US20070001954A1 - Display device and driving method of display device - Google Patents

Display device and driving method of display device Download PDF

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Publication number
US20070001954A1
US20070001954A1 US11/427,427 US42742706A US2007001954A1 US 20070001954 A1 US20070001954 A1 US 20070001954A1 US 42742706 A US42742706 A US 42742706A US 2007001954 A1 US2007001954 A1 US 2007001954A1
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sub
pixel
electrode
light
transistor
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US11/427,427
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Hideaki Shishido
Hajime Kimura
Shunpei Yamazaki
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Semiconductor Energy Laboratory Co Ltd
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Semiconductor Energy Laboratory Co Ltd
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Priority to JP2005-194699 priority
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Assigned to SEMICONDUCTOR ENERGY LABORATORY CO., LTD. reassignment SEMICONDUCTOR ENERGY LABORATORY CO., LTD. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: KIMURA, HAJIME, SHISHIDO, HIDEAKI, YAMAZAKI, SHUNPEI
Publication of US20070001954A1 publication Critical patent/US20070001954A1/en
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    • G09G3/2007Display of intermediate tones
    • G09G3/2018Display of intermediate tones by time modulation using two or more time intervals
    • G09G3/2022Display of intermediate tones by time modulation using two or more time intervals using sub-frames
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    • G09G3/2007Display of intermediate tones
    • G09G3/2018Display of intermediate tones by time modulation using two or more time intervals
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    • G09G3/2007Display of intermediate tones
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    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/28Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part
    • H01L27/32Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part with components specially adapted for light emission, e.g. flat-panel displays using organic light-emitting diodes [OLED]
    • H01L27/3206Multi-colour light emission
    • H01L27/3211Multi-colour light emission using RGB sub-pixels
    • HELECTRICITY
    • H01BASIC ELECTRIC ELEMENTS
    • H01LSEMICONDUCTOR DEVICES; ELECTRIC SOLID STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H01L27/00Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/28Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part
    • H01L27/32Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including components using organic materials as the active part, or using a combination of organic materials with other materials as the active part with components specially adapted for light emission, e.g. flat-panel displays using organic light-emitting diodes [OLED]
    • H01L27/3206Multi-colour light emission
    • H01L27/3211Multi-colour light emission using RGB sub-pixels
    • H01L27/3213Multi-colour light emission using RGB sub-pixels using more than three sub-pixels, e.g. RGBW

Abstract

It is an object of the present invention to reduce a cause of pseudo contour when display is performed with a time gray scale method. According to the present invention, one pixel is divided into m sub-pixels so that an area ratio of each sub-pixel becomes 20:21:22: . . . :2m−3:2m−2:2m−1 (m is an integer number of m≧2), and one frame is divided into n sub-frames so that a ratio of a lighting period in each sub-frame becomes 20:2m:22m: . . . :2(n−3)m:2(n−2)m:2(n−1)m (n is an integer number of n≧2). Then, a gray scale is expressed by controlling a manner of lighting in each of the m sub-pixels in each of the n sub-frames.

Description

    BACKGROUND OF THE INVENTION
  • 1. Field of the Invention
  • The present invention relates to a display device and a driving method thereof. In particular, the present invention relates to a display device to which an area gray scale method is applied and a driving method thereof.
  • 2. Description of the Related Art
  • In recent years, a so-called self-luminous type display device having a pixel that is formed of a light-emitting element such as a light-emitting diode (LED) has been attracting attention. As a light-emitting element used for such a self-luminous type display device, an organic light-emitting diode (OLED) (also called an organic EL element, an electro luminescence: EL element, and the like) has been drawing attention and used for an EL display (for example, an organic EL display or the like). Since a light-emitting element such as an OLED is a self-luminous type, it has advantages such as higher visibility of pixels than that of a liquid crystal display, and fast response without requiring a backlight. The luminance of a light-emitting element is controlled by a current value flowing through the light-emitting element.
  • As a driving method for controlling a light-emitting gray scale of such a display device, there are a digital gray scale method and an analog gray scale method. In accordance with the digital gray scale method, a light-emitting element is turned on/off in a digital manner to express a gray scale. Meanwhile, the analog gray scale method includes a method for controlling the light-emitting intensity of a light-emitting element in an analog manner and a method for controlling the light-emitting time of a light-emitting element in an analog manner.
  • In the case of the digital gray scale method, there are only two states: a light-emitting state and a non-light-emitting state. Therefore, only two gray scales can be expressed if nothing is done. Accordingly, another method is used in combination to achieve multiple gray scales. An area gray scale method and a time gray scale method are often used as a method for multiple gray scales.
  • The area gray scale method is a method for expressing a gray scale by controlling an area of a lighting portion. In other words, gray scale display is performed by dividing one pixel into a plurality of sub-pixels and controlling the number or area of lighting sub-pixels (for example, see Reference 1: Japanese Patent Application Laid-Open No. H11-73158 and Reference 2: Japanese Patent Application Laid-Open No. 2001-125526). The number of the sub-pixels cannot be increased; therefore, it is difficult to realize high definition and multiple gray scales. This can be given as a disadvantage of the area gray scale method.
  • The time gray scale method is a method for expressing a gray scale by controlling the length of a light-emitting period or the frequency of light emission. In other words, one frame is divided into a plurality of sub-frames, each of which is weighted with respect to the number of light emissions and a light-emitting period, and then the total weight (the sum of the frequency of light emission and the sum of the light-emitting time) is differentiated for each gray scale, thereby expressing a gray scale. It is known that display failure such as a pseudo contour (or a false contour) may occur when such a time gray scale method is used and a measures against the failure is considered (for example, see Reference 3: Patent Publication No. 2903984, Reference 4: Patent Publication No. 3075335, Reference 5: Patent Publication No. 2639311, Reference 6: Patent Publication No. 3322809, Reference 7: Japanese Patent Application Laid-Open No. H10-307561, Reference 8: Patent Publication No. 3585369, and Reference 9: Patent Publication No. 3489884).
  • Although various methods for reducing pseudo contour have been conventionally suggested, a sufficient effect for reducing pseudo contour has not been obtained yet and further improvement has been urged.
  • For example, it is found that a pseudo contour cannot always be prevented by focusing attention on certain two pixels in accordance with a halftone display method in Reference 4. As a specific example, it is assumed that a gray scale level of 127 is expressed in a pixel A and a gray scale level of 128 is expressed in a pixel B adjacent to the pixel A. A light-emitting state and a non-light-emitting state in each sub-frame of this case are shown in FIGS. 64A to 64B. For example, FIG. 64A shows a case where one sees only the pixel A or the pixel B without turning one's eyes away. A pseudo contour is not caused in this case. This is because the sum is taken with respect to the luminance of the place where one's eyes pass; therefore, one's eyes sense the luminance. Thus, eyes sense the gray scale level of the pixel A to be 127 (=1+2+4+8+16+32+32+32), and eyes sense the gray scale level of the pixel B to be 128 (=32+32+32+32). In other words, eyes sense an accurate gray scale.
  • On the other hand, FIG. 64B shows a case where eyes move from the pixel A to the pixel B or from the pixel B to the pixel A. In this case, eyes sometimes sense the gray scale level to be 96 (=32+32+32), and sometimes sense the gray scale level to be 159 (=1+2+4+8+16+32+32+32+32) in accordance with eyes' movement. Although the eyes are expected to sense the gray scale levels to be 127 and 128, they sense the gray scale levels to be 96 or 159. Consequently, a pseudo contour is caused.
  • FIGS. 64A to 64B show a case of 8-bit gray scales (256 gray scales). Next, FIG. 65 shows a case of 4-bit gray scales (16 gray scales). In this case also, eyes sometimes sense the gray scale level to be 4 (=4), and sometimes sense the gray scale level to be 11 (=1+2+4+4) in accordance with eyes' movement. Although the eyes are expected to sense gray scale levels to be 7 and 8, they sense the gray scale levels to be 4 or 11. Consequently, a pseudo contour is caused.
  • SUMMARY OF THE INVENTION
  • In view of such problems, it is an object of the present invention to provide a display device composed of few sub-frames and that can reduce a pseudo contour, where multiple gray scales are possible as well, and a driving method using the display device.
  • According to one aspect of the present invention, a method for driving a display device, where a pixel including m (m is an integer number of m≧2) sub-pixels provided with a light-emitting element is arranged in a plurality of pieces, includes the steps of having an area ratio of the m sub-pixels 20:21:22: . . . :2m−3:2m−2:2m−1; dividing one frame into n (n is an integer number of n≧2) sub-frames in each of the m sub-pixels; and having a ratio in a length of a lighting period of the n sub-frames 20:2m:22m: . . . :2(n−3)m:2(n−2)m:2(n−1)m, where a gray scale of the pixel is expressed by controlling the sum of a lighting period of the sub-frame when the m sub-pixels are in a lighting state, in each of the n sub-frames.
  • Here, it is also possible to select dividing at least one sub-frame of the n sub-frames into two sub-frames each having a lighting period that is half the length of the sub-frame. In addition, the sub-frame further dividing the lighting period may be a sub-frame having a longest lighting period in the n sub-frames. Moreover, the n sub-frames may be arranged in an ascending order or a descending order.
  • According to the present invention, a kind of transistor that can be applied is not limited. Therefore, a thin film transistor (TFT) using a non-single crystalline semiconductor film typified by amorphous silicon or polycrystalline silicon, a MOS transistor which is formed using a semiconductor substrate or an SOI substrate, a junction transistor, a bipolar transistor, a transistor using an organic semiconductor or a carbon nanotube, a transistor using a compound semiconductor such as ZnO, a-InGaZnO, SiGe, or GaAs, or other transistors can be applied. In addition, a kind of substrate where a transistor is disposed is not limited; therefore, a transistor can be disposed over a single-crystalline substrate, an SOI substrate, a glass substrate, or a plastic substrate.
  • In this specification, “being connected” is synonymous with being electrically connected. Therefore, in a structure disclosed by the present invention, in addition to a predetermined connection relation, other elements that enable electrical connection therebetween (for example, another element or switch) may also be disposed.
  • Note that, as for a switch shown in the present invention, switches of various modes can be used. As an example, there is an electrical switch, a mechanical switch, or the like. In other words, the switches are not particularly limited as long as current flow can be controlled and various switches can be used. For example, the switches may be a transistor, a diode (a PN diode, a PIN diode, a Schottky diode, a transistor connected as a diode, or the like) or a logic circuit that is a combination thereof. Thus, in a case of using a transistor as the switch, the transistor operates as a mere switch; therefore, the polarity (conductivity type) of the transistor is not particularly limited. However, in a case where lower off-current is desired, it is desirable to use a transistor having a polarity with lower off-current. As the transistor with low off-current, a transistor provided with an LDD region, a transistor having a multi-gate structure, or the like can be used. In addition, it is desirable to use an N-channel transistor when a transistor to be operated as a switch operates in a state where potential of a source terminal thereof is close to a lower potential side power supply (such as Vss, GND, or 0 V), whereas it is desirable to use a P-channel transistor when a transistor operates in a state where potential of a source terminal thereof is close to a higher potential side power supply (such as Vdd). This is because the absolute value of a gate-source voltage can be increased, and the transistor easily operates as a switch. Note that the switch may be of a CMOS type using both the N-channel transistor and the P-channel transistor. When the CMOS-type switch is employed, voltage outputted through the switch (that is, voltage inputted into the switch) may be high or low with respect to the outputted voltage and the switch can be operated appropriately even when the situation is changed.
  • Note that, according to the present invention, a semiconductor device refers to a device having a circuit including a semiconductor element (a transistor, a diode, or the like). In addition, a semiconductor device may also refer to devices in general that can function by utilizing semiconductor characteristics. Moreover, a display device refers to a device having a display element (a liquid crystal element, a light-emitting element, or the like). Note that a display device may also refer to a display panel body where a plurality of pixels, including a display element such as a liquid crystal element or an EL element, or a peripheral driver circuit for driving these pixels is formed over a substrate. Further, a display device may also include one with a flexible printed circuit (FPC) or a printed wiring board (PWB).
  • Note that, in this specification, a gate refers to the whole of a gate electrode and a gate wiring (also referred to as a gate line, a gate signal line, or the like), or part thereof. The gate electrode refers to a semiconductor to form a channel region, an LDD (Lightly Doped Drain) region, or the like, and a portion of a conductive film which is overlapped through a gate insulating film. The gate wiring refers to a wiring to connect between gate electrodes of each pixel or to connect to a wiring different from the gate electrode.
  • However, there is a portion that serves as a gate electrode and a gate wiring. Such a region may be referred to as a gate electrode or a gate wiring. In other words, there is a region where a gate electrode and a gate wiring cannot be distinguished apparently. For example, when there is a channel region to overlap with a gate wiring that is arranged by being extended, the region serves as a gate wiring as well as a gate electrode. Thus, such a region may be referred to as a gate electrode or a gate wiring.
  • In addition, a region formed from the same material as the gate electrode and connected to the gate electrode may also be referred to as a gate electrode. In the same manner, a region formed from the same material as the gate wiring and connected to the gate wiring may also be referred to as a gate wiring. In such a region, in a strict sense, there is a case where the region is not overlapped with a channel region or a function to connect to another gate electrode is lacked. However, with relation to a manufacturing margin or the like, there is a region formed from the same material as the gate electrode or the gate wiring and connected to the gate electrode or the gate wiring. Thus, such a region may also be referred to as a gate electrode or a gate wiring.
  • Moreover, for example, a gate electrode of one transistor and a gate electrode of another transistor in a multi-gate transistor are connected to a conductive film formed from the same material as the gate electrode in many cases. Since such a region is a region to connect the gate electrode and the gate electrode to each other, the region may also be referred to as a gate wiring; however, since the multi-gate transistor can be regarded as one transistor, the multi-gate transistor may also be referred to as a gate electrode. In other words, those formed from the same material as the gate electrode or the gate wiring and arranged by being connected thereto may also be referred to as a gate electrode or a gate wiring. Further, for example, a portion of a conductive film which is connected to the gate electrode or the gate wiring may also be referred to as a gate electrode or a gate wiring.
  • Note that a gate terminal refers to part of a region of a gate electrode or a region electrically connected to the gate electrode.
  • Note that a source refers to the whole of a source region, a source electrode, and a source wiring (also referred to as a source line, a source signal line, or the like), or part thereof. The source region refers to a semiconductor region where a P-type impurity (boron or gallium) or an N-type impurity (phosphorus or arsenic) is contained much. Therefore, the source region does not include a region where a P-type impurity or an N-type impurity is slightly contained, that is, a so-called an LDD (Lightly Doped Drain) region. The source electrode is formed from a material different from the source region, which refers to a portion of a conductive layer which is arranged by being electrically connected to the source region. However, the source electrode may include the source region to be referred to as a source electrode. The source wiring refers to a wiring to connect between source electrodes of each pixel or to connect a wiring different from the source electrode.
  • However, there is a portion that serves as a source electrode and a source wiring. Such a region may be referred to as a source electrode or a source wiring. In other words, there is a region where a source electrode and a source wiring cannot be distinguished apparently. For example, when there is a source region to overlap with a source wiring that is arranged by being extended, the region serves as a source wiring as well as a source electrode. Thus, such a region may be referred to as a source electrode or a source wiring.
  • In addition, a region formed from the same material as the source electrode and connected to the source electrode or a portion to connect a source electrode and a source electrode to each other may also be referred to as a source electrode. In addition, a portion overlapped with a source region may also be referred to as a source electrode. In the same manner, a region formed from the same material as the source wiring and connected to the source wiring may also be referred to as a source wiring. In such a region, in a strict sense, there is a case where a function to connect to another gate electrode is lacked. However, with relation to a manufacturing margin or the like, there is a region formed from the same material as the source electrode or the source wiring and connected to the source electrode or a source wiring. Thus, such a region may also be referred to as a source electrode or a source wiring.
  • Moreover, for example, a portion of a conductive film where the source electrode and the source wiring are connected may also referred to as a source wiring.
  • Note that a source terminal refers to part of a source region, a source electrode, or a region electrically connected to the source electrode.
  • Note that a drain is the same as the source.
  • Note that it is structurally difficult to distinguish between a source and a drain of a transistor. Further, there is also a case where potential levels may be interchanged depending on a circuit operation. Therefore, in this specification, the source and the drain are not specified in particular, which are referred to as a first electrode and a second electrode. For example, when the first electrode is a source, the second electrode refers to a drain, whereas, when the first electrode is a drain, the second electrode refers to a source.
  • Note that, according to the present invention, the description of “being formed over a certain object” does not necessarily refer to “being in direct contact with the certain object.” This includes a case where there is no direct contact, that is, a case where another object is sandwiched therebetween. Therefore, for example, a case where a layer B is formed over a layer A includes a case where the layer B is formed on the layer A to be in direct contact therewith and a case where another layer (for example, a layer C, a layer D, or the like) is formed on the layer A to be in direct contact therewith and the layer B is formed thereon to be in direct contact therewith. In addition, the same can also be said for the description of “above a certain object,” which does not necessarily refer to “being in direct contact with the certain object,” and a case where another object is sandwiched therebetween is included. Therefore, for example, a case where a layer B is formed above a layer A includes a case where the layer B is formed on the layer A to be in direct contact therewith and a case where another layer (for example, a layer C, a layer D, or the like) is formed on the layer A to be in direct contact therewith and the layer B is formed thereon to be in direct contact therewith. Note that the same can also be said for the description of “under a certain object” or “below a certain object,” which includes a case where there is direct contact and there is no direct contact.
  • Note that, according to the present invention, one pixel shows one color element. Therefore, in a case of a color display device including color elements of R (red), G (green), and B (blue), a minimum unit of an image includes three pixels of R, G, and B. Note that the color element is not limited to three colors and three or more colors may be used, or a color other than RGB may also be used. For example, RGBW may be employed by adding white (W). In addition, RGB may be added with one or more of yellow, cyan, magenta, and the like, for example. Moreover, for example, as for at least one color of RGB, a similar color may be added. For example, R, G, B1, and B2 may be used. Both B1 and B2 are blue but have a different wavelength. By using such a color element, it is possible to perform display that is much similar to the real and to reduce power consumption.
  • Note that, according to the present invention, a pixel includes a case where pixels are arranged in matrix. Herein, “pixels are arranged in matrix” includes a case of a so-called lattice arrangement in which a perpendicular stripe and a horizontal stripe are combined with each other, a case where dots of three color elements have a so-called delta arrangement when full color display is performed using three color elements (for example, R, G, and B), and further a case of Bayer arrangement.
  • Note that, in this specification, a light-emitting element will be explained by giving an organic EL element as an example. However, the content of the present invention can also be applied to other than a display device using an organic EL element. For example, the present invention can be applied to a display device using a display medium in which contrast varies by an electromagnetic action, such as an EL element (such as an organic EL element, an inorganic EL element, or an EL element containing an organic material and an inorganic material), an electron-emitting element, a liquid crystal element, electronic ink, a grating light valve (GLV), a plasma display (PDP), a digital micromirror device (DMD), a piezoelectric element, or a carbon nanotube. Note that an EL display is used as a display device using the EL element, a field emission display (FED), an SED (Surface-conduction Electron-emitter Display) type flat display, or the like is used as a display device using the electron-emitting element, a liquid crystal display, a transmissive liquid crystal display, a semi-transmissive liquid crystal display, or a reflective liquid crystal display is used as a display device using the liquid crystal element, and an electronic paper is used as a display device using the electronic ink.
  • According to the present invention, it is possible to reduce a pseudo contour and to perform multiple gray scales as well by combining an area gray scale method and a time gray scale method. Therefore, it becomes possible to improve display quality and to view a clear image. In addition, it is possible to improve a duty ratio (a ratio of a lighting period per one frame), and voltage applied to a light-emitting element is reduced. Thus, power consumption can be reduced, and deterioration of the light-emitting element can be suppressed.
  • BRIEF DESCRIPTION OF DRAWINGS
  • In the accompanying drawings:
  • FIG. 1 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 2 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 3 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 4 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 5 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 6 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 7 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 8 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 9 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 10 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 11 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 12 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 13 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 14 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 15 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 16 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 17 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 18 is a view showing a cause that a pseudo contour is decreased in a driving method according to the present invention;
  • FIG. 19 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 20 is a view showing one example of a selection method of a sub-frame and a sub-pixel in a case of performing gamma correction by a driving method according to the present invention;
  • FIGS. 21A and 21B are graphs each showing a relation between a gray scale level and a luminance in a case of performing gamma correction by a driving method according to the present invention;
  • FIG. 22 is a view showing one example of a selection method of a sub-frame and a sub-pixel in a case of performing gamma correction by a driving method according to the present invention;
  • FIGS. 23A and 23B are graphs each showing a relation between a gray scale level and a luminance in a case of performing gamma correction by a driving method according to the present invention;
  • FIG. 24 is a diagram showing one example of a timing chart in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 25 is a diagram showing one example of a pixel configuration in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 26 is a diagram showing one example of a pixel configuration in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 27 is a diagram showing one example of a pixel configuration in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 28 is a diagram showing one example of a timing chart in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 29 is a diagram showing one example of a pixel configuration in a case where a period where signal is written to a pixel and a lighting period are separated;
  • FIG. 30 is a diagram showing one example of a timing chart for selecting two rows in one gate selection period;
  • FIG. 31 is a diagram showing one example of a timing chart in a case where signals in pixels are erased;
  • FIG. 32 is a diagram showing one example of a pixel configuration in a case where signals in pixels are erased;
  • FIG. 33 is a diagram showing one example of a pixel configuration in a case where signals in pixels are erased;
  • FIG. 34 is a diagram showing one example of a pixel configuration in a case where signals in pixels are erased;
  • FIG. 35 is a diagram showing one example of a pixel portion layout of a display device using a driving method according to the present invention;
  • FIG. 36 is a diagram showing one example of a pixel portion layout of a display device using a driving method according to the present invention;
  • FIG. 37 is a diagram showing one example of a pixel portion layout of a display device using a driving method according to the present invention;
  • FIG. 38 is a diagram showing one example of a pixel portion layout of a display device using a driving method according to the present invention;
  • FIG. 39 is a diagram showing one example of a pixel portion layout of a display device using a driving method according to the present invention;
  • FIGS. 40A to 40C are diagrams each showing one example of a display device using a driving method according to the present invention;
  • FIG. 41 is a diagram showing one example of a display device using a driving method according to the present invention;
  • FIG. 42 is a diagram showing one example of a display device using a driving method according to the present invention;
  • FIGS. 43A and 43B are views each showing a cross-sectional structure of a transistor used for a display device according to the present invention;
  • FIGS. 44A and 44B are views each showing a cross-sectional structure of a transistor used for a display device according to the present invention;
  • FIGS. 45A and 45B are views each showing a cross-sectional structure of a transistor used for a display device according to the present invention;
  • FIGS. 46A to 46C are views each showing a structure of a transistor used for a display device according to the present invention;
  • FIGS. 47A to 47D are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 48A to 48C are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 49A to 49D are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 50A to 50D are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 51A to 51D are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 52A and 52B are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIGS. 53A and 53B are views explaining a manufacturing method of a transistor used for a display device according to the present invention;
  • FIG. 54 is a view showing a cross-sectional structure of a transistor used for a display device according to the present invention;
  • FIGS. 55A to 55E are each a top view of a transistor used for a display device according to the present invention;
  • FIGS. 56A and 56B are views each showing one example of a mask pattern of a transistor used for a display device according to the present invention;
  • FIGS. 57A and 57B are views each showing one example of a mask pattern of a transistor used for a display device according to the present invention;
  • FIGS. 58A and 58B are views each showing one example of a mask pattern of a transistor used for a display device according to the present invention;
  • FIG. 59 is a diagram showing one example of hardware for controlling a driving method according to the present invention;
  • FIG. 60 is a view showing one example of an EL module using a driving method according to the present invention;
  • FIG. 61 is a view showing a structure example of a display panel using a driving method according to the present invention;
  • FIG. 62 is a diagram showing one example of an EL television receiver using a driving method according to the present invention;
  • FIGS. 63A to 63H are views each showing one example of an electronic device to which a driving method according to the present invention is applied;
  • FIGS. 64A and 64B are diagrams each showing a cause that a pseudo contour is caused in a conventional driving method;
  • FIG. 65 is a diagram showing a cause that a pseudo contour is caused in a conventional driving method;
  • FIG. 66 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 67 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 68 is a view showing one example of a selection method of a sub-frame and a sub-pixel by a driving method according to the present invention;
  • FIG. 69 is a view showing a structure of an evaporation apparatus for forming an EL layer;
  • FIG. 70 is a view showing a structure of an evaporation apparatus for forming an EL layer; and
  • FIG. 71 is a diagram showing a structure example of a display panel using a driving method according to the present invention.
  • DETAILED DESCRIPTION OF THE INVENTION
  • Embodiment modes of the present invention will be explained below with reference to the drawings. However, it is to be easily understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.
  • (Embodiment Mode 1)
  • This embodiment mode will explain an example of applying a driving method of the present invention to a case of 4-bit display (16 gray scales) and a case of 6-bit display (64 gray scales).
  • The driving method of this embodiment mode is combined with an area gray scale method by which gray scale display is performed by dividing one pixel into a plurality of sub-pixels and controlling the number or area of lighting sub-pixels and a time gray scale method by which gray scale display is performed by dividing one frame into a plurality of sub-frames, each of which is weighted with respect to the nimber of light emissions and a light-emitting period, and then the total weight is differentiated for each gray scale. In other words, one pixel is divided into m (m is an integer number of m≧2) of sub-pixels to have an area ratio of the m sub-pixels 20:21:22: . . . :2m−3:2m−2:2m−1. In addition, one frame is divided into n (n is an integer number of n≧2) of sub-frames to have a ratio in a length of a lighting period of the n sub-frames 20:2m:22m: . . . :2(n−3)m:2(n−2)m:2(n−1)m. Then, gray scale is expressed by controlling a manner of lighting in each of the m sub-pixels in each of the n sub-frames.
  • First, a case of 4-bit gray scales (16 gray scales) is considered. Initially, a display method of each gray scale, that is, how each sub-pixel is lightedin each sub-frame corresponded a gray scale will be explained. This embodiment mode will be explained by giving, as an example, a case where one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into two sub-frames (SF1 and SF2) so that a ratio of a lighting period in each sub-frame becomes 1:4. Note that m corresponds to 2 and n corresponds to 2 in this example.
  • Here, FIG. 1 shows a display method of each gray scale in a case where the sub-pixels respectively have the following area: SP1=1 and SP2=2, and the sub-frames respectively have the following lighting periods: SF1=1 and SF2=4. Note that, as a way to see FIG. 1, FIG. 1 shows that the sub-pixels, which are lighted in each sub-frame, are indicated by ∘ marks whereas the sub-pixels, which are not lighted in each sub-frame, are indicated by × marks.
  • In this embodiment mode, it is considered that a product of an area of each sub-pixel and a lighting period of each sub-frame is substantial light-emitting intensity. For example, in a case where only the sub-pixel 1 (SP1) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frame 2 (SF2), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 4. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the gray scale is displayed.
  • For example, in a case of displaying a gray scale level of 1, the sub-pixel 1 is lighted in the sub-frame 1. In a case of displaying a gray scale level of 2, the sub-pixel 2 is lighted in the sub-frame 1. In a case of displaying a gray scale level of 3, the sub-pixels 1 and 2 are lighted in the sub-frame 1. In a case of displaying a gray scale level of 6, the sub-pixel 2 is lighted in the sub-frame 1 and the sub-pixel 1 is lighted in the sub-frame 2. Each sub-pixel that is lighted in each sub-frame is selected as well as for the other gray scale levels.
  • As described above, it is possible to display the 4-bit gray scales (16 gray scales) by selecting the sub-pixel that is lighted in each sub-frame.
  • With the driving method of this embodiment mode, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 7 is displayed in a pixel A while a gray-scale level of 8 is displayed in a pixel B in FIG. 2, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 8 (=8) or 10 (=2+8) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 7 and 8, are perceived as 8 or 10, which causes pseudo contour. However, a gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • Note that a length of a lighting period in each sub-frame is assumed as 1 and 4; however, the present invention is not limited thereto. In addition, the division number of the sub-frames is assumed as 2; however, the present invention is not limited thereto.
  • For example, generally, at least one of the n sub-frames may be further divided into two sub-frames each having a lighting period that is half of a lighting period of the sub-frame. In particular, as a sub-frame that further divides a lighting period, a sub-frame having the longest lighting period among the n of the sub-frames may be selected.
  • In other words, in the case of 4-bit gray scales (16 gray scales), the sub-frame 2 having the longest lighting period 4 in FIG. 1 may be divided into two sub-frames each having the lighting period 2 which is half of the lighting period 4. Thus, FIG. 3 shows an example that one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into three sub-frames (SF1, SF2, and SF3) so that a ratio of a lighting period in each sub-frame becomes 1:2. Here, the sub-pixels respectively have the following area: SP1=1 and SP2=2, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=2, and SF3=2.
  • In FIG. 3, in a case where only the sub-pixel 1 (SPI) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frames 2 (SF2) and 3 (SF3), the area of the sub-pixel 1 is 1; however, the lighting periods of the sub-frames 2 and 3 (SF3) are twice that of the sub-frame 1; therefore, the light-emitting intensity becomes 2. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting periods of the sub-frames 2 and 3 (SF3) are twice that of the sub-frame 1; therefore, the light-emitting intensity becomes 4. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the 4-bit gray scales (16 gray scales) are expressed.
  • With the driving method as in FIG. 3, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 7 is displayed in a pixel A while a gray-scale level of 8 is displayed in a pixel B in FIG. 4, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 6 (=4+2) or 7 (=1+2+4) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 7 and 8, are perceived as 6 or 7, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • By reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, in this manner, human eyes will be subjected to tricks as if a gray scale gap, in a case where the sight line moves, is reduced. Therefore, this has a profound effect on reducing pseudo contour. Note that the sub-frame where a lighting period is further divided is not limited to a sub-frame having the longest lighting period. However, in particular, it is desirable to further divide the sub-frame having the longest lighting period into two sub-frames each having a lighting period which is half of the lighting period because this has a profound effect on reducing pseudo contour.
  • Note that, by reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, a selection method of a sub-pixel in each sub-frame for displaying the same gray scale level is increased. Therefore, the selection method of each sub-pixel in each sub-frame is not limited thereto. For example, in a case of displaying a gray scale level of 8, in FIG. 3, the sub-pixel 2 is lighted in the sub-frames 2 and 3; however, the sub-pixels 1 and 2 may be lighted in the sub-frame 2 and the sub-pixel 1 may be lighted in the sub-frame 3. This case is shown in FIG. 5.
  • Note that, with the driving method as in FIG. 5, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 7 is displayed in a pixel A while a gray-scale level of 8 is displayed in a pixel B in FIG. 6, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 7 (=1+2+2+2) or 8 (=4+2+2) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 7 and 8, are perceived precisely. Therefore, pseudo contour is reduced compared with a conventional driving method.
  • Accordingly, it is possible to have a profound effect on reducing pseudo contour by selectively changing a selection method of a sub-pixel in each sub-frame for gray scale levels which are especially likely to cause pseudo contour.
  • Note that a sequential order of a lighting period in each sub-frame is not limited thereto. For example, in FIG. 5, the sub-pixels respectively have the following lighting periods: SF1=1, SF2=2, and SF3=2; however, the following lighting periods may also be employed: SF1=2, SF2=1, and SF3=2; or SF1=2, SF2=2, and SF3=1. Note that it is desirable that the sequential order of the lighting period of each sub-frame is an ascending order or a descending order of the lighting period. This is because it is possible to reduce a gray scale gap compared with a conventional method when the sight line moves and to reduce pseudo contour compared with a conventional method by having the lighting period of each sub-frame an ascending order or a descending order.
  • Note that the length of the lighting period is to be appropriately changed depending on total gray scale levels (the number of bits), the number of total sub-frames, or the like. Thus, there is a possibility that the length of periods that are actually lighting (for example, what μs the length is) may be changed even when the lighting periods are the same, if total gray scale levels (the number of bits) or the number of sub-frames is changed.
  • Note that a lighting period is to be used in a case of continuous lighting and lighting frequency is to be used in a case where a switch is turned on and off repeatedly in a certain period. A typical display using the lighting frequency is a plasma display, and a typical display using the lighting period is an organic EL display.
  • Note that, although the number of sub-pixels is two in this embodiment mode, the present invention is not limited thereto. For example, one pixel may be divided into three sub-pixels. In addition, although an area ratio of each sub-pixel is 1:2, the present invention is not limited thereto. For example, one pixel may be divided into sub-pixels with an area ratio of 1:4, 1:8, or 1:2:4.
  • For example, when an area ratio of each sub-pixel is 1:1, the same light-emitting intensity is obtained even when light emission of either sub-pixel is made in the same sub-frame. Therefore, in displaying the same gray scale level, light emission of which sub-pixel is to be made may be switched. Accordingly, it is possible to prevent light emission only in specific sub-pixels by being gathered thereto and to prevent an image sticking of a pixel.
  • Note that it is possible to display much more gray scales with a few sub-pixels and a few sub-frames by having an area ratio of m (m is an integer number of m≧2) of sub-pixels 20:21:22: . . . :2m−3:2m−2:2m−1 and having a lighting period of n (n is an integer number of n≧2) of sub-frames 20:2m:22m: . . . :2(n−3)m:2(n−2)m:2(n−1)m. In addition, since a rate of changing the gray scale that can be displayed by this method is constant, it is possible to display a more smooth gray scale and to improve an image quality.
  • Next, a case of 6-bit gray scales (64 gray scales) is considered. This embodiment mode will be explained by giving, as an example, a case where one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into three sub-frames (SF1, SF2, and SF3) so that a ratio of a lighting period in each sub-frame becomes 1:4:16. Note that m corresponds to 2 and n corresponds to 2 in this example.
  • Here, FIG. 7 shows a display method of each gray scale in a case where the sub-pixels respectively have the following area: SP1=1 and SP=2, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=4, and SF3=16.
  • In a case where only the sub-pixel 1 (SP1) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frame 2 (SF2), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 4. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. In the same manner, in a case where only the sub-pixel 1 is lighted in the sub-frame 3 (SF3), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 3 is sixteen times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 16. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 3 is sixteen times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 32. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the 6-bit gray scales (64 gray scales) are expressed.
  • With the driving method according to the present invention, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is displayed in a pixel B in FIG. 8, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 16 (=16) or 45 (=1+4+8+32) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 16 or 45, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • Note that a length of a lighting period in each sub-frame is assumed as 1, 4, and 16; however, the present invention is not limited thereto. In addition, the division number of the sub-frames is assumed as 3; however, the present invention is not limited thereto.
  • For example, the sub-frame 3 having the longest lighting period 16 in FIG. 7 may be divided into two sub-frames each having the lighting period 8 which is half of the lighting period 16. Thus, FIG. 9 shows an example that one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into four sub-frames (SF1, SF2, SF3, and SF4) so that a ratio of a lighting period in each sub-frame becomes 1:4:8:8. Here, the sub-pixels respectively have the following area: SP1=1 and SP2=2, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=4, SF3=8, and SF4=8.
  • In FIG. 9, in a case where only the sub-pixel 1 (SP1) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frame 2 (SF2), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 4. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. In the same manner, in a case where only the sub-pixel 1 is lighted in the sub-frames 3 (SF3) and 4 (SF4), the area of the sub-pixel 1 is 1; however, the lighting periods of the sub-frames 3 and 4 are eight times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting periods of the sub-frames 3 and 4 are eight times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 16. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the 6-bit gray scales (64 gray scales) are expressed.
  • With the driving method as in FIG. 9, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is displayed in a pixel B in FIG. 10, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 24 (=16+8) or 29 (=1+4+8+16) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 24 or 29, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • By reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, in this manner, human eyes will be subjected to tricks as if a gray scale gap, in a case where the sight line moves, is reduced. Therefore, this has a profound effect on reducing pseudo contour. Note that the sub-frame where a lighting period is further divided is not limited to a sub-frame having the longest lighting period. However, in particular, it is desirable to further divide the sub-frame having the longest lighting period into two sub-frames each having a lighting period which is half of the lighting period because this has a profound effect on reducing pseudo contour.
  • Note that, by reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, a selection method of a sub-pixel in each sub-frame for displaying the same gray scale level is increased. Therefore, the selection method of each sub-pixel in each sub-frame is not limited thereto. For example, in a case of displaying a gray scale level of 32, in FIG. 9, the sub-pixel 2 is lighted in the sub-frames 3 and 4; however, the sub-pixels 1 and 2 may be lighted in the sub-frame 3 and the sub-pixel 1 may be lighted in the sub-frame 4. This case is shown in FIG. 11.
  • With the driving method as in FIG. 11, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is displayed in a pixel B in FIG. 12, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 29 (=1+4+8+8+8) or 32 (=16+8+8) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 29 or 32, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • Accordingly, it is possible to have a profound effect on reducing pseudo contour by selectively changing a selection method of a sub-pixel in each sub-frame for gray scale levels which are especially likely to cause a pseudo contour.
  • Note that the number of sub-pixels is assumed as 2; however, the present invention is not limited thereto. In addition, an area ratio of each sub-pixel is assumed as 1:2; however, the present invention is not limited thereto.
  • For example, FIG. 13 shows an example that one pixel is divided into three sub-pixels (SP1, SP2, and SP3) so that an area ratio of each sub-pixel becomes 1:2:4 and one frame is divided into two sub-frames (SF1 and SF2) so that a ratio of a lighting period in each sub-frame becomes 1:8. Here, the sub-pixels respectively have the following area: SP1=1, SP=2, and SP3=4, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=8. Note that m corresponds to 3 and n corresponds to 2 in this example.
  • In FIG. 13, in a case where only the sub-pixel 1 (SPl) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. Moreover, in a case where only the sub-pixel 3 (SP3) is lighted, the area of the sub-pixel 3 is 4; therefore, the light-emitting intensity becomes 4. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frame 2 (SF2), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 2 is eight times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 2 is eight times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 16. Moreover, in a case where only the sub-pixel 3 is lighted, the area of the sub-pixel 3 is 4; however, the lighting period of the sub-frame 2 is eight times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 32. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the 6-bit gray scales (64 gray scales) are expressed.
  • With the driving method as in FIG. 13, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is displayed in a pixel B in FIG. 14, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 16 (=16) or 36 (=4+32) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 16 or 36, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • In addition, the sub-frame 2 having the longest lighting period 8 in FIG. 13 may be divided into two sub-frames each having the lighting period 4 which is half of the lighting period 8. Thus, FIG. 15 shows an example that one pixel is divided into three sub-pixels (SP1, SP2, and SP3) so that an area ratio of each sub-pixel becomes 1:2:4 and one frame is divided into three sub-frames (SF1, SF2, and SF3) so that a ratio of a lighting period in each sub-frame becomes 1:4:4. Here, the sub-pixels respectively have the following area: SP1=1, SP2=2, and SP3=4, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=4, SF3=4.
  • In FIG. 15, in a case where only the sub-pixel 1 (SPl) is lighted in the sub-frame 1 (SF1), the area of the sub-pixel 1 is 1; therefore, the light-emitting intensity becomes 1. In addition, in a case where only the sub-pixel 2 (SP2) is lighted, the area of the sub-pixel 2 is 2; therefore, the light-emitting intensity becomes 2. Moreover, in a case where only the sub-pixel 3 (SP3) is lighted, the area of the sub-pixel 3 is 4; therefore, the light-emitting intensity becomes 4. On the other hand, in a case where only the sub-pixel 1 is lighted in the sub-frames 2 (SF2) and 3 (SF3), the area of the sub-pixel 1 is 1; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 4. In addition, in a case where only the sub-pixel 2 is lighted, the area of the sub-pixel 2 is 2; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 8. Moreover, in a case where only the sub-pixel 3 is lighted, the area of the sub-pixel 3 is 4; however, the lighting period of the sub-frame 2 is four times as long as that of the sub-frame 1; therefore, the light-emitting intensity becomes 16. Accordingly, with a combination of an area of the sub-pixels and a lighting period of the sub-frames, different light-emitting intensity can be made, with which the 6-bit gray scales (64 gray scales) are expressed.
  • With the driving method as in FIG. 15, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is displayed in a pixel B in FIG. 16, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 27 (=1+2+8+16) or 28 (=16+8+4) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 27 or 28, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • By reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, in this manner, human eyes will be subjected to tricks as if a gray scale gap, in a case where the sight line moves, is reduced. Therefore, this has a profound effect on reducing pseudo contour. Note that the sub-frame where a lighting period is further divided is not limited to a sub-frame having the longest lighting period. However, in particular, it is desirable to further divide the sub-frame having the longest lighting period into two sub-frames each having a lighting period which is half of the lighting period because this has a profound effect on reducing pseudo contour.
  • Note that, by reducing a length of a lighting period of each sub-frame or increasing the division number of each sub-frame, a selection method of a sub-pixel in each sub-frame for displaying the same gray scale level is increased. Therefore, the selection method of each sub-pixel in each sub-frame is not limited thereto. For example, in a case of displaying a gray scale level of 32, in FIG. 15, the sub-pixel 3 is lighted in the sub-frames 2 and 3; however, the sub-pixels 1 and 3 may be lighted in the sub-frame 2 and the sub-pixels 1 and 2 may be lighted in the sub-frame 3. This case is shown in FIG. 17.
  • With the driving method as in FIG. 17, pseudo contour can be reduced. For example, it is assumed that a gray-scale level of 31 is displayed in a pixel A while a gray-scale level of 32 is expressed in a pixel B in FIG. 18, which shows the lighting/non-lighting states of each sub-pixel in each sub-frame of this case. Here, for example, if the sight line moves, human eyes perceive that the gray-scale level is 27 (=1+2+8+4+4+8) or 32 (=16+4+8+4) sometimes, depending on the movement of the sight line. It is proved that the gray scale levels, which are originally supposed to be perceived as 31 and 32, are perceived as 27 or 32, which causes pseudo contour. However, a small gray scale gap is reduced compared with a conventional driving method; thus, pseudo contour is reduced.
  • Accordingly, it is possible to have a profound effect on reducing pseudo contour by selectively changing a selection method of a sub-pixel in each sub-frame for gray scale levels which are especially likely to cause a pseudo contour.
  • Note that correspondences of areas and numbers of the sub-pixels are not limited thereto. For example, in FIG. 15, the sub-pixels respectively have the following areas: SP1=1, SP2=2, and SP3=4; however, the following areas may also be employed: SP1=1, SP2=4, and SP3=2; SP1=2, SP2=1, and SP3=4; or SP1=4, SP2=2, and SP3=1.
  • Accordingly, by using the driving method according to the present invention, it is possible to reduce pseudo contour without increasing the number of sub-frames and to display with a higher gray scale level. In addition, since it is possible to reduce the number of sub-frames compared with conventional time gray scale method, a long lighting period of each sub-frame can be provided. Accordingly, it is possible to improve a duty ratio, and voltage applied to a light-emitting element is reduced. Thus, power consumption can be reduced, and there will be few deterioration of a light-emitting element.
  • Note that a selection method of a sub-pixel in each sub-frame may be changed in terms of time or a place in a certain gray scale. In other words, a selection method of a sub-pixel in each sub-frame may be changed depending on times or a selection method of a sub-pixel in each sub-frame may be changed depending on pixels. Further, the selection method of a sub-pixel in each sub-frame may also be changed depending on times and pixels.
  • For example, in expressing a certain gray scale, different selection methods of sub-pixels may be used in odd-numbered frames and even-numbered frames. For example, in a case of 6-bit gray scales (64 gray scales), gray scales may be expressed by a selection method of sub-pixels shown in FIG. 15 in odd-numbered frames whereas gray scales may be expressed by a selection method of sub-pixels shown in FIG. 17 in even-numbered frames. Accordingly, it is possible to reduce pseudo contour by changing the selection method of sub-pixels between the odd-numbered frames and even-numbered frames in expressing a gray-scale level which is likely to cause pseudo contour.
  • Although the selection method of sub-frames is changed for the gray scale levels which are especially likely to cause pseudo contour, the selection method of sub-pixels may be changed for an arbitrary gray-scale level.
  • Alternatively, the selection method of a sub-pixel in each sub-frame may be changed between the case of displaying pixels in odd-numbered rows and pixels in even-numbered rows in order to express a certain gray scale. Further alternatively, the selection method of a sub-pixel in each sub-frame may be changed between the case of displaying pixels in odd-numbered columns and pixels in even-numbered columns in order to express a certain gray scale.
  • In addition, the division number or a ratio of the lighting periods of the sub-frames may be changed in odd-numbered frames and even-numbered frames in order to express a certain gray scale. For example, in a case of 6-bit gray scales (64 gray scales), gray scales may be expressed by a selection method of sub-pixels as in FIG. 13, where a ratio of the lighting periods of sub-frames is 1:8, in odd-numbered frames whereas gray scales may be expressed by a selection method of sub-pixels as in FIG. 15, where a ratio of the lighting periods of sub-frames is 1:4:4, in even-numbered frames.
  • Note that a sequential order of a lighting period in each sub-frame may be changed depending on times. For example, a sequential order of lighting periods in sub-frames may be changed in the first frame and the second frame. For example, the sequential order of the lighting periods in sub-frames of pixels A and B may be changed. In addition, by combining the lighting periods in sub-frames of pixels A and B, the sequential order of the lighting periods of the sub-frames may be changed depending on times and places. For example, in FIG. 15, the sub-frames may respectively have the following lighting periods: SF1=1, SF2=4, and SF3=4 in odd-numbered frames, and the sub-frames may respectively have the following lighting periods: SF1=4, SF2=1, and SF3=4 in even-numbered frames.
  • Note that the 4-bit gray scales (16 gray scales) or the 6-bit gray scales (64 gray scales) is given as an example in this embodiment mode; however, a gray scale level to be displayed is not limited thereto. For example, when one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into five sub-frames (SF1 to SF5) so that a ratio of a lighting period in each sub-frame becomes 1:4:16:32:32, it is possible to express 8-bit gray scales (256 gray scales). FIGS. 19, 66, 67, and 68 each show a selection method of a sub-pixel in each sub-frame of this case. FIG. 19 shows a selection method of a sub-pixel where gray scale levels are 0 to 63, FIG. 66 shows a selection method of a sub-pixel where gray scale levels are 64 to 127, FIG. 67 shows a selection method of a sub-pixel where gray scale levels are 128 to 191, and FIG. 68 shows a selection method of a sub-pixel where gray scale levels are 192 to 255.
  • Description has been made heretofore on the case where a lighting period increases in linear proportion to the increased gray scale levels. In this embodiment mode, description is made on a case of applying gamma correction. Gamma correction refers to a method of nonlinearly increasing the lighting period in accordance with the increased gray scale levels. When luminance increases linearly, it is difficult for human eyes to perceive that the luminance has become higher proportionally. It is even more difficult for human eyes to perceive the difference in luminance as the luminance becomes higher. Therefore, in order that the human eyes can perceive the difference in luminance, a lighting period is required to be lengthened in accordance with the increased gray scale levels, that is, gamma correction is required to be performed. Note that the relation between the luminance and gray scale levels in performing gamma correction can be expressed by the following Formula (1):
    y=A×x γ  (1)
  • However, in Formula (1), A is a constant for normalizing the luminance y to be within the range of 0=y=1, while γ which is an exponent of the gray scale level x is a parameter indicating the degree of gamma correction.
  • As the simplest method, there is a method by which display is performed with preparation of a larger number of bits (gray scale levels) than the number of bits (gray scale levels) which are actually displayed. For example, in a case of displaying 6-bit gray scales (64 gray scales), display is performed with preparation of 8-bit gray scales (256 gray scales). When actually displaying an image, display is performed with 6-bit gray scales (64 gray scales) so that the luminance and gray scale levels have a nonlinear relationship. Accordingly, gamma correction can be performed.
  • As an example, FIG. 20 shows a selection method of sub-frames in a case of displaying an image with preparation of 6-bit gray scales (64 gray scales) in order to display 5-bit gray scales (32 gray scales) by performing gamma correction. FIG. 20 shows a selection method of sub-frames in the case of displaying an image with 5-bit gray scales by performing gamma correction so that γ=2.2 is satisfied at all the gray scale levels. Note that γ=2.2 is the value which can best correct the characteristics of the human visual perception, with which human eyes can perceive the most appropriate difference in luminance even when the luminance becomes higher. With reference to FIG. 20, up to a gray-scale level of 3 in displaying 5-bit gray scales with gamma correction, display is actually performed by the selection method of sub-frames for displaying a gray-scale level of 0 in the case of 6-bit gray scales. Similarly, at a gray-scale level of 4 in displaying 5-bit gray scales with gamma correction, display is actually performed by a selection method of sub-frames for displaying a gray-scale level of 1 in the case of 6-bit gray scales, and at a gray-scale level of 6 in displaying 5 bit-gray scales with gamma correction, display is actually performed by a selection method of sub-frames for displaying a gray-scale level of 2 in the case of 6 bit-gray scales. FIGS. 21A and 21B are graphs showing the relation between the gray-scale level x and the luminance y. FIG. 21A is a graph showing the relation between the gray-scale level x and the luminance y at all gray scale levels, while FIG. 21B is a graph showing the relation between the gray-scale level x and the luminance y at low gray scale levels. In this manner, display may be performed in accordance with a correspondence table between 5-bit gray scales to be applied with gamma correction and 6-bit gray scales. Accordingly, gamma correction which can satisfy γ=2.2 can be performed.
  • However, as is apparent from FIG. 21B, the gray scale levels of 0 to 3, 4 to 5, and 6 to 7 are each displayed with the same luminance in the case of FIG. 20. This is because, since the gray scale levels is not enough in the case of displaying 6-bit gray scales, difference in luminance cannot be expressed fully. As a countermeasure against this, the following two methods can be considered.
  • The first method is a method of further increasing the number of bits which can be displayed. In other words, display is performed with preparation of not 6-bit gray scales but 7-bit or more gray scales, and preferably 8-bit or more gray scales. Consequently, a smooth image can be displayed even in the low gray scale regions.
  • The second method is a method of displaying a smooth image by not satisfying γ=2.2 in the low gray scale regions but by linearly changing the luminance. FIG. 22 shows a selection method of sub-frames of this case. In FIG. 22, in order to display a gray scale level of up to 17, the same selection method of sub-frames is used between the cases of 5-bit gray scales and 6-bit gray scales. However, at a gray-scale level of 18 in displaying 5-bit gray scales with gamma correction, pixels are actually lighted by a selection method of sub-frames for displaying a gray-scale level of 19 in the case of 6-bit gray scales. Similarly, at a gray-scale level of 19 in displaying 5-bit gray scales with gamma correction, display is actually performed by a selection method of sub-frames for displaying a gray-scale level of 21 in the case of 6-bit gray scales, and at a gray-scale level of 20 in displaying 5-bit gray scales with gamma correction, display is actually performed by a selection method of sub-frames for displaying a gray-scale level of 24 in the case of 6-bit gray scales. FIGS. 23A and 23B show the relation between the gray-scale level x and the luminance y. FIGS. 23A is a graph showing the relation between the gray-scale level x and the luminance y at all gray scale levels, while FIG. 23B is a graph showing the relation between the gray-scale level x and the luminance y at low gray scale levels. In the low gray scale regions, the luminance changes linearly. By performing such gamma correction, a smoother image can be displayed in the low gray scale regions.
  • In other words, by changing the luminance in linear proportion to the gray scale levels in the low gray scale regions while changing the luminance in nonlinear proportion to the gray scale levels in other gray scale regions, a smoother image can be displayed in the low gray scale regions.
  • Note also that the correspondence table between the 5-bit gray scales to be applied with gamma correction and the 6-bit gray scales may be appropriately modified. Thus, by modifying the correspondence table, degree of gamma correction (that is, the value of γ) can be easily changed. Accordingly, the present invention is not limited to γ=2.2.
  • Moreover, the present invention is not particularly limited to the number of bits (for example, p bits, where p is an integer) to be actually displayed, and the number of bits to be applied with gamma correction (for example, q bits, where q is an integer). In the case of displaying bits by performing gamma correction, the number of bits (p) is desirably set as large as possible in order to express gray scales smoothly. However, if the number p is set too large, a problem may arise such that the number of sub-frames is increased accordingly. Thus, the relation between the number of bits (q) and (p) desirably satisfies q+2=p=q+5. Accordingly, gray scales can be smoothly expressed while suppressing the number of sub-frames.
  • (Embodiment Mode 2)
  • This embodiment mode will describe an example of a timing chart. This embodiment mode will be explained by giving, as an example, a case (FIG. 7) where one pixel is divided into two sub-pixels (SP1 and SP2) so that an area ratio of each sub-pixel becomes 1:2 and one frame is divided into three sub-frames (SF1, SF2, and SF3) so that a ratio of a lighting period in each sub-frame becomes 1:4:16.
  • Here, the sub-pixels respectively have the following area: SP1=1 and SP2=2, and the sub-frames respectively have the following lighting periods: SF1=1, SF2=4, and SF3=16.
  • First, FIG. 24 shows a timing chart in the case where a period where a signal is written to a pixel and a lighting period where are separated.
  • Note that a timing chart is a diagram showing light emission of a pixel in one frame, and a horizontal indicates a time whereas a vertical direction indicates a row where pixels are arranged.
  • First, signals for one screen are inputted to all pixels in a signal writing period. During this period, pixels are not lighted. After the signal writing period, a lighting period starts and pixels are lighted. The length of the lighting period at this time is 1. Next, a subsequent sub-frame starts and signals for one screen are inputted to all pixels in a signal writing period. During this period, pixels are not lighted. After the signal writing period, a lighting period starts and pixels is lighted. The length of the lighting period at this time is 4.
  • By repeating similar operations, the lengths of the lighting periods are arranged in the order of 1, 4, and 16.
  • Such a driving method where a period in which a signal is written to a pixel and a lighting period are separated is preferably applied to a plasma display. Note that, in a case where the driving method is used for a plasma display, an initialization operation and the like are required, which are omitted here for simplicity.
  • In addition, this driving method is also preferably applied to an EL display (an organic EL display, an inorganic EL display, a display formed of elements including an inorganic substance and an organic substance, or the like), a field emission display, a display using a Digital Micromirror Device (DMD), or the like.
  • FIG. 25 shows a pixel configuration of this case. FIG. 25 is a configuration example of a case where a plurality of scanning lines is provided and of which scanning lines is selected is controlled so that the number of light-emitting elements lighted is changed to express a gray scale. Note that an area of each sub-pixel is expressed by the number of the light-emitting element in FIG. 25. Therefore, there is one light-emitting element in the sub-pixel 1 and two light-emitting elements in the sub-pixel 2.
  • First, a pixel configuration shown in FIG. 25 will be explained. The sub-pixel 1 includes a first select transistor 2511, a first driving transistor 2513, a first holding capacitor 2512, a signal line 2515, a first power supply line 2516, a first scanning line 2517, a first light-emitting element 2514, and a second power supply line 2518.