Active matrix light emitting diode display

• US 6,392,617 B1
• Filed: 10/27/1999
• Issued: 05/21/2002
• Est. Priority Date: 10/27/1999
• Status: Expired due to Term

• Current Assignee: InnoLux Corporation
• Original Assignee: Agilent Technologies, Inc.
• Inventors: Gleason, Robert E.
• Primary Examiner(s): SPENCER, WILLIAM C
• Application Number:
  US09/428,338
• Time in Patent Office: 937 Days
• Field of Search: 345/81, 345/82, 345/83, 345/76, 345/77, 345/80, 345/207, 315/149, 315/158
• US Class Current: 345/82
• 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 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 2320/045 Compensation of drifts in the characteristics of light emitting or modulating elements
• CPC Class Codes: G09G 2360/148 the light being detected by light detection means within each pixel
• CPC Class Codes: G09G 3/2081 with combination of amplitude modulation and time modulation space and time error diffusion G09G3/2066
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: H10K 59/13 comprising photosensors that control luminance

Abstract

An active matrix pixel within an active matrix display includes a photodiode that is optically connected to a light emitting diode within the pixel in order to detect a portion of the luminous flux that is generated by the light emitting diode. The photodiode discharges excess charge within the pixel in response to the detected portion of luminous flux. Once the excess charge is discharged, the light emitting diode stops emitting light. In an embodiment, the gate of a drive transistor is controlled by the charge on a storage node. If the charge on the storage node sets a voltage that exceeds the threshold voltage of the drive transistor then the drive transistor conducts. The amount of charge on the storage node above that which is needed to set the threshold voltage is referred to as the excess charge.

Claims

1. An active matrix pixel within an active matrix display comprising:

a light emitting diode, that is specific to a pixel of said active matrix display, for generating luminous flux;

means, that is specific to said pixel, for storing an excess charge; and

means, that is specific to said pixel and optically connected to said light emitting diode, for detecting a portion of said luminous flux that is generated by said light emitting diode, and for discharging said excess charge in response to said detected portion of said luminous flux.

2. The active matrix pixel of claim 1 further including means for preventing said generation of luminous flux when said excess charge has been discharged.

3. The active matrix pixel of claim 2 wherein said means for preventing is a drive transistor having a gate that is responsive to said means for storing.

4. The active matrix pixel of claim 1 wherein said light emitting diode is an organic light emitting diode, and wherein said means for detecting is a photodiode formed within said pixel to receive said portion of said luminous flux.

5. The active matrix pixel of claim 4 further including a reflective layer between said light emitting diode and said photodiode, with said reflective layer having an opening that allows said portion of said luminous flux to be detected by said photodiode.

6. The active matrix pixel of claim 1 further including means for isolating said means for storing from said light emitting diode while excess charge is being stored.

7. The active matrix pixel of claim 6 wherein said means for isolating includes a transistor having a gate that is connected to said storage node, a source that is connected to a complement of an address line of said pixel, and a drain that is connected to a drive transistor.

8. The active matrix pixel of claim 7 further including means for providing resistance that is located between said means for isolating and said drive transistor.

9. A method of controlling light emitted from each pixel in an active matrix array comprising steps of:

charging a storage node with a charge that sets a voltage on the storage node that exceeds the threshold voltage of a transistor, said storage node and said transistor being incorporated into a pixel of said active matrix array;

generating luminous flux from a light emitting diode that is part of said pixel while said charge on said storage node maintains said voltage that exceeds said threshold voltage of said transistor;

detecting a portion of said luminous flux that is generated by said light emitting diode;

discharging said charge on said storage node in response to said detected portion of said luminous flux; and

ceasing said generation of said luminous flux from said light emitting diode once said charge on said storage node sets a voltage on said storage node that drops below said threshold voltage of said transistor.

10. The method of claim 9 wherein said step of charging said storage node includes a step of setting an address transistor to high for a measured time interval.

11. The method of claim 9 wherein said step of discharging said charge is a step of discharging said charge in proportion to a value that is representative of said detected portion of luminous flux.

12. The method of claim 11 wherein said step of ceasing includes a step of ceasing said generation of said luminous flux from said light emitting diode when a value that is representative of said discharged charge is exceeded by a value that is representative of said detected portion of said luminous flux.

13. An active matrix pixel within an active matrix display comprising:

an address line;

a data line;

a storage node for storing an electrical charge;

an address transistor having a gate that is activated from said address line, a source that is connected to said data line, and a drain that is connected to said storage node;

a drive transistor having a gate that is responsive to said storage node;

a light emitting diode connected to a circuit that includes said drive transistor, wherein said light emitting diode emits a luminous flux when said drive transistor completes said circuit; and

a photodiode that is optically connected to said light emitting diode in order to receive a portion of said luminous flux that is emitted from said light emitting diode, and that is electrically connected to said storage node in order to discharge said electrical charge on said storage node in proportion to said portion of said luminous flux that is received by said photodiode.

14. The active matrix pixel of claim 13 wherein said gate of said drive transistor is directly connected to said storage node.

15. The active matrix pixel of claim 13 further including an isolation transistor having a gate that is connected to said storage node and a drain that is connected to said gate of said drive transistor.

16. The active matrix pixel of claim 15 wherein said isolation transistor has a source that is connected to a complement of said address line.

17. The active matrix pixel of claim 15 further including means for providing resistance between said drain of said isolation transistor and said gate of said drive transistor.

18. The active matrix pixel of claim 13 wherein said drive transistor is a bipolar transistor.

19. The active matrix pixel of claim 13 wherein said drive transistor is a thin film MOS transistor.

20. The active matrix pixel of claim 13 further including a reflective metal layer that is located between said light emitting diode and said photodiode, said reflective metal layer having an opening that enables said portion of said luminous flux to be received by said photodiode.