Light emissive active matrix display devices with optical feedback effective on the timing, to counteract ageing
View Patent ↗In an active matrix display, each pixel has a storage capacitor for storing a voltage to be used for addressing a drive transistor. A discharge transistor is provided for discharging the storage capacitor thereby to switch off the drive transistor. The timing of this is controlled by a light-dependent device which is illuminated by the display element. The drive transistor is controlled to provide a constant light output from the display element, and the duration is controlled in dependence on the data voltage. Optical feedback is used to alter further the timing of operation of the discharge transistor to provide ageing compensation of the display element and compensation for changes in the drive transistor.
1. An active matrix display device comprising an array of display pixels, each pixel comprising:
a current-driven light emitting display element;
a drive transistor for driving a current through the display element;
a storage capacitor for storing a voltage to be used for addressing the drive transistor;
a discharge transistor for discharging the storage capacitor thereby to switch off the drive transistor; and
a light-dependent device for controlling the timing of the operation of the discharge transistor by varying the gate voltage applied to the discharge transistor in dependence on the light output of the display element, wherein a discharge capacitor is provided between the gate of the discharge transistor and a constant voltage line, and the light dependent device is for charging or discharging the discharge capacitor.
2. A device as claimed in claim 1 , wherein the light-dependent device controls the timing of the switching of the discharge transistor from an off to an on state.
3. A device as claimed in claim 1 , wherein the light dependent device comprises a photodiode.
4. A device as claimed in claim 1 , wherein each pixel further comprises an address transistor connected between a data signal line and an input to the pixel.
5. A device as claimed in claim 1 , wherein the drive transistor is connected between a power supply line and the display element.
6. A device as claimed in claim 5 , wherein the storage capacitor is connected between the gate and source of the drive transistor.
7. A device as claimed in claim 1 , wherein each pixel further comprises a charging transistor connected between a charging line and the gate of the drive transistor.
8. A device as claimed in claim 7 , wherein the charging line comprises a power supply line which supplies power to the display element and the drive transistor.
9. A device as claimed in claim 1 , wherein each pixel further comprises a bypass transistor connected between the source of the drive transistor and a bypass line.
10. A device as claimed in claim 1 , wherein each pixel further comprises an isolating transistor connected in series with the drive transistor.
11. A device as claimed in claim 1 , wherein each pixel comprises first and second address transistors each connected between the data signal line and a respective one of the storage capacitor and the gate of the discharge transistor.
12. A device as claimed in claim 11 , wherein one of the first and second address transistors is an n-type transistor and the other is a p-type transistor.
13. A device as claimed in claim 1 , wherein the transistors of each pixel comprise amorphous silicon n-type transistors.
14. A device as claimed in claim 1 , wherein the transistors of each pixel comprise polysilicon transistors.
15. A device as claimed in claim 1 , wherein the transistors of each pixel comprise p-type polysilicon transistors.
16. A device as claimed in claims 1 , wherein the transistors of each pixel comprise microcrystalline transistors.
17. A device as claimed in claim 1 , wherein each pixel further comprises a threshold voltage storage capacitor for storing a threshold voltage of the drive transistor, wherein the threshold voltage storage capacitor is in series with the storage capacitor between the gate and source of the drive transistor.
18. A device as claimed in claim 17 , wherein each pixel comprises a transistor is to enable the threshold capacitor to be charged.
19. A device as claimed in claim 18 , wherein each pixel further comprises a transistor is for turning the discharge transistor on.
20. A device as claimed in claim 1 , wherein the gate insulator of the discharge transistor is thinner than the gate insulator of the drive transistor.
21. A device as claimed in claim 1 , wherein the gate insulator of the discharge transistor is thinner than the gate insulator of an address transistor connected between a data signal line and node corresponding to the gate of the drive transistor.
22. A device as claimed in claim 21 , wherein the gate insulator of the drive transistor comprises first and second gate insulator layers, and the gate insulator of the address transistor comprises only one of the first and second gate insulator layers.
23. A device as claimed in claim 1 , wherein each pixel further comprises a threshold storage capacitor for storing a threshold voltage of the discharge transistor.
24. A device as claimed in claim 1 , wherein the current-driven light emitting display element comprises an electroluminescent display element.
25. A method of driving an active matrix display device comprising an array of display pixels each comprising a drive transistor and a current-driven light emitting display element, the method comprising, for each addressing of the pixel:
applying a drive voltage to an input of the pixel;
storing a voltage derived from the drive voltage on a discharge capacitor;
driving the drive transistor using a voltage on a storage capacitor;
switching on a discharge transistor using charge flow through a light dependent device illuminated by the light output of the display element, the charge flow charging or discharging the discharge capacitor; and
discharging the storage capacitor using the discharge transistor thereby to turn off the drive transistor.