Element substrate and a light emitting device

• US 20040189615A1
• Filed: 03/24/2004
• Published: 09/30/2004
• Est. Priority Date: 03/26/2003
• Status: Active Grant

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Osame, Mitsuaki, Anzai, Aya, Yamazaki, Yu
• Granted Patent: US 7,173,586 B2
• Time in Patent Office: Days
• US Class Current: 345/173
• CPC Class Codes: G09G 2300/0426 Layout of electrodes and connections
• CPC Class Codes: G09G 2300/0809 Several active elements per pixel in active matrix panels
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2300/0861 with additional control of the display period without amending the charge stored in a pixel memory, e.g. by means of additional select electrodes
• CPC Class Codes: G09G 2300/0866 by means of changes in the pixel supply voltage
• CPC Class Codes: G09G 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 3/2022 using sub-frames
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3266 Details of drivers for scan electrodes
• CPC Class Codes: G09G 3/3291 in which the data driver supplies a variable data voltage for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: H10K 59/12 Active-matrix OLED [AMOLED] displays

Abstract

A light emitting device and an element substrate which are capable of suppressing variations in the luminance intensity of a light emitting element among pixels due to characteristic variations of a driving transistor without suppressing off-current of a switching transistor low and increasing storage capacity of a capacitor. According to the invention, a driving transistor also serves as an erasing transistor and the driving transistor is operated in a saturation region. The gate of the driving transistor is connected to an erasing scan line and it can be selected whether or not to flow current by a potential of the erasing scan line. In addition, a current controlling transistor which operates in a linear region is connected in series to the driving transistor, thus a video signal transmitting a light emission or non-emission of a pixel is input to the gate of the current controlling transistor through a switching transistor.

Claims

1. A light emitting device with a pixel comprising:

a light emitting element;

a first transistor for determining a current value flowing in the light emitting element;

a second transistor for determining a light emission or non-emission of the light emitting element according to a video signal; and

a third transistor for determining an input of the video signal,

wherein the light emitting element is connected in series to the first transistor and the second transistor between a first power supply and a third power supply,

wherein a gate electrode of the third transistor is connected to a first scan line,

wherein a gate electrode of the first transistor is connected to a second scan line, and

wherein the first transistor is operated in a saturation region and the second transistor is operated in a linear region.

2. The light emitting device according to claim 1, wherein each of the first transistor and the second transistor has the same conductivity.

3. The light emitting device according to claim 1, wherein the first transistor is a depletion mode transistor.

4. The light emitting device according to claim 1, wherein a channel length of the first transistor is longer than its channel width, and a channel length of the second transistor is equal to or shorter than its channel width.

5. The light emitting device according to claim 4, a ratio of the channel length to the channel width of the first transistor is five or more.

6. An element substrate comprising a pixel comprising:

a pixel electrode;

a first transistor for determining a current value flowing in the light emitting element;

a second transistor for determining a light emission or non-emission of the light emitting element according to a video signal; and

a third transistor for controlling an input of the video signal,

wherein the first transistor is connected in series to the second transistor between a first power supply and a third power supply,

a gate electrode of the third transistor is connected to a first scan line,

a gate electrode of the first transistor is connected to a second scan line, and

wherein the first transistor is operated in a saturation region and the second transistor is operated in a linear region.

7. The element substrate according to claim 6, wherein each of the first transistor and the second transistor has the same conductivity.

8. The element substrate according to claim 6, wherein the first transistor is a depletion mode transistor.

9. The element substrate according to claim 6, wherein a channel length of the first transistor is longer than its channel width, and a channel length of the second transistor is equal to or shorter than its channel width.

10. The element substrate according to claim 9, a ratio of the channel length to the channel width of the first transistor is five or more.