Active matrix display devices
View Patent ↗An active matrix display device uses an amorphous silicon drive transistor for driving a current through an LED display element. First and second capacitors are connected in series between the gate and source of the drive transistor, with a data input to the pixel provided to the junction between the first and second capacitors. The second capacitor is charged to a pixel data voltage, and a drive transistor threshold voltage is stored on the first capacitor. This pixel arrangement enables a threshold voltage to be stored on the first capacitor, and this can be done each time the pixel is addressed, thereby compensating for age-related changes in the threshold voltage.
1. An active matrix device comprising an array of display pixels, each pixel comprising:
a current driven light emitting display element;
an amorphous silicon drive transistor for driving a current through the display element;
first and second capacitors connected in series between gate and source or drain of the drive transistor, a data input to the pixel being provided to a junction between the first and second capacitors thereby to charge the second capacitor to a voltage derived from a pixel data voltage associated with the data input, and a voltage derived from a drive transistor threshold voltage being stored on the first capacitor; and
a further transistor connected across terminals of the second capacitor.
2. The device as claimed in claim 1 , wherein each pixel further comprises an input first transistor connected between an input data line and the junction between the first and second capacitors.
3. The device as claimed in claim 1 , wherein the drain of the drive transistor is connected to a power supply line.
4. The device as claimed in claim 1 , wherein each pixel further comprises a second transistor connected between the gate and drain of the drive transistor.
5. The device as claimed in claim 4 , wherein the second transistor is controlled by a first gate control line which is shared between a row of pixels.
6. The device as claimed in claim 5 , wherein the further transistor is controlled by a further gate control line, and the first and further gate control lines comprise a single shared control line.
7. The device as claimed in claim 1 , wherein the first and second capacitors are connected in series between the gate and source of the drive transistor.
8. The device as claimed in claim 1 , wherein the further transistor is controlled by a further gate control line which is shared between a row of pixels.
9. The device as claimed in claim 1 , wherein the first and second capacitors are connected in series between the gate and drain of the drive transistor.
10. The device as claimed in claim 1 , wherein each pixel further comprises a switching transistor connected between the drive transistor source and a ground potential line.
11. The device as claimed in claim 10 , wherein the switching transistor is controlled by a switching gate control line which is shared between a row of pixels.
12. The device as claimed in claim 11 , wherein the ground potential line is shared between a row of pixels and comprises the switching gate control line for the switching transistors of an adjacent row of pixels.
13. device as claimed in claim 1 ,
wherein a capacitor arrangement including the first and second capacitors is connected between the gate and source of the drive transistor, and the source of the drive transistor is connected to a ground line.
14. The device as claimed in claim 13 , wherein the drain of the drive transistor is connected to one terminal of the display element, the other terminal of the display element being connected to a power supply line.
15. The device as claimed in claim 13 , wherein each pixel further comprises a further transistor connected between the gate and drain of the drive transistor.
16. The device as claimed in claim 15 , wherein the further transistor is controlled by a gate control line which is shared between a row of pixels.
17. The device as claimed in claim 1 , wherein each pixel further comprises a second drive transistor.
18. The device as claimed in claim 17 , wherein the second drive transistor is provided between a power supply line and the first drive transistor.
19. The device as claimed in claim 18 , wherein the gate and drain of the second drive transistor are connected together.
20. The device as claimed in claim 17 , wherein the second drive transistor is provided between the first drive transistor and the display element.
21. The device as claimed in claim 20 , wherein another transistor is connected between the gate and drain of the second drive transistor.
22. The device as claimed in claim 20 , wherein each pixel further comprises another transistor connected between the gate of the second drive transistor and a ground potential line.
23. The device as claimed in claim 1 , wherein the drive transistor comprises an n-type transistor.
24. The device as claimed in claim 1 , wherein the display element comprises an electroluminescent display element.
25. The device as claimed in claim 24 , wherein the electroluminescent display element comprises an electrophosphorescent organic electroluminescent display element.
26. An active matrix device comprising an array of display pixels, each pixel comprising:
a current driven light emitting display element;
an amorphous silicon drive transistor for driving a current through the display element;
first and second capacitors connected in series between gate and source or drain of the drive transistor, a data input to the pixel being provided to a junction between the first and second capacitors thereby to charge the second capacitor to a voltage derived from a pixel data voltage associated with the data input, and a voltage derived from a drive transistor threshold voltage being stored on the first capacitor; and
a further transistor connected between the junction between the first and second capacitors, and the source of the drive transistor.
27. The device as claimed in claim 26 , wherein the further transistor is controlled by a further gate control line which is shared between a row of pixels.
28. An active matrix device comprising an array of display pixels, each pixel comprising:
a current driven light emitting display element;
an amorphous silicon drive transistor for driving a current through the display element;
first and second capacitors connected in series between gate and source or drain of the drive transistor, wherein the source of the drive transistor is connected to a round line, a data input to the pixel being provided to a junction between the first and second capacitors thereby to charge the second capacitor to a voltage derived from a pixel data voltage associated with the data input, and a voltage derived from a drive transistor threshold voltage being stored on the first capacitor; and
a shorting transistor connected across terminals of the second capacitor.
29. An active matrix device comprising an array of display pixels, each pixel comprising:
a current driven light emitting display element;
an amorphous silicon drive transistor for driving a current through the display element;
first and second capacitors connected in series between gate and source or drain of the drive transistor, a data input to the pixel being provided to a junction between the first and second capacitors thereby to charge the second capacitor to a voltage derived from a pixel data voltage associated with the data input, and a voltage derived from a drive transistor threshold voltage being stored on the first capacitor; and
a charging transistor connected between a power supply line and the drain of the drive transistor.
30. A method of driving an active matrix display device comprising an array of current driven light emitting display pixels, each pixel comprising an display element and an amorphous silicon drive transistor for driving a current through the display element, the method comprising, for each pixel:
driving a current through the drive transistor ground, and charging a first capacitor to the resulting gate-source voltage;
discharging the first capacitor until the drive transistor turns off, the first capacitor thereby storing a threshold voltage;
charging a second capacitor, in series with the first capacitor between the gate and source or drain of the drive transistor, to a data input voltage; and
using the drive transistor to drive a current through the display element using a gate voltage that is derived tram the voltages, across the first and second capacitors.
31. The method as claimed in claim 30 , wherein the step of charging a second capacitor is carried out by switching on an address transistor connected between a data line, and an input to the pixel.
32. The method as claimed in claim 31 , wherein the address transistor for each pixel in a row is switched on simultaneously by a common row address control line.
33. The method as claimed in claim 32 , wherein the address transistors for one row of pixels are turned on substantially immediately after the address transistors for an adjacent row are turned off.
34. The method as claimed in claim 30 , wherein, the first capacitor of each pixel is charged to store a respective threshold voltage of the pixel drive transistor at an initial threshold measurement period of a display frame period, a pixel driving period of the frame period following the threshold measurement period.