Patent Publication Number: US-8125419-B2

Title: Electro-optical device, method for driving electro-optical device, and electronic apparatus

Description:
The entire disclosure of Japanese Application No. 2006-115433, filed Apr. 19, 2006 is expressly incorporated by reference herein. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a technique for controlling gradation levels of electro-optical elements such as organic light emitting diode (OLED) elements. 
     2. Related Art 
     Electro-optical devices having multiple electro-optical elements have been proposed. Each of the electro-optical elements is controlled to a gradation level corresponding to the level (such as the voltage value or the current value) of a data signal output from a driving circuit. The driving circuit generates a data signal having a level corresponding to a gradation value D specified by image data. A characteristic curve F C1  shown in  FIG. 19  indicates the relationship between the voltage of the data signal and the gradation of the electro-optical elements (e.g., the brightness of the OLED elements). 
     JP-A-2003-255900 discloses a display device in which the relationship between a gradation value D and an actual gradation level of an electro-optical element is adjusted by a gamma correction.  FIG. 20  is a graph showing the relationship between a gradation value D and a gradation level of an electro-optical element when the gamma value is set to 2.0. 
     There is a demand for an electro-optical device capable of multiple gradation display. However, the step width of levels of a data signal (that is, the minimum change amount) needs to be reduced to finely change the gradation of electro-optical elements. Therefore, a problem occurs in that a high-performance large-scale driving circuit is needed, resulting in an increase in the cost of the electro-optical device. 
     The above-described problem becomes noticeable when the luminous efficiency of the electro-optical elements increases. That is, as indicated by a characteristic curve F C2  shown in  FIG. 19 , the change amount of the gradation of the electro-optical elements with respect to the level (e.g., the voltage value) of the data signal increases as the luminous efficiency of the electro-optical elements increases. Thus, if the gradation of the electro-optical elements changes by a value of ΔG shown in  FIG. 19 , it is necessary to improve the performance of the driving circuit so that a step width ΔV 2  of levels of the data signal becomes smaller than a step width ΔV 1  in the characteristic curve F C1 . 
     Further, in a case where the gamma correction is performed using a gamma value higher than 1, as shown in  FIG. 20 , it is necessary to reduce the step width ΔG of the gradation levels of the electro-optical elements, in particular within a low-gradation range. In this case, it is also necessary to finely change the voltage of the data signal, and there arises a problem of increasing the cost of the electro-optical device. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a technique for fine control of the gradation of electro-optical elements while maintaining a certain step width of levels of a data signal. 
     According to an aspect of the invention, an electro-optical device includes a unit circuit including a first element section that controls a first electro-optical element to a gradation level corresponding to a level of a data signal, and a second element section that controls a second electro-optical element to a gradation level corresponding to a level of a data signal, the gradation level of the first electro-optical element being lower than the gradation level of the second electro-optical element when data signals having an identical level are applied to the first element section and the second element section; and a signal generating circuit that generates data signals having different levels according to a gradation value specified for the unit circuit. When the gradation value is within a first gradation range, the signal generating circuit applies to the first element section a data signal whose level is determined so that the first electro-optical element is controlled to a gradation level corresponding to the gradation value. When the gradation value is within a second gradation range higher than the first gradation range, the signal generating circuit applies to the second element section a data signal whose level is determined so that the second electro-optical element is controlled to a gradation level corresponding to the gradation value. 
     According to the invention, the gradation level of the first electro-optical element is lower than the gradation level of the second electro-optical element when data signals having an identical level are applied to the first element section and the second element section (that is, the first element section and the second element section have different gradation change rates). With this structure, when a gradation value in the first gradation range is specified, the first electro-optical element is controlled by the data signal corresponding to the gradation value. Therefore, when a gradation value in the first gradation range is specified, the step width of levels of the data signal can be sufficiently maintained compared with a structure in which one electro-optical element having a characteristic equivalent to that of the second electro-optical element is controlled regardless of the gradation value specified for the unit circuit. When a gradation value in the second gradation range is specified, the second electro-optical element is controlled. Therefore, a wide range of multiple gradation levels can be represented while the levels of the data signals are suppressed (that is, the power consumption is reduced) compared with a structure in which one electro-optical element having a characteristic equivalent to that of the first electro-optical element is controlled regardless of the gradation value specified for the unit circuit. 
     In the invention, each of the electro-optical elements is an element whose optical characteristics, such as brightness and transmittance, vary in accordance with electric energy applied thereto (such as a supplied current or a applied voltage). Each of the electro-optical elements may be a self-emission element that emits light or a non-emission element (such as a liquid crystal element) that variably controls the transmittance of ambient light, or may be a current-driven element that is driven by a supplied current or a voltage-driven element that is driven by an applied voltage. Various electro-optical elements can be used such as an OLED element, an inorganic EL element, a field emission (FE) element, a surface-conduction electron-emitter (SE) element, a ballistic electron surface emitting (BS) element, a light emitting diode (LED) element, a liquid crystal element, an electrophoresis element, and an electrochromic element. 
     In the invention, each of the data signals may be a current signal or a voltage signal. When the data signal is a current signal, the data signal has a level indicative of a current value. When the data signal is a voltage signal, the data signal has a level indicative of a voltage value. While the unit circuit is formed of the first element section and the second element section, the unit circuit may include three or more element sections including the first element section and the second element section. 
     It is preferable that the area of a region of the first electro-optical element from which light is output is different from the area of a region of the second electro-optical element from which light is output. Therefore, the first electro-optical element and the second electro-optical element can be manufactured commonly using a process while the first element section and the second element section have different gradation change rates. The structure in which element sections having different gradation change rates are provided can be achieved by the following approaches. 
     In a first approach, each of the first electro-optical element and the second electro-optical element is a light-emitting element including a first electrode, a second electrode, and a light-emitting layer between the first electrode and the second electrode, wherein a distance between the first electrode and the second electrode of the first electro-optical element is different from a distance between the first electrode and the second electrode of the second electro-optical element. In other words, the thickness of a portion that is provided between the first electrode and the second electrode of the first electro-optical element and that includes the light-emitting layer is different from that of the second electro-optical element. 
     In a second approach, each of the first electro-optical element and the second electro-optical element is a light-emitting element including a first optically transparent electrode, a second optically reflective electrode facing the first electrode, and a light-emitting layer between the first electrode and the second electrode, and the first electrode of the first electro-optical element and the first electrode of the second electro-optical element have different thicknesses. 
     In a third approach, the electro-optical device further includes an optically transparent insulation layer defined on a surface of a substrate. Each of the first electro-optical element and the second electro-optical element is a light-emitting element including a first optically transparent electrode defined on a surface of the insulation layer, a second optically reflective electrode facing the first electrode, and a light-emitting layer between the first electrode and the second electrode, wherein the thickness of a region of the insulation layer through which light output from the first electro-optical element is transmitted is different from the thickness of a region of the insulation layer through which light output from the second electro-optical element is transmitted. 
     In a fourth approach, the electro-optical device further includes a first light-transmitting member through which light output from the first electro-optical element is transmitted, and a second light-transmitting member through which light output from the second electro-optical element is transmitted, wherein the first light-transmitting member and the second light-transmitting member have different transmittances. 
     In the first to fourth approaches described above, the area of the first electro-optical element and the area of the second electro-optical element can be equal to each other. That is, the area of the second electro-optical element does not need to be larger than the area of the first electro-optical element. Therefore, advantageously, high-definition electro-optical elements can be easily realized. 
     The structure in which the gradation change rate of the first element section is different from the gradation change rate of the second element section is not limited to those described above. For example, the first element section may include a first driving transistor that generates a drive current corresponding to a voltage at a gate of the first driving transistor and that supplies the drive current to the first electro-optical element, and the second element section may include a second driving transistor that generates a drive current corresponding to a voltage at a gate of the second driving transistor and that supplies the drive current to the second electro-optical element, wherein the drive current generated by the first driving transistor and the drive current generated by the second driving transistor have different current values when the same voltage is applied to the gate of the first driving transistor and the gate of the second driving transistor. Therefore, advantageously, the conditions of the electro-optical elements (such as the area of the electro-optical elements and the thickness of layers) do not need to be different for each of element section. 
     Further, the characteristics of elements (such as an electro-optical element and a driving transistor) included in each element section do not need to be differently set. For example, the first element section may control the first electro-optical element to emit light at a brightness corresponding to the level of the data signal in a first period, and the second element section may control the second electro-optical element to emit light at a brightness corresponding to the level of the data signal in a second period longer than the first period. With this structure, the gradation change rates can be different for each of the first element section and the second element section according to the time length of the first period and the second period. A specific example of this structure is described below with respect to a third embodiment of the invention. 
     It is preferable that the first element section controls the first electro-optical element to a gradation level corresponding to a voltage value of the data signal; the second element section controls the second electro-optical element to a gradation level corresponding to a current value of the data signal; and the signal generating circuit includes a voltage generating circuit that outputs a data signal having a voltage value corresponding to the gradation value specified for the unit circuit to the first element section when the gradation value is within the first gradation range, and a current generating circuit that supplies a data signal having a current value corresponding to the gradation value to the second element section when the gradation value is within the second gradation range. With this structure, the first electro-optical element is driven according to the voltage value of the data signal when the gradation value is within the second nigh-gradation range, and the second electro-optical element is driven according to the current value of the data signal when the gradation value is within the first low-gradation range. Therefore, even if a transmission channel of the data signal has a high time constant, the first electro-optical element can be reliably set to a predetermined gradation level. A specific example of this structure is described below with respect to a fourth embodiment of the invention. 
     The electro-optical device according to the invention can be used in various electronic apparatuses. The electronic apparatuses are typically apparatuses using the electro-optical device as a display device. Examples of the electronic apparatuses include personal computers and mobile phones. However, the use of the electro-optical device according to the invention is not limited to the display of images. The electro-optical device according to the invention can be used in various applications such as an exposure apparatus (namely, an exposure head) for irradiating an image bearing member such as a photoconductive drum with light to form a latent image on the image bearing member, an apparatus (such as a backlight) disposed on a back surface of a light crystal device for lighting the light crystal device, and various lighting apparatuses such as apparatuses included in an image reading apparatus such as a scanner for irradiating a document with light. 
     According to another aspect, the invention provides a method for driving the electro-optical device. The method includes determining which of a plurality of gradation ranges including a first gradation range and a second gradation range higher than the first gradation range a gradation value specified for the unit circuit belongs to; and generating data signals having different levels according to the gradation value, wherein when it is determined that the gradation value is within the first gradation range, a data signal whose level is determined so that the first electro-optical element is controlled to a gradation level corresponding to the gradation value is applied to the first element section, and when it is determined that the gradation value is within the second gradation range, a data signal whose level is determined so that the second electro-optical element is controlled to a gradation level corresponding to the gradation value is applied to the second element section. The above-described method can also achieve similar advantages to those of the electro-optical device according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a block diagram showing the structure of an electro-optical device according to the invention. 
         FIG. 2  is a circuit diagram showing the structure of each unit circuit. 
         FIG. 3  is a timing chart showing the operation of the electro-optical device. 
         FIG. 4  is a plan view showing the arrangement of electro-optical elements and lines. 
         FIG. 5  is a graph showing the relationship between voltage values of a data signal and the gradation (amount of light emission) of each of the electro-optical elements. 
         FIG. 6  is a cross-sectional view showing the structure of an element array section according to a first method in a second embodiment of the invention. 
         FIG. 7  is a cross-sectional view snowing the structure of an element array section according to a second method in the second embodiment. 
         FIG. 8  is a graph showing the spectral characteristic of light beams output from electro-optical elements. 
         FIG. 9  is a cross-sectional view showing the structure of an element array section according to a third method in the second embodiment. 
         FIG. 10  is a cross-sectional view showing the structure of an element array section according to a fourth method in the second embodiment. 
         FIG. 11  is a circuit diagram showing the structure of a unit circuit according to a third embodiment of the invention. 
         FIG. 12  is a timing chart showing the operation of the electro-optical device. 
         FIG. 13  is a circuit diagram showing the structure of a unit circuit according to a fourth embodiment of the invention. 
         FIGS. 14A and 14B  are timing charts showing the operation of the electro-optical device. 
         FIG. 15  is a circuit diagram showing the structure of a unit circuit according to a modification of the invention. 
         FIG. 16  is a perspective view showing an electronic apparatus (e.g., a personal computer) according to the invention. 
         FIG. 17  is a perspective view showing an electronic apparatus (e.g., a mobile phone) according to the invention. 
         FIG. 18  is a perspective view showing an electronic apparatus (e.g., a portable information terminal) according to the invention. 
         FIG. 19  is a graph showing the relationship between voltage values of a data signal and the gradation of an electro-optical element. 
         FIG. 20  is a graph showing the relationship between gradation values and the actual gradation of an electro-optical element. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a block diagram showing the structure of an electro-optical device  100  according to a first embodiment of the invention. As shown in  FIG. 1 , the electro-optical device  100  includes an element array section A having a large number of unit circuits P, a scanning line driving circuit  22  and data line driving circuit  24  that drive each of the unit circuits P, and a control circuit  20  that controls the scanning line driving circuit  22  and the data line driving circuit  24 . The large number of unit circuits P are arranged in a matrix of m rows and n columns in X- and Y-axes orthogonal to each other (where each of m and n is a natural number of 2 or more). 
       FIG. 2  is a circuit diagram showing the structure of each of the unit circuits P. In  FIG. 2 , one of the unit circuits P that is positioned at the j-th column (where j is an integer satisfying 1≦j≦n) in the i-th row (where i is an integer satisfying 1≦i≦m) is illustrated. All the unit circuits P have a similar structure. As shown in  FIGS. 1 and 2 , the element array section A includes m scanning lines  120  extending along the X-axis, and n line groups  14  extending along the Y-axis. Each of the unit circuits P is located at a position corresponding to an intersection of each of the scanning lines  120  and each of the line groups  14 . As shown in  FIG. 2 , the line group  14  at the j-th column includes three data lines LD 1 [j] to LD 3 [j] each extending along the Y-axis. A power supply potential V EL  is supplied to each of the unit circuits P via a power supply-line  17 . 
     The scanning line driving circuit  22  shown in  FIG. 1  is a circuit (e.g., an m-bit shift register) operable to generate scanning signals G[ 1 ] to G[m] for sequentially selecting the m rows of the element array section A (i.e., the scanning lines  120 ) and to output the scanning signals G[ 1 ] to G[m] to the scanning lines  120 . As shown in  FIG. 3 , the control signal G[i] output to the i-th scanning line  120  is at a high (selection) level for the i-th horizontal scanning period H within a period of one frame, and is maintained at a low (non-selection) level for the remaining period. 
     The control circuit  20  controls the timing of the operation of the scanning line driving circuit  22  and the data line driving circuit  24  according to an output of various signals such as a clock signal. Further, the control circuit  20  sequentially outputs image data for specifying a gradation value D of each of the unit circuits P to the data line driving circuit  24 . As shown in  FIG. 1 , the data line driving circuit  24  includes a data determining unit  241  that determines a range R to which the gradation value D of each of the unit circuits P belongs, and n signal generating circuits  25 , where n denotes the total number of line groups  14  (i.e., the number of columns of the unit circuits P). The data determining unit  241  determines which of three ranges R (namely, R L , R M , and R H ) the gradation value D supplied from the control circuit  20  belongs to. The range of the gradation value D from the minimum value to the maximum value is divided into the ranges R L , R M , and R H  that do not overlap. The range R L  includes the minimum value of the gradation value D, and the range R H  includes the maximum value of the gradation value D. The range R M  is a gradation range higher than the range R L , and the range R H  is a gradation range higher than the range R M . 
     The signal generating circuit  25  at the j-th column generates data signals S 1 [j] to S 3 [j], and outputs the data signals S 1 [j] to S 3 [j] to the line group  14  at the j-th column. Each of the data signals S 1 [j] to S 3 [j] is a voltage signal whose voltage value V d  is designated according to the gradation value D of the j-th column and a determination result of the data determining unit  241 . The data signal Sk[j] (where k is an integer satisfying 1≦k≦3) is output to the data line LDk[j]. The operation of the signal generating circuits  25  is described in detail below. 
     The structure of the unit circuits P will be described in detail. As shown in  FIG. 2 , each of the unit circuits P includes three element sections U 1  to U 3 , the number of element sections corresponding to the number of sections the range R is divided. The element section Uk includes an electro-optical element Ek arranged on a path extending from the power supply line  17  to a ground line (a ground potential Gnd). In the first embodiment, the electro-optical element Ek is an OLED element having a light-emitting layer formed of an organic electroluminescent (EL) material between electrodes facing each other. The light-emitting layer emits light when a current (hereinafter referred to as a “drive current”) I EL  is supplied. 
     The element section Uk further includes a p-channel driving transistor Qdr on the path of the drive current I EL  (between the power supply line  17  and the electro-optical element Ek). The driving transistor Qdr is a thin-film transistor that generates the drive current I EL  having a current amount corresponding to the voltage at a gate of the driving transistor Qdr and that supplies the drive current I EL  to the electro-optical element Ek. The element section Uk further includes a selection transistor Qsl between the gate of the driving transistor Qdr and the data line LDk[j] for controlling the electrical connection (conduct ion/non-conduction) therebetween. The gates of the selection transistors Qsl included in the element sections U 1  to U 3  of each of the unit circuits P in the i-th row are commonly connected with the scanning line  120  in the i-th row. A capacitor element C is provided between the gate and source (on the side of the power supply line  17 ) of the driving transistor Qdr. 
     When the scanning signal G[i] changes to a high level in a horizontal scanning period H, the selection transistors Qsl included in the element sections U 1  to U 3  of each of the unit circuits P in the i-th row are turned on at the same time. Therefore, the gate of the driving transistor Qdr of the element section Uk is set to the voltage value V d  of the data signal Sk[j] supplied to the data line LDk[j] in that horizontal scanning period H. During this period, electric charge corresponding to the voltage value V d  is accumulated in the capacitor element C. Thus, even if the scanning signal G[i] changes to a low level and the selection transistors Qsl are turned off, the gates of the driving transistors Qdr are maintained at the voltage value V d . The drive current I EL  corresponding to the voltage value V d  is continuously supplied to the electro-optical element Ek until the next time the scanning signal G[i] changes to the high level. Accordingly, the electro-optical element Ek is set to the gradation level (i.e., the amount of light emission) corresponding to the voltage value V d  of the data signal Sk[j]. 
       FIG. 4  is a plan view of each of the unit circuits P, showing the arrangement of the electro-optical elements E 1  to E 3  and the lines. As shown in  FIG. 4 , the areas of the electro-optical elements E 1  to E 3  are different from each other. Specifically, the area of the electro-optical element E 2  is larger than that of the electro-optical element E 1 , and the area of the electro-optical element E 3  is larger than that of the electro-optical element E 2 . The electro-optical elements E 1  and E 2  are arranged along the X-axis and are disposed in the negative direction of the Y-axis with respect to the scanning line  120 . The electro-optical element E 3  is arranged in the positive direction of the Y-axis with respect to the scanning line  120 . The data lines LD 1 [j] and LD 3 [j] extend along the Y-axis and are disposed in the negative direction of the X-axis as viewed from the electro-optical elements E 1  to E 3 . The data line LD 2 [j] and the power supply line  17  extend along the Y-axis and are disposed in the positive direction of the X-axis as viewed from the electro-optical elements E 1  to E 3 . 
       FIG. 5  is a graph showing the relationship between the voltage value V d  of the data signal Sk[j] and the gradation of the electro-optical element Ek. In  FIG. 5 , a characteristic curve F Ak  indicates the relationship between the absolute value of the voltage value V d  of the data signal Sk[j] and the actual gradation level (i.e., the amount of light emission) of the electro-optical element Ek. In the first embodiment, as shown in  FIG. 4 , the areas of the electro-optical elements E 1  to E 3  are different from each other. Thus, even if the data signals S 1 [j] to S 3 [j] having the same voltage value V d  are supplied to the element sections U 1  to U 3 , as shown in  FIG. 5 , the gradation levels (i.e., amounts of light emission) of the electro-optical elements E 1  to E 3  are different from each other. Specifically, when the data signals S 1 [j] to S 3 [j] having the same voltage value V d  are supplied, the gradation level of the electro-optical element E 1  is lower than that of the electro-optical element E 2 , and the gradation level of the electro-optical element E 3  is higher than that of the electro-optical element E 2 . In other words, relative ratios of the change amounts of the gradation levels of the electro-optical elements E 1  to E 3  with respect to the change amounts of the voltage values V d  of the data signals S 1 [j] to S 3 [j] (the relative ratio is hereinafter referred to as a “gradation change rate”) are as follows: the gradation change rate for the electro-optical element E 3  is maximum, and the gradation change rate for the electro-optical element E 1  is minimum. The gradation change rate is defined by “(the change amount of the gradation)/(the change amount of the voltage value V d )”, and is a numerical value used as an index of sensitivity, which measures a change in the gradation level of the electro-optical element Ek in accordance with the voltage value V d  (that is, if the gradation change rate is higher, the gradation level of the electro-optical element Ek varies more sensitively to a change of the voltage value V d ). 
     The signal generating circuit  25  at the j-th column determines the voltage values V d  of the data signals S 1 [j] to S 3 [j] so that one electro-optical element Ek corresponding to the range R to which the gradation value D belongs can be selectively driven to the gradation level corresponding to the gradation value D from among the electro-optical elements E 1  to E 3  of the unit circuits P at the j-th column. 
     For example, when the data determining unit  241  determines that the gradation value D is within the range R L , the signal generating circuit  25  generates the data signal S 1 [j] having a voltage value V d  within a range B 1  shown in  FIG. 5  in accordance with the gradation value D, and sets the data signals S 2 [j] and S 3 [j] to a voltage value V d  for turning off the corresponding electro-optical elements E 2  and E 3  (i.e., the power supply potential V EL ). When the gradation value D is within the range R M , the signal generating circuit  25  generates the data signal S 2 [j] having a voltage value V d  within a range B 2  shown in  FIG. 5  in accordance with the gradation value D, and generates the data signals S 1 [j] and S 3 [j] having a voltage value V d  for turning off the electro-optical elements E 1  and E 3 . When the gradation value D is within the range R H , the signal generating circuit  25  generates the data signal S 3 [j] having a voltage value V d  within a range B 3  shown in  FIG. 5  in accordance with the gradation value D, and generates the data signals S 1 [j] and S 2 [j] having a voltage value V d  for turning off the electro-optical elements E 1  and E 2 . 
     For example, a gradation value D within the range R H  is designated for the unit circuit P at the j-th column in the i-th row, a gradation value D within the range R L  is designated for the unit circuit P at the j-th column in the (i+1)-th row, and a gradation value D within the range R M  is designated for the unit circuit P at the j-th column in the (i+2)-th row. In this case, as shown in  FIG. 3 , in the horizontal scanning period H during which the scanning signal G[i] is at the high level, the data signal S 3 [j] is set to the voltage value V d  (which has a potential lower than the power supply potential V EL ) for turning on the electro-optical element E 3  at the gradation level corresponding to the gradation value D, and the data signals S 1 [j] and S 2 [j] are set to the voltage value V d  (equal to the power supply potential V EL ) for turning off the electro-optical elements E 1  and E 2 . In the horizontal scanning period H during which the scanning signal G[i+1] is at the high level, the data signal S 1 [j] is set to the voltage value V d  corresponding to the gradation value D, and the data signals S 2 [j] and S 3 [j] are set to the power supply potential V EL . In the horizontal scanning period H during which the scanning signal G[i+2] is at the high level, the data signal S 2 [j] is set to the voltage value V d  corresponding to the gradation value D, and the data signals S 1 [j] and S 3 [j] are set to the power supply potential V EL . 
     Accordingly, the voltage value V d  of one data signal Sk[j] selected from among the data signals S 1 [j] to S 3 [j] according to the range R of the gradation value D is determined according to the gradation value D. Therefore, in  FIG. 5 , a curve portion fk indicated by a solid line from among the characteristic curve F Ak  of the electro-optical element Ek is used. That is, light is output (or displayed) at a gradation level within the range R L  by light emission of the electro-optical element E 1  (indicated by the curve portion f 1 ), at a gradation level within the range R M  by light emission of the electro-optical element E 2  (indicated by the curve portion f 2 ), and at a gradation level within the range R H  by light emission of the electro-optical element E 3  (indicated by the curve portion f 3 ). 
     In the first embodiment, therefore, the electro-optical element E 1  having the minimum gradation change rate is driven when a gradation value D within the low-gradation range R L  is designated, and the electro-optical element E 3  having the maximum gradation change rate is driven when a gradation value D within the high-gradation range R H  is designated. Therefore, advantageously, the voltage values V d  of the data signals S 1 [j] to S 3 [j] can be reduced while the step width of the voltage values V d  of the data signals S 1 [j] to S 3 [j] is sufficiently maintained. This advantage will be described in detail. 
     A structure in which each of the unit circuits P includes only the element section U 3  (that is, a structure in which all the gradation values D are represented by the electro-optical element E 3  having a high gradation change rate) is given as a first comparative example. With the structure of the first comparative example, as shown in  FIG. 5 , it is necessary to change the voltage value V d  of the data signal S 3 [j] by a fine change amount ΔV 1  in order to change the gradation level of the electro-optical element E 3  by a value ΔG within the range R L . In this case, the data line driving circuit  24  that is expensive to allow for fine adjustment of the voltage value V d  is required. In the first embodiment, on the other hand, since a gradation value D within the range R L  is represented by the electro-optical element E 1  having a low gradation change rate, a change amount ΔV 2  of the voltage value V d  required for changing the gradation value D by the value ΔG is larger than the change amount ΔV 1  in the first comparative example. In the first embodiment, therefore, the need for fine adjustment of the change amount of the voltage value V d  of the data signal Sk[j] can be eliminated or reduced, and the data line driving circuit  24  can be less expensive than that in the first comparative example. 
     A structure in which each of the unit circuits P includes only the element section U 1  (that is, a structure in which all the gradation values D are represented by the electro-optical element E 1  having a low gradation change rate) is given as a second comparative example. With the structure of the second comparative example, as shown in  FIG. 5 , it is necessary to increase the data signal S 1 [j] to a voltage value V d1  in order to control the electro-optical element E 1  to a gradation level GH within the range R H , resulting in a problem of excessive power consumption in the data line driving circuit  24 . In the first embodiment, on the other hand, gradation values D within the ranges R M  and R H  are represented by the electro-optical elements E 2  and E 3  having a higher gradation change rate than the electro-optical element E 1 . Thus, for example, a voltage value V d  of the data signal S 3 [j] required to control the electro-optical element E 3  to the gradation level GH is equal to a voltage value V d2  that is greatly lower than the voltage value V d1  in the second comparative example. According to the first embodiment, since the voltage value V d  required for nigh-gradation output is reduced, advantageously, the data line driving circuit  24  can achieve lower power consumption than the second comparative example. 
     Second Embodiment 
     In the first embodiment, the electro-optical elements E 1  to E 3  have different gradation change rates according to the areas of the electro-optical elements E 1  to E 3 . A specific method for selecting a gradation change rate for each electro-optical element Ek can be modified in various ways as below. The following description will be given while focusing on the electro-optical elements E 1  and E 2 . A similar structure can be used to adjust the gradation change rate of the electro-optical element E 3  to a predetermined value. In the following description, the electro-optical elements E 1  to E 3  are referred to simply as “electro-optical elements E” unless they are separately identified. In the figures described in conjunction with the following methods, elements having the same or similar advantages and functions are represented by the same reference numerals. 
     First Method 
       FIG. 6  is a cross-sectional view of an element array section A according to a first method in a second embodiment of the invention. As shown in  FIG. 6 , lines  31  electrically connected with the drains of the driving transistors Qdr are defined on a surface of an optically transparent substrate  30 . The surface of the substrate  30  having elements such as the driving transistors Qdr and the lines  31  defined thereon is overlaid by an insulation layer  32 . On a surface of the insulation layer  32 , first electrodes  33  serving as anodes of the electro-optical elements E are arranged apart from each other to define the electro-optical elements E. 
     The first electrodes  33  are formed of an optically-transparent conductive material such as indium tin oxide (ITO), and are electrically connected to the lines  31  (and then the driving transistors Qdr) via contact holes in the insulation layer  32 . On the surface of the insulation layer  32  having the first electrodes  33  defined thereon, a partition layer  34  is defined. The partition layer  34  is an insulating film having openings  341  in regions where the partition layer  34  and the first electrodes  33  overlap. 
     In recesses surrounded by the inner periphery of the openings  341  in the partition layer  34 , of which the bottom surfaces correspond to surfaces of the first electrodes  33 , light-emitting function layers  35  are defined. The light-emitting function layers  35  include a light-emitting layer formed of an organic EL material. Each of the light-emitting function layers  35  may be formed of a laminate of various function layers (such as a hole injection layer, a hole transporting layer, an electron injection layer, an electron transporting layer, a hole block layer, and an electron block layer) for facilitating or efficiently performing light emission of the light-emitting layer. A second electrode  36  serving as cathodes of the electro-optical elements E is defined on a surface of the partition layer  34  and the light-emitting function layers  35 . The second electrode  36  is a continuous conductive layer defined over the plurality of electro-optical elements E. The second electrode  36  has light reflectivity. Therefore, as indicated by arrows shown in  FIG. 6 , a light beam output from the light-emitting function layers  35  to the substrate  30  and a light beam reflected from a surface of the second electrode  36  are transmitted through the insulation layer  32  and the substrate  30 , and are emitted outside the electro-optical device  100 . 
     In the first embodiment, the gradation change rates of the electro-optical elements E 1  to E 3  are different from each other according to the areas of the light-emitting function layers  35  (that is, the areas of the regions where a current flows between the first electrodes  33  and the second electrode  36 ). In the first method of the second embodiment, on the other hand, the areas of the light-emitting function layers  35  of the electro-optical elements E are substantially equal to each other, whereas the thickness of the light-emitting function layers  35  (in other words, the distance between the first electrodes  33  and the second electrode  36 ) is adjusted for each of the electro-optical elements E to obtain different gradation change rates of the electro-optical elements E. As shown in  FIG. 6 , a thickness Ta 1  of the light-emitting function layer  35  of the electro-optical element E 1  is larger than a thickness Ta 2  of the light-emitting function layer  35  of the electro-optical element E 2 . The smaller the thickness of the light-emitting function layer  35 , the larger the amount of light emission when a predetermined voltage is applied between the first electrodes  33  and the second electrode  36 . In the structure shown in  FIG. 6 , therefore, as in the first embodiment, the gradation change rate of the electro-optical element E 1  is lower than that of the electro-optical element E 2 . 
     Second Method 
       FIG. 7  is a cross-sectional view of an element array section A according to a second method in the second embodiment. As shown in  FIG. 7 , elements forming the electro-optical elements E and the lamination order of the elements are similar to those shown in  FIG. 6 . In the second method, however, the first electrodes  33  of the electro-optical elements E have different thicknesses. For example, as shown in  FIG. 7 , a thickness Tb 1  of the first electrode  33  of the electro-optical element E 1  is larger than a thickness Tb 2  of the first electrode  33  of the electro-optical element E 2 . 
     In the structure shown in  FIG. 7 , the insulation layer  32  is formed of a material having a different refractive index from the substrate  30 . Thus, the interface between the insulation layer  32  and the substrate  30  serves as a transflective surface that allows a portion of light incident to the interface to pass to the substrate  30  and another portion to be reflected in the direction opposite to the substrate  30 . Therefore, a resonator structure in which light beams output from the light-emitting function layers  35  resonate between the transflective surface and the surface of the second electrode  36  is obtained. That is, the light beams output from the light-emitting function layers  35  reciprocate between the transflective surface and the surface of the second electrode  36 , and a component in a frequency band (or resonant wavelength) corresponding to the distance between both interfaces is selectively transmitted through the substrate  30  and is emitted. 
     In the second method of the second embodiment, the thickness of the first electrodes  33  forming the resonator structure (i.e., the optical path length of light output from the light-emitting function layers  35  until it is transmitted through the transflective surface) is different from one electro-optical element E to another. Hence, the spectral characteristic of light output from the light-emitting function layers  35  and transmitted through the substrate  30  when a predetermined voltage is applied between the first electrodes  33  and the second electrode  36  is different between the electro-optical elements E 1  and E 2 . For example, as shown in  FIG. 8 , the light output from the electro-optical element E 1  exhibits a characteristic curve F B1  in which the intensity is distributed uniformly over a wide range, while the light output from the electro-optical element E 2  exhibits a characteristic curve F B2  in which the intensity is high within a narrow range including the resonant wavelength. With this structure, as in the first embodiment, the gradation change rate of the electro-optical element E 1  can be set lower than that of the electro-optical element E 2 . 
     Third Method 
       FIG. 9  is a cross-sectional view of an element array section A according to a third method in the second embodiment. As shown in  FIG. 9 , in the third method, the insulation layer  32  has different thicknesses for the electro-optical elements E. For example, as shown in  FIG. 9 , a thickness Tc 1  of a portion of the insulation layer  32  corresponding to the electro-optical element E 1  is larger than a thickness Tc 2  of a portion of the insulation layer  32  corresponding to the electro-optical element E 2 . Also in the structure shown in  FIG. 9 , the optical path length of light output from the light-emitting function layers  35  until it is transmitted through the transflective surface is different from one electro-optical element E to another. Hence, the spectral characteristic of the light transmitted through the substrate  30  is different between the electro-optical elements E 1  and E 2  in the manner shown in  FIG. 8 . The gradation change rate of the electro-optical element E 1  can therefore be set lower than that of the electro-optical element E 2 . 
     Fourth Method 
       FIG. 10  is a cross-sectional view of an element array section A according to a fourth method in the second embodiment. As shown in  FIG. 10 , according to the fourth method, the electro-optical device  100  further includes a neutral density (ND) filter  37  bonded to the surface of the substrate  30  in addition to the elements shown in  FIG. 6 . The insulation layer  32  is adhered to a surface of the ND filter  37  using an optical transparent adhesive  38 . The light beams output from the electro-optical elements E are transmitted through the ND filter  37  and the substrate  30 , and are emitted to the outside. 
     Portions of the ND filter  37  that overlap the electro-optical elements E 1  to E 3  have different transmittances. For example, in the ND filter  37 , as shown in  FIG. 10 , the transmittance of a portion  371  overlapping the electro-optical element E 1  is lower than the transmittance of a portion  372  overlapping the electro-optical element E 2 . Therefore, as in the first embodiment, the gradation change rate of the electro-optical element E 1  is lower than that of the electro-optical element E 2 . 
     According to the second embodiment, therefore, the gradation change rates of the electro-optical elements E can be individually set with the areas of the electro-optical elements E being equal to each other. Therefore, the space required to install the unit circuits P can be reduced compared with the first embodiment in which the area of the electro-optical element E 3  is relatively large. Therefore, advantageously, a high-definition image can be easily achieved. 
     The structure according to the first to third methods of the second embodiment in which the elements on the substrate  30  have different thicknesses for the electro-optical elements E is manufactured by a method such as by using a different number of laminated layers of the elements depending on each of the electro-optical elements E or by forming the elements so as to have predetermined thicknesses using processes different for the electro-optical elements E. For example, in  FIG. 7 , the first electrode  33  of the electro-optical element E 1  is manufactured by laminating a larger number of conductive layers than the first electrode  33  of the electro-optical element E 2 . In the manufacturing of the element array section A according to the first to third methods, therefore, the step of forming an element defining the gradation change rate is changed depending on the electro-optical elements E. In the first embodiment, however, since the gradation change rates of the electro-optical elements E are individually determined according to the area of the electro-optical elements E, a method is commonly used to manufacture the elements of each of the electro-optical elements E, thus achieving the advantage of a simplified manufacturing process of the element array section A. 
     Third Embodiment 
     A third, embodiment of the invention will be described. In the first embodiment, the electro-optical elements E 1  to E 3  have different gradation change rates according to the characteristics of the electro-optical elements E 1  to E 3 . In the third embodiment, the gradation change rates are differently set according to the time length during which each of the electro-optical elements E actually emits light. In the third embodiment, elements having the same or similar advantages and functions to those of the first embodiment are represented by the same reference numerals, and a detailed description thereof is appropriately omitted. 
       FIG. 11  is a circuit diagram showing the structure of the unit circuit P at the j-th column in the i-th row. As shown in  FIG. 11 , an element array section A of the third embodiment includes a scanning line  120  and three control lines  121  to  123  extending in parallel to the scanning line  120 . The scanning line driving circuit  22  outputs a scanning signal G[i] to the scanning line  120 , and outputs control signals G 1 [i], G 2 [i], and G 3 [i] to the control lines  121 ,  122 , and  123 , respectively. Specific waveforms of those signals are described below. 
     As shown in  FIG. 11 , each of the unit circuits P includes two element sections U 1  and U 2 . The element section Uk (where k is 1 or 2 in the third embodiment) includes an electro-optical element Ek. The areas of the electro-optical elements E 1  and E 2  are equal to each other, and the thicknesses of the layers of the electro-optical elements E 1  and E 2  are equal to each other. In the third embodiment, the range of the gradation value D from the minimum value to the maximum value is divided into a low-gradation range R L  and a high-gradation range R H . The electro-optical element E 1  is driven when the gradation value D is within the range R L , and the electro-optical element E 2  is driven when the gradation value D is within the range R H . 
     The element section Uk includes an n-channel transistor (hereinafter referred to as a “light-emission control transistor”) Qel between the drain of the driving transistor Qdr and the anode of the electro-optical element Ek for controlling the electrical connection therebetween. The control signal G 2 [i] is supplied to a gate of the light-emission control transistor Qel of the element section U 1  from the control line  122 . The control signal G 3 [i] is supplied to the gate of the light-emission control transistor Qel of the element section U 2  from the control line  123 . 
     The element section Uk further includes an n-channel transistor Qsw 1  between the gate and drain of the driving transistor Qdr for controlling the electrical connection therebetween. The control signal G 1 [i] is commonly supplied to the gates of the transistors Qsw 1  in the element sections U 1  and U 2  from the control line  121 . 
     The element section Uk further includes a capacitor element C 1  (with a capacitance value c 1 ) having electrodes Ec 1  and Ec 2  facing each other with a dielectric member therebetween. The electrode Ec 1  is connected with the gate of the driving transistor Qdr. The selection transistor Qsl of the element section Uk is provided between the electrode Ec 2  and the data line LDk[j] to control the electrical connection therebetween. As in the first embodiment, a capacitor element C (with a capacitance value c) is provided between the gate and source (on the side of the power supply line  17 ) of the driving transistor Qdr. 
       FIG. 12  is a timing chart showing specific waveforms of the respective signals. As shown in  FIG. 12 , an initial setting period P 0  and a compensation period P CP  are designated before the beginning of each horizontal scanning period H. The control signal G 1 [i] is set to a nigh level in the initial setting period P 0  and the compensation period P CP  immediately before the horizontal scanning period H during which the scanning signal G[i] is at a high level, and is maintained at a low level in the remaining period. The control signal G 2 [i] is set to a high level in the initial setting period P 0  immediately before the horizontal scanning period H and in a light-emission period P EL1  after the lapse of the horizontal scanning period H, and is maintained at a low level in the remaining period. The control signal G 3 [i] is set to a high level in the initial setting period P 0  immediately before the horizontal scanning period H and in a light-emission period P EL2  after the lapse of the horizontal scanning period H, and is maintained at a low level in the remaining period. As shown in  FIG. 12 , the light-emission period P EL2  is longer than the light-emission period P EL1 . 
     The operation of one of the unit circuits P will be described. First, in the initial setting period P 0 , the control signals G 2 [i] and G 3 [i] change to the high level to thereby turn on the light-emission control transistors Qel of the element sections U 1  and U 2 . Since the control signal G 1 [i] also changes to the high level, the transistors Qsw 1  of the element sections U 1  and U 2  are turned on. Thereby, the driving transistors Qdr of the element sections U 1  and U 2  are diode-connected, and the gates of the driving transistors Qdr are initialized to voltages corresponding to the characteristics of the electro-optical elements E 1  and E 2 . 
     When the compensation period P CP  begins, the control signals G 2 [i] and G 3 [i] change to the low level to thereby turn off the light-emission control transistors Qel of the element sections U 1  and U 2 . Therefore, by the time when the end of the compensation period P CP  has arrived, the voltage at the gate of the driving transistor Qdr of each of the element sections U 1  and U 2  reaches to a difference value (V EL -V th ) between the power supply potential V EL  of the power supply line  17  and a threshold voltage V th  of the driving transistor Qdr. 
     When the scanning signal G[i] changes to the high level after the lapse of the compensation period P CP , the selection transistors Qsl are turned on, and the voltage at the electrodes Ec 2  changes from the previous voltage value, i.e., V 0 , to the voltage value V d  of the data signal Sk[j]. The voltage value V d  is set to a voltage value lower than the voltage value V 0  and corresponding to the gradation value D. Further, the control signal G 1 [i] changes to the low level to thereby release the diode connection of the driving transistors Qdr. Since the impedance at the gates of the driving transistors Qdr is sufficiently high, if the electrodes Ec 2  decrease from the voltage value V 0  to the voltage value V d  by a change amount ΔV(=V 0 −V d ), the voltage at the electrodes Ec 1  changes (decreases) from the voltage value (V EL -V th ), which is designated during the compensation period P CP , by a value of ΔV·c 1 /(c 1 +c). That is, the gates of the driving transistors Qdr are set to a voltage V g  given by Eq. (1) as follows:
 
 V   g   =V   EL   −V   th   −k·ΔV   Eq. (1)
 
where k=c 1 /(c 1 +c)
 
     In the light-emission period P EL1  during which the control signal G 2 [i] is maintained at the high level, the light-emission control transistor Qel of the element section U 1  is turned on. In the light-emission period P EL2 , the light-emission control transistor Qel of the element section U 2  is turned on. In the light-emission period P ELk , therefore, a drive current I EL  corresponding to the voltage at the gate of the driving transistor Qdr of the element section Uk is supplied to the electro-optical element Ek. 
     In the horizontal scanning period H during which the scanning signal G[i] is at the high level, the signal generating circuit  25  at the j-th column sets one of the data signals S 1 [j] and S 2 [j] to the voltage value V d  corresponding to the gradation value D, and sets the other to the voltage value V 0 . For example, when the data determining unit  241  determines that the gradation value D is within the range R L , as shown in  FIG. 12 , the signal generating circuit  25  sets the data signal S 1 [j] to the voltage value V d  (which has potential lower than the voltage value V 0 ) corresponding to the gradation value D, and sets the data signal S 2 [j] to the voltage value V d  (equal to the voltage value V 0 ) for turning off the electro-optical element E 2 . When the gradation value D is within the range R H , the signal generating circuit  25  generates the data signal S 2 [j] having a voltage value V d  corresponding to the gradation value D and the data signal S 1 [j] having a voltage value V d  (equal to the voltage value V 0 ) for turning off the electro-optical element E 1 . 
     When the gradation value D is within the range R L , therefore, the electro-optical element E 1  emits light at a brightness corresponding to the gradation value D while the electro-optical element E 2  is turned off during a period from the beginning to the end of the light-emission period P EL1 . When the gradation value D is within the range R H , the electro-optical element E 2  emits light at a brightness corresponding to the gradation value D while the electro-optical element E 1  is turned off during a period from the beginning to the end of the light-emission period P EL2 . 
     The gradation level of the electro-optical element Ek (which is a time integral value of the brightness (i.e., the amount of light emission)) is determined according to the brightness in the light-emission period P ELk  and the time length of the light-emission period P ELk . Since the light-emission period P EL1  is set shorter than the light-emission period P EL2 , the gradation change rate of the electro-optical element E 1  is lower than the gradation change rate of the electro-optical element E 2 . Therefore, the third embodiment can also achieve similar advantages to those of the first embodiment. 
     In a case where the driving transistors Qdr operate in a saturation region, the drive current I EL  supplied to the electro-optical element Ek in the light-emission period P ELk  is represented by Eq. (2) as follows: 
                           I   EL     =       (     β   /   2     )     ⁢       (       V   gs     -     V   th       )     2                   =       (     β   /   2     )     ⁢       (       V   EL     -     V   g     -     V   th       )     2                     Eq   .           ⁢     (   2   )                 
where β denotes the gain coefficient of the driving transistor Qdr, and V gs  denotes the voltage between the gate and source of the driving transistor Qdr.
 
     By substituting Eq. (1) into Eq. (2), Eq. (2) is modified as follows:
 
 I   EL =(β/2)( k·ΔV ) 2  
 
     That is, the drive current I EL  supplied to the electro-optical element Ek does not depend on the threshold voltage V th  of the driving transistor Qdr. According to the third embodiment, therefore, unevenness in the gradation of the electro-optical element Ek caused by variations in the threshed voltages V th  of the driving transistors Qdr (deviation from a prescribed value or a difference from the other driving transistors Qdr) can be suppressed. 
     Fourth Embodiment 
     A fourth embodiment of the invention will be described. 
     In the first embodiment, a voltage programming method in which the gradation level of the electro-optical element Ek is determine according to the voltage value V d  of the data signal Sk[j] is employed. In the fourth embodiment, a current programming method in which the gradation level of the electro-optical element Ek is determined according to a current value Id of the data signal Sk[j] is employed in combination with the voltage programming method. In the fourth embodiment, elements having the same or similar advantages and functions to those of the first embodiment are represented by the same reference numerals, and a detailed description thereof is appropriately omitted. 
       FIG. 13  is a circuit diagram showing the structure of the unit circuit P at the j-th column in the i-th row. As shown in  FIG. 13 , the unit circuit P includes two element sections U 1  and U 2 . The element section Uk (where k is 1 or 2 in the fourth embodiment) includes an electro-optical element Ek. As in the first embodiment, the gradation change rate of the electro-optical element E 1  is lower than that of the electro-optical element E 2  (for example, the area of the electro-optical element E 2  is larger than that of the electro-optical element E 1 ). In the fourth embodiment, as in the third embodiment, the electro-optical element E 1  is driven when the gradation value D is within the low-gradation range R L , and the electro-optical element E 2  is driven when the gradation value D is within the high-gradation range R H . 
     As shown in  FIG. 13 , the element array section A of the fourth embodiment includes a scanning line  120  and a control line  121  extending in parallel to the scanning line  120 . The scanning line driving circuit  22  outputs a control signal G 1 [i] to the control line  121 . The element section Uk includes a light-emission control transistor Qel between the drain of the driving transistor Qdr and the anode of the electro-optical element Ek. A control signal G 1 [i] is supplied from the control line  121  to the gates of the light-emission control transistors Qel in the element sections U 1  and U 2 . 
     As in the first embodiment, the element section U 1  includes a selection transistor Qsl between the gate of the driving transistor Qdr and the data line LD 1 [j]. The element section U 2 , on the other hand, includes a selection transistor Qsl between the drain of the driving transistor Qdr and the data line LD 2 [j]. The element section U 2  further includes a transistor Qsw 2  between the gate and drain of the driving transistor Qdr for controlling the electrical connection therebetween. A gate of the transistor Qsw 2  is connected with the scanning line  120 . 
     As shown in  FIG. 13 , each of the signal generating circuits  25  includes a voltage generating circuit  251 , a current generating circuit  252 , and switches SW 1  and SW 2 . The switch SW 1  of the signal generating circuit  25  at the j-th column is provided between the data line LD 2 [j] and the voltage generating circuit  251 , and the switch SW 2  is provided between the data line LD 2 [j] and the current generating circuit  252 . The voltage generating circuit  251  is also connected with the data line LD 1 [j]. 
       FIGS. 14A and 14B  are timing charts showing the operation of the fourth embodiment.  FIG. 14A  shows the operation when a gradation value D within the low-gradation range R L  is designated for the unit circuit P at the j-th column in the i-th row, and  FIG. 14B  shows the operation when a gradation value D within the high-gradation range R H  is designated for the same unit circuit P. As shown in  FIGS. 14A and 14B , the control signal G 1 [i] is set to a high level after the lapse of a horizontal scanning period H during which the scanning signal G[i] is at a high level. 
     When the data determining unit  241  determines that the gradation value D is within the range R L , as shown in  FIG. 14A , the signal generating circuit  25  turns on the switch SW 1  and turns off the switch SW 2  in the horizontal scanning period H during which the scanning signal G[i] is at the nigh level. When the gradation value D is within the range R H , as shown in  FIG. 14B , the signal generating circuit  25  turns off the switch SW 1  and turns on the switch SW 2  in the horizontal scanning period H. 
     When the gradation value D is within the range R L , the voltage generating circuit  251  outputs a data signal S 1 [j] having a voltage value V d  corresponding to the gradation value D, and outputs the power supply voltage V EL  to the switch SW 1 . When the gradation value D is within the range R H , the voltage generating circuit  251  outputs the power supply voltage V EL  to the data line LD 1 [j]. The current generating circuit  252  outputs a current having the current value Id corresponding to the gradation value D to the switch SW 2  when the gradation value D is within the range R H , and stops outputting the current when the gradation value D is within the range R L . 
     When the gradation value D is within the range R L , therefore, as shown in  FIG. 14A , the data signal S 1 [j] having the voltage value V d  is output to the data line LD 1 [j], and the data signal S 2 [j] having the voltage value V EL  is output to the data line LD 2 [j] via the switch SW 1 . When the gradation value D is within the range R H , as shown in  FIG. 14B , the data signal S 1 [j] having the voltage value V EL  is output to the data line LD 1 [j], and the data signal S 2 [j] having the current value Id is output to the data line LD 2 [j] via the switch SW 2 . 
     As in the first embodiment, the data signal S 1 [j] is supplied to the gate of the driving transistor Qdr of the element section U 1  when the selection transistor Qsl is turned on. Therefore, as shown in  FIG. 14A , when the data signal S 1 [j] has the voltage value V d , the electro-optical element E 1  is controlled to the gradation level corresponding to the voltage value V d  (i.e., the gradation value D) in the period during which the control signal G 1 [i] is at the high level, and, as shown in  FIG. 14B , when the data signal S 1 [j] has the voltage value V EL , the electro-optical element E 1  is turned off. 
     In the horizontal scanning period H during which the scanning signal G[i] is turned on, the selection transistor Qsl and the transistor Qsw 2  of the element section U 2  are turned on. In the case shown in  FIG. 14A , since the gate of the driving transistor Qdr is set to the voltage value V EL  of the data signal S 2 [j] in the horizontal scanning period H, the electro-optical element E 2  is turned off in the period during which the control signal G 1 [j] is at the nigh level. In the case shown in  FIG. 14B , in the horizontal scanning period H, as indicated by a dotted arrow shown in  FIG. 13 , the data signal S 2 [j] having the current value Id flows from the power supply line  17  via the driving transistor Qdr and the selection transistor Qsl, and the voltage corresponding to the current value Id is held in the capacitor element C. The electro-optical element E 2  is therefore controlled to the gradation level corresponding to the current value Id in the period during which the control signal G 1 [j] is at the high level. 
     According to the fourth embodiment, therefore, the electro-optical elements Ek having different gradation change rates are selectively driven according to the range R of the gradation values D, and similar advantages to those of the first embodiment, can also be achieved. Furthermore, in the fourth embodiment, when the gradation value D is high, the gradation level of the electro-optical element E 2  is determined according to the current value Id of the data signal S 2 [j] (that is, the current programming method), whereas when the gradation value D is low, the gradation level of the electro-optical element E 1  is determined according to the voltage value V d  of the data signal S 1 [j] (that is, the voltage programming method). Therefore, even if the gradation value D is low, advantageously, the electro-optical element E 1  can be reliably controlled to the gradation level corresponding to the gradation value D. The details of this advantage are described below. 
     The data line LDk[j] involves resistance and capacitance. When a low gradation level is specified (that is, when the current value Id is low), the current programming method has a problem in that a considerable amount of time is required to set the data signal Sk[j] to the current value Id corresponding to the gradation value D. In other words, if the time for supplying the data signal Sk[j] is insufficient, the gate of the driving transistor Qdr is not correctly set to the voltage corresponding to the gradation value D. In the fourth embodiment, in contrast, when the gradation value D is within the low-gradation range R L , the voltage at the gate of the driving transistor Qdr is set using the voltage programming method. With this structure, the problem of insufficient writing of the voltage at the gate of the driving transistor Qdr can be overcome. Therefore, even if the data line LDk[j] has a high time constant, the electro-optical element E 1  can be controlled to a predetermined gradation level with high accuracy. 
     Modifications 
     A variety of modifications can be made to the foregoing embodiments. Followings are specific modifications. The following modifications may be combined as necessary. 
     First Modification 
     In the first and second embodiments, the gradation change rates of the electro-optical elements Ek are different according to the conditions of the electro-optical elements Ek (such as the area of the electro-optical elements Ek and the thickness of layers). A variety of modifications may be made to a structure for designating different gradation change rates for the element sections U. More specifically, different gradation change rates may be designated for the element sections U by providing the same configuration for the electro-optical elements E 1  to E 3  included in each of the unit circuits P and selecting the characteristics of the driving transistors Qdr (the relationship between the voltage at the gates and the drive current I EL ) for each of the element sections U. 
     For example, in the structure of the first embodiment (see  FIG. 2 ), if the same voltage is applied to the gates of the driving transistors Qdr of the element sections U 1  to U 3 , the characteristics (such as the channel width and the channel length) of the driving transistors Qdr in the element sections U 1  to U 3  are determined so that the drive current I EL  of the electro-optical element E 1  is smaller than the drive current I EL  of the electro-optical element E 2  and the drive current I EL  of the electro-optical element E 2  is smaller than the drive current I EL  of the electro-optical element E 3 . With this structure, similar advantages to those of the first and second embodiments can also be achieved. 
     In the embodiments of the invention, therefore, it is sufficient to provide a structure in which the gradation level (i.e., the gradation change rate) of the electro-optical element Ek differs from one element section U to another when data signals Sk[j] having the same level (such as the voltage value V d  or the current value Id) are supplied to the element sections Uk, and there is no limit to a specific structure for achieving this difference in gradation level. 
     Second Modification 
     In the foregoing modifications, separate data signals Sk[j] are supplied to the element sections Uk. However, as shown in  FIG. 15 , one data line LD[j] (i.e., one data signal S[j]) can be shared between a plurality of element sections Uk in one unit circuit P. The unit circuit P shown in  FIG. 15  includes element sections U 1  and U 2  and a selection transistor Qsl. The element section U 1  includes a p-channel driving transistor Qdr_p that controls a drive current I EL  supplied to an electro-optical element E 1  according to the voltage at a gate of the driving transistor Qdr_p. The element section U 2  includes an n-channel driving transistor Qdr_n that controls a drive current I EL  supplied to an electro-optical element E 2  according to the voltage at a gate of the driving transistor Qdr_n. The selection transistor Qsl is provided between the gates of the driving transistors Qdr_p and Qdr_n and the data line LD[j]. 
     When the gradation value D is within the range R L , the data signal S[j] supplied to the gates of the driving transistors Qdr_p and Qdr_n in the horizontal scanning period H during which the selection transistor Qsl is turned on is set to the voltage value V d  corresponding to the gradation value D within the range that allows the driving transistor Qdr_p to be turned on. Therefore, while the drive current I EL  corresponding to the gradation value D is supplied to the electro-optical element E 1  from the driving transistor Qdr_p, the driving transistor Qdr_n is turned off, thereby turning off the electro-optical element E 2 . When the gradation value D is within the range R H , the data signal S[j] set to the voltage value V d  corresponding to the gradation value D within the range that allows the driving transistor Qdr_n to be turned on is supplied. Therefore, the electro-optical element E 2  is controlled to the gradation level corresponding to the gradation value D, and the electro-optical element E 1  is turned off. With the structure shown in  FIG. 15 , similar advantages to those of the foregoing embodiments can also be achieved by setting different gradation change rates for the element sections U 1  and U 2 . 
     APPLICATION EXAMPLES 
     An electronic apparatus including an electro-optical device according to the invention will be described.  FIGS. 16 to 18  show electronic apparatuses including the electro-optical device  100  according to any of the embodiments and modifications described above as a display device. 
       FIG. 16  is a perspective view showing the structure of a mobile personal computer  2000  using the electro-optical device  100 . The personal computer  2000  includes the electro-optical device  100  that displays various images, and a main body  2010  having a power supply switch  2001  and a keyboard  2002 . Since OLED elements are used as the electro-optical elements E, the electro-optical device  100  can display an easy-to-read screen having a wide angle of view. 
       FIG. 17  is a perspective view showing the structure of a mobile phone  3000  using the electro-optical device  100 . The mobile phone  3000  includes a plurality of operation buttons  3001  and scroll buttons  3002 , and the electro-optical device  100  that displays various images. By operating the scroll buttons  3002 , the screen displayed on the electro-optical device  100  can be scrolled. 
       FIG. 18  is a perspective view showing the structure of a personal digital assistant (PDA)  4000  using the electro-optical device  100 . The PDA  4000  includes a plurality of operation buttons  4001 , a power supply switch  4002 , and the electro-optical device  100  that displays various images. By operating the power supply switch  4002 , various types of information such as an address book or a schedule book are displayed on the electro-optical device  100 . 
     Electronic apparatuses using an electro-optical device according to the invention include, not only the apparatuses shown in  FIGS. 16 to 18 , but also various electronic apparatuses such as digital still cameras, television sets, video camcorders, car navigation systems, pagers, electronic notebooks, electronic paper, electronic calculators, word processors, workstations, videophones, point-of-sale (POS) terminals, printers, scanners, copying machines, video players, and apparatuses equipped with touch panels. The use of the electro-optical device according to the invention is not limited to the display of images. For example, image forming apparatuses such as optical recording printers or electronic copying machines include an optical head (or recording head) that exposes a photosensitive member to light according to an image to be formed onto a recording material such as a sheet of paper, and an electro-optical device of the invention can be used as the optical head.