WO2009098802A1 - 画素回路および表示装置 - Google Patents
画素回路および表示装置 Download PDFInfo
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- WO2009098802A1 WO2009098802A1 PCT/JP2008/068381 JP2008068381W WO2009098802A1 WO 2009098802 A1 WO2009098802 A1 WO 2009098802A1 JP 2008068381 W JP2008068381 W JP 2008068381W WO 2009098802 A1 WO2009098802 A1 WO 2009098802A1
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- gradation
- switching element
- current
- voltage
- pixel circuit
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G3/00—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes
- G09G3/20—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters
- G09G3/22—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources
- G09G3/30—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels
- G09G3/32—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED]
- G09G3/3208—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED]
- G09G3/3225—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix
- G09G3/3233—Control arrangements or circuits, of interest only in connection with visual indicators other than cathode-ray tubes for presentation of an assembly of a number of characters, e.g. a page, by composing the assembly by combination of individual elements arranged in a matrix no fixed position being assigned to or needed to be assigned to the individual characters or partial characters using controlled light sources using electroluminescent panels semiconductive, e.g. using light-emitting diodes [LED] organic, e.g. using organic light-emitting diodes [OLED] using an active matrix with pixel circuitry controlling the current through the light-emitting element
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2300/00—Aspects of the constitution of display devices
- G09G2300/08—Active matrix structure, i.e. with use of active elements, inclusive of non-linear two terminal elements, in the pixels together with light emitting or modulating elements
- G09G2300/0809—Several active elements per pixel in active matrix panels
- G09G2300/0842—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor
- G09G2300/0852—Several active elements per pixel in active matrix panels forming a memory circuit, e.g. a dynamic memory with one capacitor being a dynamic memory with more than one capacitor
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- G—PHYSICS
- G09—EDUCATION; CRYPTOGRAPHY; DISPLAY; ADVERTISING; SEALS
- G09G—ARRANGEMENTS OR CIRCUITS FOR CONTROL OF INDICATING DEVICES USING STATIC MEANS TO PRESENT VARIABLE INFORMATION
- G09G2310/00—Command of the display device
- G09G2310/02—Addressing, scanning or driving the display screen or processing steps related thereto
- G09G2310/0262—The addressing of the pixel, in a display other than an active matrix LCD, involving the control of two or more scan electrodes or two or more data electrodes, e.g. pixel voltage dependent on signals of two data electrodes
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
- H10K59/00—Integrated devices, or assemblies of multiple devices, comprising at least one organic light-emitting element covered by group H10K50/00
- H10K59/10—OLED displays
- H10K59/12—Active-matrix OLED [AMOLED] displays
- H10K59/121—Active-matrix OLED [AMOLED] displays characterised by the geometry or disposition of pixel elements
- H10K59/1213—Active-matrix OLED [AMOLED] displays characterised by the geometry or disposition of pixel elements the pixel elements being TFTs
Definitions
- the present invention relates to an active matrix display device such as an organic electroluminescence (hereinafter referred to as organic EL) display device, and more specifically to a pixel circuit configuration and a gradation driving method thereof.
- organic EL organic electroluminescence
- a semiconductor switching element 106 is electrically connected to an organic EL thin film 108 which is a light emitter, and a power supply electrode having a power supply voltage Vp. 103 and a series circuit between the common electrode 104 of the power supply voltage VC.
- the gradation signal VD is applied from the data signal line 101 to the gate terminal of the semiconductor switching element 106 so that the semiconductor switching element 106 that controls the light emission current I has a predetermined conductivity.
- the gradation signal VD is applied to the gate terminal when the switching element 105 is ON-controlled by the wiring 102 to which the voltage VS is applied.
- the gate voltage Vg for a predetermined period is held by the capacitor 107 having the capacitance value Cs in the pixel, and light emission at a predetermined gradation level can be maintained.
- the pixel configuration in Patent Document 1 realizes gradation driving within a predetermined signal voltage range.
- a driving system is used that switches between two current supply paths, a path passing through the driving TFT 1101a and a path passing through the driving TFT 1101b. .
- a high Vgs voltage can be applied to the TFT even when the supply current is small, and the influence of threshold variation. Can be displayed.
- the current-voltage characteristics of the current control TFT for determining the light emission current of the organic EL element must be designed so that a predetermined gradation display can be performed within the range of the current voltage of the control signal from the data signal driver or the like. There is.
- gradation display when performing gradation display within a predetermined voltage range, for example, when it is desired to increase the luminance output of a pixel to a predetermined level or more according to the image content, that is, when it is desired to display so-called peak luminance.
- the signal voltage range supplied from a driver or the like is limited, and the gradation display range for generating higher peak luminance is limited.
- the change in the TFT drain current is approximately proportional to the square of the change in the gate voltage.
- the grayscale signal applied to the gate voltage is supplied from the driver circuit.
- the amplitude is, for example, about 5 Vpp due to the design voltage output limitation
- the peak luminance is 2 of the maximum luminance in the normal grayscale display.
- the gradation range is set so that a light emission current that is twice the maximum brightness when outputting 5 Vpp and one time the maximum brightness when outputting about 3.5 Vpp can be generated. There is a need.
- it is desired to output a luminance equivalent to 3 times at 5 Vpp it is necessary to perform normal gradation display within a range of about 2.9 Vpp.
- the voltage range for normal gradation display is limited to a narrower range.
- the frequency of occurrence of gradations that require peak luminance is often the case when the display is generally low gradations, and most of the display gradations are within the normal display gradation range. Are concentrated on the low gradation side output, and the voltage range cannot be used effectively.
- the driver element has an output specification of 5 Vpp, if it is actually used only up to 3 Vpp for normal display, if the driver is adapted to a specification of about 3.5 Vpp, power consumption and This is advantageous in terms of module cost, but the voltage range for obtaining peak luminance needs to be sufficiently wide as described above.
- the present invention has been made in view of the above-described conventional problems, and the object thereof is higher while ensuring a sufficient voltage range for performing normal gradation display within a predetermined driver output range.
- a pixel circuit of the present invention includes a display element whose emission luminance is current-controlled, and at least one first switching element whose output current characteristic with respect to an input variable as a gradation signal exhibits a saturation characteristic. And a first current path that outputs a current determined by the first switching element unit, and at least one second switching element unit whose output current characteristic with respect to an input variable as a gradation signal exhibits a linear characteristic And a second current path that outputs a current determined by the second switching element unit merges and is connected to the current path of the display element.
- the first switching element unit outputs a current in the saturation characteristic region
- the second switching element unit outputs a current in the linear region, and supplies these combined currents to the display element. Therefore, the current output from the first switching element portion is dominant in the low gradation region side, and the current output from the second switching element portion is dominant in the current flowing through the display element in the high gradation region side. A large current change that can follow the luminance change can be given to the current flowing through the display element.
- the signal voltage amplitude for displaying the normal gradation within the signal voltage amplitude range limited by the driver or the like can be made sufficiently wide.
- each of the first switching element unit and the second switching element unit includes one semiconductor element having two or more terminals, or a plurality of semiconductor elements It is characterized by comprising parallel circuits.
- the first switching element portion and the second switching element portion can be easily configured using a conventional semiconductor element having two or more terminals.
- each of the first switching element portion and the second switching element portion includes one thin film transistor having three terminals or more, or a plurality of thin film transistors in parallel. It consists of a circuit.
- the first switching element portion and the second switching element portion can be easily configured using the conventional thin film transistor having three or more terminals.
- the pixel circuit of the present invention is configured such that the total gate channel width of the thin film transistors constituting the first switching element portion is W1, the gate channel length is L1, and the second switching element portion is It is characterized in that W1 / L1 ⁇ W2 / L2 is satisfied, where W2 is the total gate channel width of the thin film transistor and L2 is the gate channel length.
- the first switching element portion can be easily made to have saturation characteristics and the second switching element portion can be made to have linear characteristics.
- the pixel circuit of the present invention is a thin film transistor that constitutes the first switching element part, wherein the thin film transistors that constitute the first switching element part and the second switching element part are P-type.
- the first thin film transistor has a saturation characteristic of the threshold voltage Vth1 and generates an output current from the first power supply line that outputs the voltage V1
- the second thin film transistor that constitutes the second switching element unit has the threshold voltage
- An output current is generated from a second power supply line having a linear characteristic of Vth2 and outputting the voltage V2, and the current output terminal of the first thin film transistor and the current output terminal of the second thin film transistor are one terminal electrode of the current path of the display element.
- the other terminal electrode of the display element is connected to the common electrode It is, is characterized in that meets V1 + Vth1 ⁇ V2 + Vth2.
- the contribution of the output current of the first thin film transistor becomes dominant in the normal gradation display, and the contribution of the output current of the second thin film transistor can be increased in the high gradation display and the peak luminance display. Play.
- the pixel circuit of the present invention is characterized by satisfying V1 ⁇ V2 in order to solve the above-mentioned problems.
- the pixel circuit of the present invention is an N-type thin film transistor that constitutes the first switching element portion and the second switching element portion, and is a thin film transistor that constitutes the first switching element portion.
- the first thin film transistor has a saturation characteristic of the threshold voltage Vth1 and generates an output current from the first power supply line that outputs the voltage V1
- the second thin film transistor that constitutes the second switching element unit has the threshold voltage
- An output current is generated from a second power supply line having a linear characteristic of Vth2 and outputting the voltage V2, and the current output terminal of the first thin film transistor and the current output terminal of the second thin film transistor are one terminal electrode of the current path of the display element.
- the other terminal electrode of the display element is connected to the common electrode It is, is characterized in that meets V1 + Vth1 ⁇ V2 + Vth2.
- the contribution of the output current of the first thin film transistor becomes dominant in the normal gradation display, and the contribution of the output current of the second thin film transistor can be increased in the high gradation display and the peak luminance display. Play.
- the pixel circuit of the present invention is characterized by satisfying V1 ⁇ V2 in order to solve the above-mentioned problem.
- the pixel circuit of the present invention has the same voltage applied to the gate terminal of the thin film transistor constituting the first switching element portion and the gate terminal of the thin film transistor constituting the second switching element portion. Is applied.
- the wiring for applying the gate voltage can be shared by the first switching element unit and the second switching element unit, the circuit configuration can be simplified.
- the pixel circuit of the present invention includes a path from the first power supply line to the common electrode via the first switching element portion and the display element, and the second power supply line to the first power supply line.
- One switching element unit and at least one thin film transistor that conducts and cuts off a current flowing through the display element is further provided on a path that is combined with a current path that reaches the common electrode via the display element. It is a feature.
- display timing can be arbitrarily performed within a frame by further including at least one thin film transistor that conducts and cuts off a current flowing through the display element.
- a pixel circuit includes a gate terminal of a thin film transistor that constitutes the first switching element portion and a second switching element in a non-light-emitting process in a lighting display process using the display element. It is characterized by having a process in which an initial voltage is set to at least one of a gate terminal of a thin film transistor that constitutes a part.
- the first switching element portion and the second switching element portion usually include process variations, and even if adjacent elements are adjacent to each other, the current-voltage characteristics vary.
- the gate voltage is shifted in accordance with the data signal using the initial voltage as a reference point, thereby producing an effect that light emission current variations due to the threshold can be reduced.
- the pixel circuit of the present invention has a gradation range that is standardized so that the normal gradation range is a value from 0 to 1 and the peak gradation range is a value exceeding 1.
- the gradation signal voltage amplitude for displaying the normal gradation range is set to a value from 0 to 1
- the gradation signal voltage amplitude for displaying the peak gradation range is set to a value exceeding 1.
- the lighting contribution ratio by the second switching element portion is 20% or more
- the display gradation level is It is characterized in that the lighting contribution ratio increases with respect to a change in the gradation signal voltage amplitude corresponding to the increase.
- the lighting contribution ratio by the second switching element portion is 20% or more.
- the lighting contribution ratio by the first switching element unit at a value of 1 in the normalized gradation signal voltage amplitude range is 45% or more and 55% or less. It is characterized by.
- the brightness index curve can be made substantially linear in the high gradation region.
- the pixel circuit of the present invention provides the second switching element unit with a grayscale signal voltage amplitude having a value of 1/3 or more and less than 2/3 in the grayscale signal voltage amplitude range.
- the lighting contribution ratio is 0% or more and less than 20%, and the lighting contribution ratio increases with respect to the change in the gradation signal voltage amplitude corresponding to the increase in the display gradation level.
- the pixel circuit of the present invention reduces the lighting contribution ratio by the second switching element portion to 0 at a gradation signal voltage amplitude that is less than one third in the gradation signal voltage amplitude range. % To less than 20%, which is characterized in that the lighting contribution ratio increases with respect to the change in the gradation signal voltage amplitude corresponding to the increase in the display gradation level.
- the pixel circuit of the present invention has a gradation range that is standardized so that the normal gradation range is a value from 0 to 1 and the peak gradation range is a value exceeding 1.
- the gradation signal voltage amplitude for displaying the normal gradation range is set to a value from 0 to 1
- the gradation signal voltage amplitude range for displaying the peak gradation range is set to a value exceeding 1.
- the brightness index characteristic with respect to the gradation signal voltage amplitude is an ideal linear relationship when the gradation signal voltage amplitude is in the range of 2/3 or more, and the slope error of the brightness index curve is ideal. It is characterized by being within 5%.
- the display device of the present invention is characterized in that the display element is an organic light emitting diode.
- a display device of the present invention is provided with a pixel selection signal circuit section that includes a plurality of the pixel circuits and supplies a selection signal that permits the gradation signals to be supplied to the pixel circuits.
- a gradation signal supply circuit unit for supplying a gradation signal to the pixel circuit, a first power supply line for supplying power for generating an output current by the first switching element unit, and an output current for the second switching element unit
- a second power supply line for supplying power to be generated; a scanning signal line for transmitting the selection signal output from the pixel selection signal circuit unit to the pixel circuit; and the floor output from the gradation signal supply circuit unit.
- a data signal line for transmitting a tone signal to the pixel circuit.
- FIG. 1 is a circuit block diagram showing a basic configuration of a pixel circuit.
- FIG. FIG. 2 is a circuit diagram illustrating a first embodiment of the pixel circuit of FIG. 1. It is a characteristic view which shows the relationship between the output current and gate voltage of a 1st thin-film transistor and a 2nd thin-film transistor.
- FIG. 3 is a circuit diagram illustrating a second embodiment of the pixel circuit of FIG. 1.
- FIG. 5 is a characteristic diagram showing a relationship between a light emitting element current and a gradation signal voltage for the pixel circuit of FIG. 4.
- FIG. 5 is a characteristic diagram showing the relationship between normal gradation current magnification and gradation signal voltage for the pixel circuit of FIG. 4.
- FIG. 1 is a circuit block diagram showing a basic configuration of a pixel circuit.
- FIG. 2 is a circuit diagram illustrating a first embodiment of the pixel circuit of FIG. 1. It is a characteristic view which shows the relationship between the output current and gate voltage of a
- FIG. 5 is a characteristic diagram showing a relationship between normalized luminance and normalized brightness index and normalized gradation signal voltage amplitude for the pixel circuit of FIG. 4.
- FIG. 2 is a characteristic diagram showing a relationship between normalized luminance and normalized brightness index and normalized gradation signal voltage amplitude for the pixel circuit of FIG. 1.
- FIG. 2 is a characteristic diagram showing a relationship between a normalized lightness index and a normalized gradation signal voltage amplitude for the pixel circuit of FIG. 1.
- FIG. 2 is a characteristic diagram illustrating a relationship between a contribution rate of luminance increase and a normalized gradation signal voltage amplitude in the pixel circuit of FIG.
- FIG. 5 is a block diagram illustrating a configuration of a display device including the pixel circuit of FIG. 4 according to the embodiment of the present invention.
- First semiconductor element First switching element unit, first thin film transistor
- Second semiconductor element Second semiconductor element (second switching element unit, second thin film transistor) 306, 400, 600
- Light emitting element display element
- 401a TFT first switching element portion, first thin film transistor
- 401b TFT second switching element portion, second thin film transistor
- 601a TFT first switching element portion, first thin film transistor
- 601b TFT second switching element portion, second thin film transistor
- Power supply electrode first power line
- Power supply electrode second power supply line
- 602a Current supply voltage line first power line
- 602b Current supply voltage line
- Gradation signal line data signal line
- Line selection line scanning signal line
- 1404a Power supply line Power supply line (first power line)
- Power supply line Second power supply line
- 1406 Source driver circuit (gradation signal supply circuit) 1407 Gate driver circuit (pixel selection signal circuit portion)
- FIG. 1 shows a basic configuration of a pixel circuit 1 according to the present embodiment.
- the pixel circuit 1 has a circuit configuration for controlling a light emission current related to gradation display, and includes a first semiconductor element (first switching element portion, first thin film transistor) 304, a second semiconductor element (second switching element portion, A second thin film transistor) 305 and a light emitting element (display element) 306.
- first semiconductor element first switching element portion, first thin film transistor
- second semiconductor element second switching element portion, A second thin film transistor
- a light emitting element display element 306.
- the number of the semiconductor elements is two, but the present invention is not limited to this.
- the first semiconductor element 304 and the second semiconductor element 305 are light emission current control switching elements that control the light emission current Ie flowing through the light emitting element 306, respectively.
- the light emitting element 306 is an element whose luminance is controlled by current, for example, an organic light emitting diode, a path through which the light emitting current I1 controlled by the first semiconductor element 304 flows, and a light emitting current controlled by the second semiconductor element 305.
- the path through which I2 flows joins at each current output terminal side, and is connected to one end of a light emitting element 306 provided as a single light emitting element.
- first semiconductor element 304 on the opposite side to the junction is the supply power electrode (first power line) 301
- second semiconductor element 305 on the opposite side to the junction is the supply power electrode (second power line).
- Power supply line) 302 respectively.
- the voltage of the power supply electrode 301 and the voltage of the power supply electrode 302 may be the same, but generally may be different from each other.
- the other end of the light emitting element 306 is connected to the common electrode 303.
- each of the first semiconductor element 304 and the second semiconductor element 305 and the light emitting element 306 are connected in series between the power supply electrode 301 or 302 and the common electrode 303.
- each semiconductor element and the light emitting element 306 are connected in series.
- At least one of the plurality of current paths connected to the light emitting element 306 has mainly saturation characteristics (especially when the current is considered to be constant as in a saturation region in a MOS transistor, steady current characteristics, that is, constant current characteristics). It is configured using a semiconductor element having current-voltage characteristics (also referred to as current characteristics).
- the first semiconductor element 304 is used.
- at least one of the current paths is constituted by a semiconductor element having a current-voltage characteristic mainly showing a linear characteristic, and here is constituted by using the second semiconductor element 305.
- Each of these semiconductor elements has a structure having two or more terminals, and a switch in which a current ratio between conduction and non-conduction is at least 100 or more at a maximum applied voltage and a minimum applied voltage in a predetermined voltage range. Has characteristics.
- the first semiconductor element 304 when a voltage of a certain level or higher is applied between the two terminals of the first semiconductor element 304, the first semiconductor element 304 has saturation characteristics.
- the first semiconductor element 304 is composed of a P-type TFT 1 having three terminals (that is, three terminals of a source, a drain, and a gate), as shown in Expression (1), the TFT 1 is interposed between the drain and source.
- a voltage difference Vds1 sufficiently larger than the threshold voltage Vth1 is applied, and a voltage difference Vgs1 smaller than the drain-source voltage difference Vds1 is applied between the gate and the source.
- the gate voltage of the first semiconductor element 304 becomes an input variable as a gradation signal, and the first semiconductor element 304 outputs a current determined by this gate voltage.
- Vds1 Vs1-Vd1 >>-Vth1
- Vgs1 Vs1-Vg1 ⁇ Vds1 (1)
- Vth1 is a threshold voltage when the TFT1 changes from the non-conductive state to the conductive state, and has a negative value as a difference from the gate voltage value Vg1 when the source voltage value Vs1 is used as a reference.
- the value of the general threshold voltage Vth may be a gate-source voltage Vgs0 with respect to Ids0 that is a predetermined source-drain current that can be substantially regarded as a boundary between on-current and off-current. The derived value by the current voltage approximate expression may be used.
- the voltage difference between the terminals is such that the drain voltage value Vd1 is lower than the source voltage value Vs1 as shown in Equation (2) when the source voltage value is used as a reference.
- the gate voltage value Vg2 is a voltage value in the range shown in Expression (3) when the TFT1 is in a non-conducting state, and indicates the voltage range shown in Expression (4) when in the conductive state.
- the conduction current between the drain and the source changes in proportion to the square of the voltage difference between the gate and the source.
- the saturation characteristic region the range of conditions showing such voltage-current characteristics.
- an inter-terminal current generally proportional to the voltage between the two terminals flows.
- the second semiconductor element 305 is configured by a P-type TFT 2 having three terminals (that is, three terminals of a source, a drain, and a gate), as shown in Expression (5), the TFT 2 is interposed between the drain and the source. Is equal to or smaller than the threshold voltage Vth2, and a voltage difference Vgs2 sufficiently larger than the drain-source voltage difference Vds2 is applied between the gate and the source.
- Vth2 is a threshold voltage when the TFT2 changes from the non-conductive state to the conductive state, and has a negative value as a difference from the gate voltage value Vg2 when the source voltage value Vs2 is used as a reference.
- the gate voltage of the second semiconductor element 305 becomes an input variable as a gradation signal, and the second semiconductor element 305 outputs a current determined by this gate voltage.
- Vds2 Vs2 ⁇ Vd2 ⁇ ⁇ Vth2
- Vgs2 Vs2-Vg >> Vds2 (5)
- the gate voltage value Vg2 is a voltage value in the range shown in Expression (7) when the TFT 2 is in a non-conducting state, and indicates the voltage range shown in Expression (8) when in the conductive state.
- the TFT 2 has a source-drain conduction current that is generally proportional to the source-drain voltage difference Vds2.
- a range of conditions showing such voltage-current characteristics is called a linear characteristic region.
- the device currents I1 and I2 of the TFT1 and the TFT2 obtained from the power supply electrodes 301 and 302 are combined and flow as the light emission current Ie to the common electrode through the light emitting device.
- the gradation display is output and displayed with a luminance that is approximately proportional to the applied current.
- the light emitting element 306 mainly emits light of current contribution by the TFT 1 in the case of relatively low gradation display, and emits light to which the current contribution of the TFT 2 is added in high gradation display. Furthermore, in a higher luminance peak luminance display region, gradation display can be performed with a current contribution several times that of TFT 2 as compared to the current contribution of TFT 1 alone.
- the same gate signal is compared with the case where the light emission current Ie which is roughly proportional to the square of the gate voltage amplitude is applied to the light emitting element 306. Higher emission luminance can be obtained with a voltage amplitude, and normal gradation display can be performed in a sufficiently wide voltage range.
- each of TFT1 and TFT2 for controlling the light emission current Ie may be a single semiconductor element, or each of TFT1 and TFT2 may be replaced by a parallel circuit of a plurality of semiconductor elements.
- the current output function for the gradation signal can be set arbitrarily, and the gradation display quality can be improved.
- One or a plurality of third TFTs 3 for controlling the interruption or conduction of the light emission current Ie may be provided.
- the display timing can be performed at any point in the frame.
- the source voltage Vs1 of the TFT 1 and the source voltage Vs2 of the TFT 2 satisfy the formula (9) in the relationship of the TFT 2 that is a linear current output with respect to the TFT 1 that is a constant current output. Is desirable.
- normal gradation display used in this specification means that, for example, if the display gradation is an 8-bit gradation, there are 256 luminance levels from the 0 gradation level to the 255 gradation level. This is the gradation range displayed by the output signal voltage amplitude range.
- the range up to 2.5 Vpp is the normal gradation display. This is a signal voltage range in which can be performed.
- the relationship between the source voltage Vs1 and threshold voltage Vth1 of the TFT1 and the source voltage Vs2 and threshold voltage Vth2 of the TFT2 satisfies the formula (10).
- the semiconductor element is represented as a three-terminal TFT element in the above configuration, it may be a two-terminal element or a switching element having four or more terminals.
- a process of setting a predetermined gate voltage to at least one of the TFT 1 showing constant current characteristics and the TFT 2 showing linear characteristics It is desirable to contain. This is because the above-described light emission current control element usually includes process variations, and current-voltage characteristics vary even if adjacent elements are adjacent to each other. Even if the same gate voltage is applied to each pixel, the threshold characteristics and mobility characteristics of the elements are slightly different, so that there is a high possibility that the emission current will vary.
- the TFT1 and the TFT2 have a circuit that can set an initial voltage of the gate voltage in the vicinity of the threshold voltage, for example.
- the TFT1 and the TFT2 have a circuit that can set an initial voltage of the gate voltage in the vicinity of the threshold voltage, for example.
- the linear range in a gradation range normalized with a value in which the normal gradation range is 0 to 1 and the peak gradation range exceeds 1, the linear range is obtained in a gradation range of 2/3 (0.667) or more. It is desirable that the current contribution indicating the lighting contribution of the operating TFT 2 is 20% or more.
- the normalized normal gradation range (0 to 1) is an 8-bit gradation
- the region where the normalized gradation level is 0 or more and less than 1/3 ( ⁇ 0.333) is The low gradation area of 0 to 84 gradations, and the area where the standardized gradation level is 1/3 ( ⁇ 0.333) or more and less than 2/3 ( ⁇ 0.667) is among the 85 to 169 gradations
- the gradation area and the area where the normalized gradation level is 2/3 ( ⁇ 0.667) or more and 1 or less are the high gradation area of 170 to 255 gradations and the gradation whose normalized gradation level exceeds 1.
- the 256 gradation or more is set as a peak gradation area.
- the gradation range is defined based on the same concept even if the expression bit number of the normal gradation level is other than 8 bits.
- the L * v * u * color system (JIS Z8729) in the CIE 1976 CUS chromaticity diagram is a color space (uniform color space) with a perceptually almost uniform rate recommended by the CIE (International Lighting Commission) in 1976.
- L * represents a lightness index
- u * and v * are indices indicating chromaticity. This color system is widely used in device devices such as monitors that emit light.
- the brightness index L * which is defined as follows by the ratio of the emission luminance Y to the background luminance Yn.
- Natural image display can be realized by controlling the light emission luminance so that the brightness index is approximately proportional to the gradation signal voltage amplitude.
- FIG. 8 shows the luminance curve Y0 and the brightness index curve L0 * corresponding to the luminance curve Y0 when the luminance changes in a quadratic function with respect to the input of the gradation signal, and the luminance having the characteristics shown in this embodiment.
- a characteristic curve Yt and a lightness index curve Lt * corresponding thereto are shown.
- the gradation signal voltage is standardized within a predetermined output amplitude range.
- the gradation signal voltage range for displaying standardized normal gradation is a range of 0 to 1, and a signal voltage range exceeding 1 is a range for displaying a peak gradation.
- a signal voltage range exceeding 1 is a range for displaying a peak gradation.
- the driver output voltage amplitude is 5 Vpp
- the normal gradation display range is given as 3 Vpp
- the peak gradation display range exceeds 3 Vpp and falls within the range of 5 Vpp.
- the current-voltage characteristic in the saturation characteristic region of a typical TFT is approximated by a quadratic function. Therefore, the luminance Y0 of the light emitting element 306 is also a quadratic function with respect to the signal voltage. In contrast, the lightness index L0 * is an increasing function in which the slope decreases as the gray level changes from low to high.
- the contribution ratio of the luminance increment by the TFT 2 is 20% at the normalized gradation signal voltage amplitude 0.667 and about 50% at the normalized gradation signal voltage amplitude 1 with respect to the entire emission luminance. . That is, the contribution ratio of the light emission current by the TFT 2 is also the same.
- FIG. 9 shows the value of the normalized brightness index with respect to the normalized gradation signal voltage amplitude
- L1 * indicates the normalized brightness index when the luminance is proportional to the square of the gradation signal voltage amplitude.
- L2 * is an example in which the lightness index is set to have a substantially linear relationship with respect to a range exceeding a specific voltage amplitude by the above design.
- L3 * is a reference value when the lightness index has a completely linear relationship with respect to the voltage amplitude.
- FIG. 10 shows the luminance increment of the contribution of the linear TFT 2 with respect to the normalized gradation signal voltage amplitude.
- the brightness index L2 * indicates that the contribution ratio of luminance increase by TFT 2 (ie, the lighting contribution) is 20% in the normalized gradation signal voltage amplitude 0.667 in FIG. 10, and the normalized gradation signal voltage amplitude increases. This is a characteristic when the contribution rate of the luminance increment is set to increase as the value increases.
- the above-described characteristics can be realized by adjusting parameters of the TFT 2 operating with linear characteristics as described above.
- the current contribution of the TFT 2 of 0% or more and less than 20% (ie, lighting) so that the luminance Yt is continuous in the range where the normalized gradation signal voltage amplitude exceeds 0.333 and less than 0.667. If the contribution) is included, the change in the brightness index becomes a smoother characteristic with respect to the change in luminance, so that the unnaturalness of display is reduced.
- the current contribution of the TFT 2 of less than 20% may be at a position where the normalized gradation signal voltage amplitude is less than 0.333.
- the gradation luminance level increases as the gradation signal voltage amplitude increases.
- the gradation luminance level increases.
- the concept of the present invention can be applied.
- the semiconductor elements such as the first semiconductor element 304 and the second semiconductor element 305 are shown as switching elements.
- the input signals similar to those of the TFT 1 having the constant current characteristic and the TFT 2 having the linear current characteristic are described.
- it is a switching element having the characteristic of the amount of light, it may not be a semiconductor element. Further, it may be a solid thin film having similar electro-optical transmission characteristics and light emission characteristics.
- an organic light emitting diode is shown as the light emitting element 306.
- an element that modulates transmittance in a transmissive liquid crystal pixel circuit may be used.
- a polarizing film that can electrically control the polarization direction of transmitted light may be used, or a solid thin film whose transmittance changes electrically.
- an element that electrically refracts or reflects light to control the light scattering direction may be used.
- the light modulation element may be an element that can control the amount of light by a combination with a semiconductor switch element.
- FIG. 2 shows a configuration of a pixel circuit 2 that is an embodiment of the pixel circuit 1.
- the first semiconductor element 304 of FIG. 1 is the TFT 401a
- the second semiconductor element 305 is the TFT 401b
- the light emitting element 306 is the light emitting element 400
- the power supply electrode 301 is the current supply voltage line 402a
- the common electrode 303 corresponds to the common voltage supply line 403. It is assumed that all TFTs used in the pixel circuit 2 are P-type.
- Each of the drain terminals of the TFT 401 a and the TFT 401 b is connected to one of the light emitting elements 400 so that a light emitting current Ie obtained by combining two drain currents of the drain current Idsa of the TFT 401 a and the drain current Idsb of the TFT 401 b is applied to the light emitting element 400.
- the source terminals of the TFT 401a and the TFT 401b are connected to the input terminal in parallel, and are sequentially connected to the current supply voltage lines 402a and 402b.
- the common voltage supply line 403 is connected to the other terminal of the light emitting element 400.
- a signal voltage is applied from the data signal line 404a through the selection TFT 405a to the gate terminal of the TFT 401a that functions as a light emission current control element.
- a signal voltage is applied from the data signal line 404b through the selection TFT 405b to the gate terminal of the TFT 401b functioning as a light emission current control element.
- a storage capacitor 406a is connected between the gate terminal and the source terminal of the TFT 401a, and a storage capacitor 406b is connected between the gate terminal and the source terminal of the TFT 401b.
- a selection signal electrode 407a is connected to the gate terminal of the selection TFT 405a, and a selection signal electrode 407b is connected to the gate terminal of the selection TFT 405b.
- the gate channel dimensions (gate channel width Wa, gate channel length La) of the TFT 401a for light emission current control and the gate channel dimensions (gate channel width Wb, gate channel length Lb) of the TFT 401b are the conditions of the expression (12). Shall be satisfied.
- the TFT 401a operates in a saturation characteristic region according to the range of the gate signal voltage and the source / drain voltage
- the TFT 401b has a substantially linear characteristic according to the range of the gate signal voltage and the source / drain voltage. Work in the region. That is, when the TFT 401a and the TFT 401b are in a conductive state, the drain current Idsa of the TFT 401a proportional to the square of the gate voltage change and the drain current Idsb of the TFT 401b substantially proportional to the gate voltage change are applied to the light emitting element 400.
- the drain current Idsa of the TFT 401a is predominant, and the light emission current Ie proportional to the square of the gate voltage change is applied to the light emitting element 400.
- the gate voltage level applied to the TFT 401b decreases, the drain current Idsb of the TFT 402b becomes dominant, and the change of the entire light emission current Ie becomes steeper with respect to the change of the gate voltage.
- a light emission current Ie larger than that of the TFT 401a alone can be obtained by adding the light emission current Idsb of the TFT 401b to the light emission current Idsa of the TFT 401a at a gate voltage at a sufficiently high gradation display.
- FIG. 5 shows the characteristics between the light emission current Ie and the gradation signal voltage Vg in the pixel circuit 2.
- Vg is a dark gradation level in the vicinity of 10 V, and a range of about 8 to 10 V is low gradation display.
- the high gradation display is around 7V, and the peak gradation display is in the Vg range of less than 7V, realizing a steep current change.
- FIG. 4 shows a configuration of a pixel circuit 3 which is another embodiment of the pixel circuit 1.
- the storage capacitor, the data signal line, and the selection signal line are shared.
- the first semiconductor element 304 of FIG. 1 is the TFT 601a
- the second semiconductor element 305 is the TFT 601b
- the light emitting element 306 is the light emitting element 600
- the power supply electrode 301 is the current supply voltage line 602a
- the common electrode 303 corresponds to the common voltage supply line 603. It is assumed that all TFTs used in the pixel circuit 3 are P-type.
- Each of the drain terminals of the TFT 601a and the TFT 601b is connected to one of the light emitting elements 600 so that a light emitting current Ie obtained by combining two drain currents of a drain current Ia of the TFT 601a and a drain current Ib of the TFT 601b is applied to the light emitting element 600.
- the source terminals of the TFT 601a and the TFT 601b are connected in parallel to the input terminal, and are sequentially connected to the current supply voltage lines 602a and 602b.
- a common voltage supply line 603 is connected to the other terminal of the light emitting element 600.
- a signal voltage is applied from the data signal line 604 via the selection TFT 605 to the gate terminals of the TFTs 601a and 601b that function as light emitting current control elements.
- a storage capacitor 606 is connected between the gate terminal and the source terminal of the TFT 601a.
- a selection signal electrode 607 is connected to the gate terminal of the selection TFT 605.
- TFT 601a corresponds to TFT 401a of Example 1
- TFT 601b corresponds to TFT 401b of Example 1.
- the relationship is the same as in Example 1.
- the TFT 601a operates in a saturation characteristic region according to the range of the gate signal voltage and the drain-source voltage
- the TFT 601b corresponds to the range of the gate signal voltage and the drain-source voltage. Operates in a generally linear characteristic region.
- the light emission current from the TFT 601a is mainly applied to the light emitting element 600 in the low gradation region
- the light emission current of the TFT 601b is also applied to the light emitting element 600 in the higher gradation region.
- the light emission current control TFT is a P-type, but each is an N-type, and each is a combination of an N-type and a P-type. Can also be applied in accordance with the spirit of the present invention. However, since the operation polarities of the elements are different in the above formula, the polarity of the reference voltage needs to be corrected so as to match each polarity.
- a TFT that operates in a linear characteristic that controls a light emission current and a TFT that operates in a saturation characteristic are provided on different current paths.
- either one of the drain end and the source end of the TFT that operates in a linear characteristic and the TFT that operates in a saturation characteristic may be configured to be combined with the same material.
- a circuit configuration for compensating for characteristic variations between other pixels may be combined with a circuit configuration inside or outside the pixel circuit.
- display variations caused by variations in light emission current due to differences in TFT characteristic parameters such as threshold characteristics and mobility characteristics of individual TFTs are different from pixel to pixel so that they are not visible at a predetermined viewing distance.
- Means for reducing variations in the light emission current are used.
- the drain current Ia and the drain current Ib of each TFT when conducting are approximately in the form of the following equation (16).
- Va a source voltage of the TFT 601a
- Vb a source voltage of the TFT 601b
- Vdsb linear characteristic region
- Va is a source voltage of the TFT 601a
- Vb is a source voltage of the TFT 601b
- Vga is the gate voltage of the TFT 601a
- Vgb is the gate voltage of the TFT 601b
- Vtha is a threshold voltage of the TFT 601a
- Vthb is a threshold voltage of the TFT 601b, and has the above-described threshold relationship.
- the light emission current Ie of the light emitting element 600 generally takes the form of the equation (17), and a combined current of the light emission current represented by the equation (17) is applied during driving.
- K is a proportional constant indicating the characteristics of the light emitting element 600
- Ve is a voltage applied to both ends of the light emitting element 600
- Vthe is a threshold voltage of the light emitting element 600. Note that Vdsb and Ve have the following relationship.
- FIG. 5 shows changes in the light emission current Ie when the gradation signal voltage Vg is changed on the horizontal axis.
- the power supply voltage Va is 12V
- the power supply voltage Vb is 9V
- the threshold voltages Vtha, Vthb, and Vthe of the TFT 601a and TFT 601b and the light emitting element 600 are ⁇ 1.5V, ⁇ 1.0V, and + 0.8V, respectively.
- the voltage starting point Vs at which a steep current change is obtained in FIG. 5 is approximately 8.0 V, and the signal voltage level for setting the 0 gradation is 10.5 V. That is, when the signal voltage of 0 gradation level is set as the starting point, the current voltage amplitude is set to 2.5 Vpp or more so that a steep current change occurs.
- the light emission current Ie is set to about 130 nA at a high gradation level (255 gradation level in the case of 8-bit gradation) at the time of normal gradation display.
- the maximum amplitude of the gradation signal voltage at the time of peak display is set to 5 Vpp
- the gradation amplitude range of the normal gradation range is set to 3 Vpp at maximum
- the normal gradation current ratio C and the peak current ratio P are expressed by the following formula (19 ) In FIG.
- Vo in parentheses in the function notation is a signal voltage for displaying a dark gradation in driving.
- 3Vpp is used when high gradation display is performed with an amplitude of a maximum of 3 Vpp based on the dark gradation signal. The voltage is shown.
- Idsa corresponds to Ia
- Idsb corresponds to Ib.
- the respective current levels in the case of applying a gate voltage of 7.5 V where the maximum gradation level in the normal gradation range is 3 Vpp starting from 10.5 V are as follows.
- the gradation level at which the contribution of the current amount Ib of the TFT 601b occurs is approximately 127 gradation levels, and the signal voltage amplitude is in the range of 2.5 Vpp or more.
- FIG. 6 shows how much peak magnification can be obtained in this characteristic.
- the dark gradation signal voltage Vo is in the range of 10.2 V or more
- the current ratio C at the time of normal gradation display defined by the equation (19) is 200 or more, and it can be seen that the contrast is sufficient.
- the dark gradation signal voltage value V0 is in the range of 10.2V to 10.5V
- the peak current ratio P is approximately 5 to 8 times.
- the peak luminance display is 8 times the normal gradation display in the display with the maximum amplitude of 5 Vpp. It can be performed.
- FIG. 7 shows a normalized luminance Y and a normalized brightness index L * normalized from the light emission current Ie.
- the lightness index L * in the normal gradation display range and the lightness index L * in the peak gradation display range provide the contribution of the current amount Ib of the TFT 601b to 28.8% at the signal voltage amplitude of 3 Vpp.
- the characteristic is approximately proportional to the signal voltage amplitude. Therefore, it is possible to provide a display device having a sharp display characteristic from low gradation display to peak gradation display.
- FIG. 11 shows a conventional typical 2TFT1C type pixel circuit.
- the gate of the TFT 106 is obtained.
- a light emission current I corresponding to the square of the change in the voltage Vg is applied to the light emitting element 108.
- the gate voltage Vg is supplied from the data signal line 101 to the storage capacitor 107 when the selection TFT 105 is turned on by the voltage VS of the selection signal electrode 102, and the selection TFT 105 is turned off by the voltage VS of the selection signal electrode 102. This voltage is held in the holding capacitor 107 when the state is reached.
- FIG. 12 is a diagram showing a change in the drain current Ids of the TFT 106, that is, the light emission current I when the gate signal voltage Vg is swept in the pixel circuit configuration of FIG.
- the gate signal voltage Vg has a voltage-current characteristic that is approximately a quadratic function when the gate signal voltage Vg is between 11V and 6V. Below about 5.5V, the current changes almost linearly with the gate voltage. This is because the light emitting element 108 becomes a high luminance voltage condition as the gate signal voltage is lowered, so that the voltage between the drain and the source of the TFT 106 gradually decreases, and finally operates in the linear characteristic region. It is to become.
- the light emission current I is approximately approximated by the following equation (20) indicating the saturation characteristic by the voltage difference Vgs between the held gate voltage Vg and the source voltage Vs of the driving TFT 106.
- Vgs ⁇ Vth Vgs ⁇ Vth 2 (20)
- Vds Vth
- ⁇ is a parameter constant unique to the TFT, and includes mobility, gate channel dimensions, and gate-silicon capacitance parameters.
- the signal voltage V0 in the dark level light emission is expressed as the applied voltage as shown in the equation (21)
- the bright level signal voltage VL can be expressed as the equation (22).
- ⁇ V corresponds to the amplitude of the gradation signal voltage.
- the peak luminance magnification P is approximately proportional to the light emission current. If (23) is used, it can be expressed as in equation (24).
- V0 indicates a voltage value in the vicinity of the threshold value, but the ratio of the light emission current I when the gate voltage Vg is ⁇ Vn + V0 and the light emission current I when the gate voltage Vg is V0 is at least 100 or more. Solving for ⁇ Vn, equation (25) is obtained.
- the maximum signal voltage amplitude of 5 Vpp can only obtain a peak magnification of twice, but in the configuration of the first embodiment, When the normal gradation signal voltage amplitude is 3 Vpp, a peak magnification of about 6 times can be obtained at the maximum signal voltage amplitude of 5 Vpp.
- FIG. 14 shows a configuration of a display device using the present invention.
- the display device in FIG. 14 includes a source driver circuit (gradation signal supply circuit portion) 1406, a gate driver circuit (pixel selection signal circuit portion) 1407, and a pixel region 1401 having the pixel circuits in FIG. 4 in a matrix. This is the area that has been
- a gradation signal line (data signal line) 1402 (604), a line selection signal line (scanning signal line) 1403 (607), a power supply line (first power line) 1404a (602a), and power supply A line (second power supply line) 1404b (602b) and a power supply line 1405 (603) are provided side by side and extend outside the pixel region 1401. Note that the numbers and symbols in parentheses represent the corresponding members in FIG.
- a source driver circuit 1406 provided outside the pixel region 1401 includes a plurality of register circuits and sample hold circuits that temporarily store supplied gradation signals, a buffer circuit that amplifies signal strength, and the like.
- the voltage held on the gradation signal line 1402 is output at the timing.
- the gate driver circuit 1407 includes a shift register and a buffer circuit that amplifies signal strength. Each row output is connected to a line selection signal line 1403 in the pixel region 1401, and the gate circuit in the pixel is turned on / off at a fixed timing. The signal voltage to be output is sequentially output. A plurality of control signal lines 1408 and a plurality of power supply lines 1409 are connected to the gate driver circuit 1407, and a pulse voltage signal, a DC voltage signal, or the like is applied thereto.
- a plurality of gradation signal lines 1410 and a plurality of control signal lines 1411 are connected to the source driver circuit 1406, and a pulse signal voltage or a DC voltage is applied thereto.
- a power supply line 1412 is connected to the source driver circuit 1406 and a power supply voltage is supplied.
- a signal applied to the gradation signal line 1410 may be a pulse voltage signal or an analog voltage signal.
- the power supply voltage applied to the power supply lines 1404a, 1404b, and 1405 in the pixel region 1401 is connected to the voltage value applied to the plurality of power supply voltage lines 1412 connected to the source driver circuit 1406 or to the gate driver circuit 1407.
- the voltage value applied to the plurality of power supply voltage lines 1409 may be a common voltage value or may be different.
- a gradation signal of pixels for one row is applied to the source driver circuit 1406 within a predetermined period.
- the scanning period per row is about 34.72 ⁇ s.
- the grayscale signal voltage for 640 pixels in one row that is, the grayscale signal voltage for 1920 pixels is normally stored in the source driver circuit 1406 because one pixel is composed of three RGB subpixels.
- the voltages held in the source driver circuit 1406 are output to the gradation signal line 1402 all at once.
- the selection TFT 605 is turned ON, and the gradation signal voltage output from the source driver circuit 1406 is applied to the auxiliary capacitor 606 inside the pixel. Is done.
- the TFTs 601a and TFT 601b which are driving TFTs, are turned on according to the gradation signal voltage, and the light emitting element 600 is turned on according to the level of the conductive state.
- the selection TFT 605 in the pixel is turned off, and the gradation signal voltage level is held in the auxiliary capacitor 606.
- the output of the gate driver circuit 1407 is selected and output to a different row in accordance with the timing signal, and thereafter the same operation is sequentially repeated.
- the light emission corresponding to the voltage level held in the auxiliary capacitor 606 is maintained until the next scanning is performed.
- the display device having the pixel circuit according to the present invention is driven.
- the source driver circuit 1406 and the gate driver circuit 1407 are configured as one set. However, for example, the same display can be performed by combining a plurality of driver circuits in order to perform screen division scanning.
- the pixel circuit in the pixel region 1401 can be applied to any pixel circuit as long as it has the functions of the present invention.
- a row selection signal may be used at a timing different from that of the gate driver 1407. May be additionally provided.
- TFTs are used for the first semiconductor element 304 and the second semiconductor element 305 in FIG. 1 .
- the present invention is not limited to this, and a normal field effect transistor formed on a silicon substrate may be used. Good.
- MOS transistors can be used for the first semiconductor element 304 and the second semiconductor element 305.
- the organic light emitting diode can be replaced with a normal light emitting diode.
- the pixel circuit of the present invention includes a display element whose emission luminance is current-controlled, and at least one first switching element unit in which an output current characteristic with respect to an input variable as a gradation signal exhibits a saturation characteristic; A first current path for outputting a current determined by the first switching element unit, the output current characteristic with respect to an input variable as a gradation signal having at least one second switching element unit exhibiting a linear characteristic; A second current path that outputs a current determined by the second switching element unit merges and is connected to the current path of the display element.
- the present invention can be particularly suitably used for an active matrix light-emitting element array display device in which luminance is current-controlled.
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Abstract
Description
304 第1半導体素子(第1スイッチング素子部、第1薄膜トランジスタ)
305 第2半導体素子(第2スイッチング素子部、第2薄膜トランジスタ)
306、400、600
発光素子(表示素子)
401a TFT(第1スイッチング素子部、第1薄膜トランジスタ)
401b TFT(第2スイッチング素子部、第2薄膜トランジスタ)
601a TFT(第1スイッチング素子部、第1薄膜トランジスタ)
601b TFT(第2スイッチング素子部、第2薄膜トランジスタ)
301 供給電源電極(第1電源線)
302 供給電源電極(第2電源線)
402a、602a
電流供給電圧線(第1電源線)
402b、602b
電流供給電圧線(第2電源線)
1402 階調信号線(データ信号線)
1403 ライン選択線(走査信号線)
1404a 電源供給線(第1電源線)
1404b 電源供給線(第2電源線)
1406 ソースドライバ回路(階調信号供給回路部)
1407 ゲートドライバ回路(画素選択信号回路部)
Vgs1=Vs1-Vg1≦Vds1
・・・・・・・・・・(1)
なお、ここでVth1はTFT1が非導通状態から導通状態に変化する際の閾値電圧であり、ソース電圧値Vs1を基準にしたときのゲート電圧値Vg1との差として、負の値を持つ。また、一般的な閾値電圧Vthの値としては、実質的にオン電流とオフ電流との境界と見なせる所定のソース・ドレイン電流となるIds0に対するゲート・ソース間電圧Vgs0であっても良いし、通常の電流電圧近似式による導出値であっても良い。
Vg1>Vs1+Vth1 ・・・・・・・・・・(3)
Vg1≦Vs1+Vth1 ・・・・・・・・・・(4)
このような構成であれば、ゲート・ソース間電圧が一定であるときは、ドレイン・ソース間電圧が僅かに変動してもドレイン・ソース端子間の電流変化は非常に小さく、定電流特性を示す。例えば、ドレイン・ソース間電圧が1V程度変動しても、ゲート・ソース間電圧が一定であれば、ドレイン・ソース間電流は数%程度以下の電流変動である。
Vgs2=Vs2-Vg>>Vds2
・・・・・・・・・・(5)
ここで、各端子間の電圧差は、TFT2がP型であるため、ソース電圧値を基準にすると式(6)のようにドレイン電圧値Vd2がソース電圧値Vs2よりも低くなる。また、ゲート電圧値Vg2は、TFT2が非導通状態である場合は式(7)に示す範囲の電圧値であり、導通状態であるときは式(8)に示す電圧の範囲を示す。
Vg2>Vs2+Vth2 ・・・・・・・・・・(7)
Vg2≦Vs2+Vth2 ・・・・・・・・・・(8)
TFT2は、このような電圧条件を満たせばおおむねソース・ドレイン間電圧差Vds2に比例したソース・ドレイン間の導通電流となる。一般にはこのような電圧電流特性を示す条件の範囲を線形特性領域と呼んでいる。
この様にする事で、高階調表示における階調信号対階調輝度出力の変化率を大きくすることができ、通常階調表示での数倍の発光電流Ieを少ない増分の信号電圧変化で得ることができる。
この様にすることで、通常階調表示ではTFT1の寄与が優勢になり、より高階調表示やピーク輝度表示ではTFT2の寄与が大きくなるように設定することができる。
・・・・・・・・・・(11)
つまり明度指数L*は、背景輝度Ynに対する発光輝度Yの比の3分の1乗に比例して感じられることになる。式(11)によれば、輝度Yが大きくなるにつれて明るい部分の明度指数L*の変化は緩慢になることが分かる。このことにより、高階調レベルでの表示では、輝度を多少変化させた程度では明るさが変化したようにあまり感じることができない。
なお、TFT401aが複数のTFTの並列回路で置き換えられるときには、それらの各TFTのゲートチャネル長は等しくLaであり、上記Waは各TFTのゲートチャネル幅の合計となる。
この条件を満たすことによって、TFT401aはゲート信号電圧およびソース・ドレイン間電圧の範囲に応じて概ね飽和特性領域で動作し、TFT401bはゲート信号電圧およびソース・ドレイン間電圧の範囲に応じて概ね線形特性領域で動作する。つまり、TFT401aおよびTFT401bが導通状態となるときには、ゲート電圧変化の2乗に比例したTFT401aのドレイン電流Idsaおよび、ゲート電圧変化にほぼ比例したTFT401bのドレイン電流Idsbが発光素子400に印加される。
(TFTがP型であるので、Vtha、Vthbは共に負の値。)
・・・・・・・・・(14)
また、信号電圧振幅の上限が固定されている場合などでは、通常階調を表示する電圧振幅△Vnとピーク表示を行う電圧振幅△Vpとが信号ドライバなどの所定の電圧出力範囲△Voutで設定できることが望ましいため、次式の様にTFTや電源電圧の条件を満たすことで上記の特性の設計が容易になる。
△Vn=(Vpa+Vtha)-(Vpb+Vthb)
ピーク表示階調信号電圧振幅:△Vp
△Vn≦△Vp≦△Vout
・・・・・・・・・(15)
つまり、本設計では、通常階調表示における最大階調レベル(8ビット階調であれば255階調)を出力する信号電圧振幅を△Vnとなる様に設定し、△Vn以上の階調信号電圧振幅でピーク階調を表示する様に設定されている。
Ib=βb(Vb-Vgb+Vthb-Vdsb/2)Vdsb (線形特性領域)
・・・・・・・・・・(16)
ここで、VaはTFT601aのソース電圧、VbはTFT601bのソース電圧であり、電流供給電圧源である。また、VgaはTFT601aのゲート電圧、VgbはTFT601bのゲート電圧である。また、VthaはTFT601aの閾値電圧、VthbはTFT601bの閾値電圧であり、前述した閾値の関係を有している。また、VdsbはTFT601bのドレイン・ソース間電圧である。また、TFT601aのゲート電圧VgaとTFT601bのゲート電圧Vgbとは共通電圧であるため、Vgb=Vgaと表現することができる。
=Ia+Ib
・・・・・・・・・・(17)
ここで、Kは発光素子600の特性を示す比例定数であり、Veは発光素子600の両端に印加される電圧、Vtheは発光素子600の閾値電圧である。なお、VdsbとVeとには次の関係がある。
該画素回路3において、図5に階調信号電圧Vgを横軸に変化させた場合の発光電流Ieの変化を示す。電源電圧Vaは12V、電源電圧Vbは9V、TFT601aおよびTFT601bおよび発光素子600の各閾値電圧Vtha、VthbおよびVtheはそれぞれ、-1.5V、-1.0V、+0.8Vである。
ピーク電流比P=Ie(5Vpp)/Ie(3Vpp)
・・・・・・・・・・(19)
ここで、関数表記における括弧内のVoは、駆動において暗階調を表示するための信号電圧であり、例えば3Vppとは暗階調信号を基準にして最大3Vppの振幅で高階調表示を行う際の電圧を示している。
発光素子600の電流量Ie:126.7nA
TFT601aの電流量Ia: 90.2nA、寄与率:71.2%
TFT601bの電流量Ib: 36.5nA、寄与率:28.8%
これによれば、通常階調表示範囲での最大階調レベル、すなわち信号電圧振幅3VppにおけるTFT601bの電流量Ibの寄与は28.8%である。
但し、TFT106がP型であるので、TFT106の導通状態ではVgs≦Vth、Vds<<Vthである。なお、Vdsは電源電圧Vpを基準電圧に置くため、負の値である。ここで、βはTFT固有のパラメータ定数であって、移動度、ゲートチャネル寸法、ゲート-シリコン間の電気容量パラメータを含む。
V0=Vg-Vp-Vth
・・・・・・・・・・(21)
VL=△V+V0 ・・・・・・・・・・(22)
このようにして、式(20)に示される発光電流Iは式(23)の様に簡略化され、階調信号の振幅の関数として表すことができる。
なお、△V≒0であるときはI≒0であり、発光電流Iは視認可能な発光に寄与しないレベルで発光体に印加されている。
ここで、V0は閾値近傍の電圧値を示すが、ゲート電圧Vgが△Vn+V0であるときの発光電流Iとゲート電圧VgがV0であるときの発光電流Iとの比は少なくとも100以上である。△Vnについて解けば、式(25)の様になる。
式(25)において、通常の信号ドライバの出力振幅を最大5Vppとして考えた場合に、△Vp=5Vが最大となる。ここでピーク倍率を2倍、および、3倍とることが必要であれば、V0=1.5Vと置くと、
2倍の場合:
△Vn=3.536-0.293・V0
=3.097
3倍の場合:
△Vn=2.887-0.423・V0
=2.253
の様に通常階調表示のための信号電圧範囲を設定する必要がある。これは実施例1の構成と比較すると、通常階調表示を行うための電圧範囲がより狭い構成となっていることを示している。
Claims (18)
- 輝度が電流制御される表示素子と、
階調信号としての入力変量に対する出力電流特性が飽和特性を示す、少なくとも1つの第1スイッチング素子部と、
階調信号としての入力変量に対する出力電流特性が線形特性を示す、少なくとも1つの第2スイッチング素子部とを備え、
前記第1スイッチング素子部によって決定される電流を出力する第1電流経路と、前記第2スイッチング素子部によって決定される電流を出力する第2電流経路とが合流して、前記表示素子の電流経路に接続されていることを特徴とする画素回路。 - 前記第1スイッチング素子部および前記第2スイッチング素子部は、それぞれ2端子以上を有する、1つの半導体素子からなる、あるいは、複数の半導体素子の並列回路からなることを特徴とする請求の範囲第1項に記載の画素回路。
- 前記第1スイッチング素子部および前記第2スイッチング素子部は、それぞれ3端子以上を有する、1つの薄膜トランジスタからなる、あるいは、複数の薄膜トランジスタの並列回路からなることを特徴とする請求の範囲第1項に記載の画素回路。
- 前記第1スイッチング素子部を構成する薄膜トランジスタのゲート端子と、前記第2スイッチング素子部を構成する薄膜トランジスタのゲート端子とには、同一の電圧が印加されることを特徴とする請求の範囲第3項に記載の画素回路。
- 前記第1スイッチング素子部を構成する薄膜トランジスタの各ゲートチャネル幅の合計をW1、各ゲートチャネル長をL1とし、前記第2スイッチング素子部を構成する薄膜トランジスタの各ゲートチャネル幅の合計をW2、各ゲートチャネル長をL2とするとき、W1/L1≦W2/L2を満たしていることを特徴とする請求の範囲第3項または第4項に記載の画素回路。
- 前記第1スイッチング素子部および前記第2スイッチング素子部を構成する薄膜トランジスタがP型であって、
前記第1スイッチング素子部を構成する薄膜トランジスタである第1薄膜トランジスタは、閾値電圧Vth1の飽和特性を有するとともに電圧V1を出力する第1電源線から出力電流を生成し、
前記第2スイッチング素子部を構成する薄膜トランジスタである第2薄膜トランジスタは、閾値電圧Vth2の線形特性を有するとともに電圧V2を出力する第2電源線から出力電流を生成し、
第1薄膜トランジスタの電流出力端子および第2薄膜トランジスタの電流出力端子は前記表示素子の電流経路の一方の端子電極に接続され、前記表示素子の他方の端子電極は共通電極に接続されており、
V1+Vth1≧V2+Vth2を満たしていることを特徴とする請求の範囲第5項に記載の画素回路。 - V1≧V2を満たしていることを特徴とする請求の範囲第6項に記載の画素回路。
- 前記第1スイッチング素子部および前記第2スイッチング素子部を構成する薄膜トランジスタがN型であって、
前記第1スイッチング素子部を構成する薄膜トランジスタである第1薄膜トランジスタは、閾値電圧Vth1の飽和特性を有するとともに電圧V1を出力する第1電源線から出力電流を生成し、
前記第2スイッチング素子部を構成する薄膜トランジスタである第2薄膜トランジスタは、閾値電圧Vth2の線形特性を有するとともに電圧V2を出力する第2電源線から出力電流を生成し、
第1薄膜トランジスタの電流出力端子および第2薄膜トランジスタの電流出力端子は前記表示素子の電流経路の一方の端子電極に接続され、前記表示素子の他方の端子電極は共通電極に接続されており、
V1+Vth1≦V2+Vth2を満たしていることを特徴とする請求の範囲第5項に記載の画素回路。 - V1≦V2を満たしていることを特徴とする請求の範囲第8項に記載の画素回路。
- 前記第1電源線から前記第1スイッチング素子部および前記表示素子を経由して前記共通電極に至る経路と前記第2電源線から前記第1スイッチング素子部および前記表示素子を経由して前記共通電極に至る電流経路とを合わせた経路上に、前記表示素子に流す電流の導通および遮断を行う少なくとも1つの薄膜トランジスタをさらに備えていることを特徴とする請求の範囲第6項から第9項までのいずれか1項に記載の画素回路。
- 前記表示素子を用いた点灯表示プロセスにおける非点灯プロセスにおいて、前記第1スイッチング素子部を構成する薄膜トランジスタのゲート端子と、前記第2スイッチング素子部を構成する薄膜トランジスタのゲート端子との少なくともいずれか一方に初期電圧が設定されるプロセスを有することを特徴とする請求の範囲第3項から第10項までのいずれか1項に記載の画素回路。
- 通常階調範囲を0から1までの値とするとともに、ピーク階調範囲を1を越える値とするように規格化された階調範囲を有し、前記通常階調範囲を表示する階調信号電圧振幅を0から1までの値とするとともに、前記ピーク階調範囲を表示する階調信号電圧振幅を1を越える値とするように規格化された階調信号電圧振幅範囲において、値が3分の2以上となる階調信号電圧振幅では、前記第2スイッチング素子部による点灯寄与率を20%以上有し、表示階調レベルの上昇に対応する階調信号電圧振幅の変化に対して点灯寄与率が増加することを特徴とする請求の範囲第1項に記載の画素回路。
- 前記規格化された階調信号電圧振幅範囲の1の値における第1スイッチング素子部による点灯寄与率が45%以上55%以下であることを特徴とする請求の範囲第12項に記載の画素回路。
- 前記階調信号電圧振幅範囲において値が3分の1以上3分の2未満となる階調信号電圧振幅において、前記第2スイッチング素子部による点灯寄与率を0%以上20%未満有し、表示階調レベルの上昇に対応する階調信号電圧振幅の変化に対して点灯寄与率が増加することを特徴とする請求の範囲第12項または第13項に記載の画素回路。
- 前記階調信号電圧振幅範囲において値が3分の1未満となる階調信号電圧振幅において、第2スイッチング素子部による点灯寄与率を0%~20%未満有し、表示階調レベルの上昇に対応する階調信号電圧振幅の変化に対して点灯寄与率が増加することを特徴とする請求の範囲第12項から第14項までのいずれか1項に記載の画素回路。
- 通常階調範囲を0から1までの値とするとともに、ピーク階調範囲を1を越える値とするように規格化された階調範囲を有し、通常階調範囲を表示する階調信号電圧振幅を0から1までの値とするとともに、ピーク階調範囲を表示する階調信号電圧振幅範囲を1を越える値とするように規格化された階調信号電圧振幅範囲において、階調信号電圧振幅に対する明度指数の特性が、階調信号電圧振幅が3分の2以上の範囲では、明度指数曲線の傾きの誤差が理想の線形関係に対して5%以内であることを特徴とする請求の範囲第1項に記載の画素回路。
- 前記表示素子は有機発光ダイオードであることを特徴とする請求の範囲第1項から第16項までのいずれか1項に記載の画素回路。
- 請求の範囲第1項から第17項までのいずれか1項に記載の画素回路を複数備え、
前記画素回路に前記階調信号の供給を許可する選択信号を供給する画素選択信号回路部と、
供給される階調信号を前記画素回路に供給する階調信号供給回路部と、
前記第1スイッチング素子部が出力電流を生成する電源を供給する第1電源線と、
前記第2スイッチング素子部が出力電流を生成する電源を供給する第2電源線と、
前記画素選択信号回路部から出力された前記選択信号を前記画素回路に伝達する走査信号線と、
前記階調信号供給回路部から出力された前記階調信号を前記画素回路に伝達するデータ信号線とをさらに備えていることを特徴とする表示装置。
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Cited By (4)
Publication number | Priority date | Publication date | Assignee | Title |
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JP2012058443A (ja) * | 2010-09-08 | 2012-03-22 | Hitachi Displays Ltd | 画像表示装置およびその駆動方法 |
JP2012247540A (ja) * | 2011-05-26 | 2012-12-13 | Mitsubishi Electric Corp | 映像表示装置 |
WO2015016007A1 (ja) * | 2013-07-30 | 2015-02-05 | シャープ株式会社 | 表示装置およびその駆動方法 |
US11521557B2 (en) | 2019-10-30 | 2022-12-06 | Canon Kabushiki Kaisha | Display apparatus, information display apparatus, photoelectric conversion apparatus, electronic apparatus, lighting apparatus, and mobile body |
Also Published As
Publication number | Publication date |
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EP2239723B1 (en) | 2015-01-28 |
US8878756B2 (en) | 2014-11-04 |
US20100302285A1 (en) | 2010-12-02 |
EP2239723A1 (en) | 2010-10-13 |
JP5294274B2 (ja) | 2013-09-18 |
JPWO2009098802A1 (ja) | 2011-05-26 |
CN101903934B (zh) | 2013-04-24 |
CN101903934A (zh) | 2010-12-01 |
EP2239723A4 (en) | 2011-03-09 |
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