PIXEL CIRCUIT, LIGHT EMITTING DISPLAY DEVICE AND DRIVING METHOD THEREOF

• US 20100053041A1
• Filed: 08/27/2009
• Published: 03/04/2010
• Est. Priority Date: 09/03/2008
• Status: Active Grant

• Current Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Original Assignee: Canon Kabushiki Kaisha (Canon Inc.)
• Inventors: Takahashi, Kenji, Abe, Katsumi, Hayashi, Ryo, Kumomi, Hideya
• Granted Patent: US 8,659,519 B2
• Time in Patent Office: Days
• US Class Current: 345/76
• CPC Class Codes: G09G 2300/0426 Layout of electrodes and connections
• CPC Class Codes: G09G 2300/0439 Pixel structures
• CPC Class Codes: G09G 2300/0814 used for selection purposes, e.g. logical AND for partial update
• CPC Class Codes: G09G 2300/0852 being a dynamic memory with more than one capacitor
• CPC Class Codes: G09G 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2310/0262 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
• CPC Class Codes: G09G 2320/0223 Compensation for problems related to R-C delay and attenuation in electrodes of matrix panels, e.g. in gate electrodes or on-substrate video signal electrodes
• CPC Class Codes: G09G 3/325 the data current flowing through the driving transistor during a setting phase, e.g. by using a switch for connecting the driving transistor to the data driver

Abstract

A pixel circuit including at least a light emitting element, and a thin film transistor that supplies to the light emitting element a first current controlling a gray scale according to luminance-current characteristics of the light emitting element, wherein the thin film transistor has a back gate electrode, at least a driving period in which the thin film transistor supplies the first current to the light emitting element, and a writing period in which a second current is written to the thin film transistor before the driving period in order to pass the first current to the thin film transistor during the driving period are included, and by changing voltages which are applied to the back gate electrode in the driving period and the writing period, current capability to a gate voltage of the thin film transistor is made to differ.

Claims

1. A pixel circuit comprising a light emitting element, and a thin film transistor that supplies to the light emitting element a first current controlling a gray scale according to luminance-current characteristics of the light emitting element,

wherein the thin film transistor has

a back gate electrode,

a driving period in which the thin film transistor supplies the first current to the light emitting element and

a writing period in which a second current is written to the thin film transistor before the driving period in order to supply the first current from the thin film transistor during the driving period are set,

the difference between the driving period and the writing period in voltage to be applied to the back gate electrode rendering the driving period and the writing period different from each other in a current capability determined by a gate voltage of the thin film transistor.

2. The pixel circuit according to claim 1, wherein the second current is larger than the first current.

3. The pixel circuit according to claim 1, wherein a voltage applied to the back gate electrode in the writing period is set so that the current capability is higher than that controlled by the voltage applied to the back gate electrode in the driving period.

4. The pixel circuit according to claim 1, wherein a change in mobility of the thin film transistor by variation of the voltage applied to the back gate electrode is 5% or less.

5. The pixel circuit according to claim 4, wherein a relationship between the voltage applied to the back gate electrode and a threshold voltage of the thin film transistor is expressed by linear relation.

6. The pixel circuit according to claim 1, wherein the second current from outside of the pixel circuit in the writing period controls the gray scale.

7. The pixel circuit according to claim 1, wherein the voltage provided to the back gate electrode in the writing period controls the gray scale.

8. A light emitting display device, comprising:

the pixel circuits according to claim 1 arranged two-dimensionally; and

a scanning unit applying a voltage to the back gate electrodes of a plurality of the pixel circuits arranged in a row direction for each row.

9. A camera, comprising:

the light emitting display device according to claim 8;

an image taking unit taking an image of a subject;

an image signal processing unit processing a signal of an image taken in the image taking unit, wherein an image signal subjected to signal processing in the image signal processing unit is displayed in the light emitting display device.

10. A driving method of a pixel circuit comprising a light emitting element, and a thin film transistor that supplies to the light emitting element a first current controlling a gray scale according to luminance-current characteristics of the light emitting element,

wherein the thin film transistor has

a back gate electrode,

a driving period in which the thin film transistor supplies the first current to the light emitting element and

a writing period in which a second current is written to the thin film transistor before the driving period in order to supply the first current from the thin film transistor during the driving period are set,

the difference between the driving period and the writing period in voltage to be applied to the back gate electrode rendering the driving period and the writing period different from each other in a current capability determined by a gate voltage of the thin film transistor.

11. The driving method of a pixel circuit according to claim 10, wherein the second current is larger than the first current.

12. The driving method of a pixel circuit according to claim 10, wherein a voltage applied to the back gate electrode in the writing period is set so that the current capability is higher than that controlled by the voltage applied to the back gate electrode in the driving period.

13. The driving method of a pixel circuit according to claim 10, wherein the second current from outside of the pixel circuit in the writing period controls the gray scale.

14. The driving method of a pixel circuit according to claim 10, wherein the voltage provided to the back gate electrode in the writing period controls the gray scale.

15. A driving method of a light emitting display device using the driving method of a pixel circuit according to claim 10,

wherein the pixel circuits are arranged two-dimensionally, and

a voltage is provided to the back gate electrodes of a plurality of the pixel circuits arranged in a row direction for each row.