light-emitting device that uses the present invention.  FIG. 9A  shows the light-emitting device that includes: a pixel region in which pixels provided with light-emitting elements are arranged in matrix; a signal line driver circuit having a current source circuit; a first scanning line driver circuit; and a second scanning line driver circuit. The first scanning line driver circuit is connected with the first scanning line (Ga), and the second scanning line driver circuit is connected with the second scanning line (Gb). Note that the first and second scanning line driver circuits may be provided on the same side with respect to the pixel region, although being arranged symmetrically, while sandwiching the pixel region. 
   The structures of the first scanning line driver circuit and the second scanning line driver circuit are described with reference to FIG.  9 B. The first scanning line driver circuit and the second scanning line driver circuit each have a shift register and a buffer. An operation thereof is simply explained. The shift register sequentially outputs sampling pulses in accordance with a clock signal (G-CLK), a start pulse (S-SP), and a clock inversion signal (G-CLKb). Thereafter, the sampling pulses amplified by the buffer are input to the scanning lines to select rows on a one-by-one basis. Then, the signal current is sequentially written from the signal line into the pixel controlled by the selected scanning line. 
   Such a structure may be adopted in which a level shifter circuit is arranged between the shift register and the buffer. Voltage amplitude can be extended by additionally arranging the level shifter circuit. 
   According to the driving method of the present invention, the area of the signal line driver circuit, particularly the area of the current source circuit can be reduced. Note that the number of scanning line driver circuits is increased to two, but the area of the scanning line driver circuit is small compared with the area of the signal line driver circuit. Therefore, miniaturization, reduction in weight, and reduction in size of a frame of the light-emitting device can be attained. 
   Furthermore, plural signal line driver circuits may be provided in order to more speedily conduct the (writing) operation of writing the image signal current to the pixel and the (reading) operation of reading the signal current to the current source circuit. 
   [Embodiment 4] 
   Given as examples of electronic apparatuses using a light-emitting device of the present invention include a video camera, a digital camera, a goggles-type display (head mount display), a navigation system, a sound reproduction device (such as a car audio equipment and an audio set), a lap-top computer, a game machine, a portable information terminal (such as a mobile computer, a mobile telephone, a portable game machine, and an electronic book), an image reproduction apparatus including a recording medium (more specifically, an apparatus which can reproduce a recording medium such as a digital versatile disc (DVD) and so forth, and includes a display for displaying the reproduced image), or the like. In particular, in the case of the portable information terminal, use of the light-emitting device is preferable, since the portable information terminal that is likely to be viewed from a tilted direction is often required to have a wide viewing angle.  FIGS. 11A  to  11 H respectively shows various specific examples of such electronic apparatuses. 
     FIG. 11A  illustrates a light-emitting device which includes a casing  2001 , a support table  2002 , a display portion  2003 , a speaker portion  2004 , a video input terminal  2005  and the like. The present invention is applicable to the display portion  2003 . Also, the light-emitting device shown in  FIG. 11A  is completed by the present invention. The light-emitting device is of the self-emission-type and therefore requires no backlight. Thus, the display portion thereof can have a thickness thinner than that of the liquid crystal display device. The light-emitting device is including the entire display device for displaying information, such as a personal computer, a receiver of TV broadcasting and an advertising display. 
     FIG. 11B  illustrated a digital still camera which includes a main body  2101 , a display portion  2102 , an image receiving portion  2103 , an operation key  2104 , an external connection port  2105 , a shutter  2106 , and the like. The light-emitting device of the present invention can be used as the display portion  3102 . Also, the digital still camera shown in  FIG. 11B  is completed by the present invention. 
     FIG. 11C  illustrates a lap-top computer which includes a main body  2201 , a casing  2202 , a display portion  2203 , a keyboard  2204 , an external connection port  2205 , a pointing mouse  2206 , and the like. The light-emitting device of the present invention can be used as the display portion  2203 . Also, the lap-top computer shown in  FIG. 11C  is completed by the present invention. 
     FIG. 11D  illustrated a mobile computer which includes a main body  2301 , a display portion  2302 , a switch  2303 , an operation key  2304 , an infrared port  2305 , and the like. The light-emitting device of the present invention can be used as the display portion  2302 . The mobile computer shown in  FIG. 11D  is completed by the present invention. 
     FIG. 11E  illustrates a portable image reproduction apparatus including a recording medium (more specifically, a DVD reproduction apparatus), which includes a main body  2401 , a casing  2402 , a display portion A  2403 , another display portion B  2404 , a recording medium (DVD or the like) reading portion  2405 , an operation key  2406 , a speaker portion  2407  and the like. The display portion A  2403  is used mainly for displaying image information, while the display portion B  2404  is used mainly for displaying character information. The light-emitting device of the present invention can be used as these display portions A  2403  and B  2404 . The image reproduction apparatus including a recording medium further includes a domestic game machine or the like. Also, the portable image reproduction apparatus shown in  FIG. 11E  is completed by the present invention. 
     FIG. 11F  illustrates a goggle type display (head mounted display) which includes a main body  2501 , a display portion  2502 , arm portion  2503 , and the like. The light-emitting device of the present invention can be used as the display portion  2502 . Also, the goggle type display shown in  FIG. 11F  is completed by the present invention. 
     FIG. 11G  illustrates a video camera which includes a main body  2601 , a display portion  2602 , a casing  2603 , an external connecting port  2604 , a remote control receiving portion  2605 , an image receiving portion  2606 , a battery  2607 , a sound input portion  2608 , an operation key  2609 , and the like. The light-emitting device of the present invention can be used as the display portion  2602 . Also, the video camera shown in  FIG. 11G  is completed by the present invention. 
     FIG. 11H  illustrates a mobile telephone which includes a main body  2701 , a casing  2702 , a display portion  2703 , a sound input portion  2704 , a sound output portion  2705 , an operation key  2706 , an external connecting port  2707 , an antenna  2708 , and the like. The light-emitting device of the present invention can be used as the display portion  2703 . Note that the display portion  2703  can reduce power consumption of the mobile telephone by displaying white-colored characters on a black-colored background. Also, the mobile telephone shown in  FIG. 11H  is completed by the present invention. 
   When a brighter luminance of light-emitting materials becomes available in the future, the light-emitting device in accordance with the present invention will be applicable to a front-type or rear-type projector in which light including output image information is enlarged by means of lenses or the like to be projected. 
   The aforementioned electronic apparatuses are more likely to be used for display information distributed through a telecommunication path such as Internet, a CATV (cable television system), and in particular likely to display moving picture information. The light-emitting device is suitable for displaying moving pictures since the organic light-emitting material can exhibit high response speed. 
   A portion of the light-emitting device that is emitting light consumes power, so it is desirable to display information in such a manner that the light-emitting portion therein becomes as small as possible. Accordingly, when the light-emitting device is applied to a display portion which mainly displays character information, e.g., a display portion of a portable information terminal, and more particular, a portable telephone or a sound reproduction device, it is desirable to drive the light-emitting device so that the character information is formed by a light-emitting portion while a non-emission portion corresponds to the background. 
   As set forth above, the present invention can be applied variously to a wide range of electronic apparatuses in all fields. Moreover, the electronic apparatuses in this embodiment can be implemented by using any structure of the signal line drive circuit in Embodiments 1 to 3. 
   According to the present invention, one current source circuit in the signal line driver circuit is provided for each column. Then, the row selection period (horizontal period) is divided into plural periods. In each of the divided periods, the (writing) operation of writing the image signal current to the pixel is performed in a certain column of the row while the (reading) operation of reading the image signal current to the current source circuit in the signal line driver circuit in another column of the row. The columns for conducting the writing operation and the reading operation differ for each divided period. As described above, the number of current source circuits in the signal line driver circuit is limited to one for each column. Thus, the signal line driver circuit that includes the current source circuit having a small area can be provided, and therefore, the reduction in size of the frame of the light-emitting device can be attained. 
   Further, according to the present invention, the image signal current input line is shared by the plural current source circuits in the signal line driver circuit. Thus, the number of terminals for taking in the image signals from the outside can be reduced. As a result of the reduction in the number of the connection terminals with respect to the outside, the degradation in yield due to connection failure can also be avoided.