Electroluminescent display device having pixels with NMOS transistors

1. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

an electroluminescent display element;

an amorphous silicon or microcrystalline silicon first drive NMOS transistor having a source directly connected to the anode of the display element and having a drain connected to a power supply line;

a storage capacitor between the anode of the display element and a gate of the first drive transistor; and

an amorphous silicon or microcrystalline silicon second drive NMOS transistor having a source directly connected to the anode of the display element and having a drain directly connected to a holding voltage for supplying the holding voltage, minus a drain to source voltage drop across the second drive transistor, to the anode of the display element,

wherein when the holding voltage is supplied to the anode of the display element, a gate-source voltage, for the first drive transistor, is stored on the storage capacitor.

2. The device as claimed in claim 1 , wherein the holding voltage is provided by the power supply line.

3. The device as claimed in claim 1 , wherein the power supply line is a first power supply line and the holding voltage is provided by a second power supply line that is separate from the first power supply line.

4. The device as claimed in claim 3 , wherein the second power supply line is shared between pixels in a row of the array.

5. The device as claimed in any one of claims 1 to 4 , wherein the gate of the first drive transistor is coupled to a data signal line through an address transistor.

6. The device as claimed in claim 1 , wherein the gate of the first drive transistor is coupled to a data signal line through an address transistor, and wherein the data signal line comprises a column conductor shared between pixels in a column of the array.

7. The device as claimed in claim 6 , wherein the gate of the address transistor is coupled to a row conductor shared between pixels in a row of the array.

8. The device as claimed in claim 1 , wherein the first and second drive transistors comprise microcrystalline silicon TFTs comprising silicon crystallites of size 40 nm-140 nm in an amorphous silicon matrix.

9. A method of driving the pixels of an active matrix electroluminescent display device comprising an array of display pixels each having an electroluminescent display element, the method comprising:

holding the voltage across the display element by applying a holding voltage through a second amorphous silicon or microcrystalline silicon NMOS transistor having a source directly connected to an anode of the electroluminescent display element and having a drain directly connected to the holding voltage, the holding voltage holding a source voltage of a first amorphous silicon or microcrystalline silicon NMOS transistor that is directly connected to the anode of the electroluminescent display element;

while holding the first voltage across the display element, storing a desired gate-source voltage, for the first transistor, on a storage capacitor connected between the gate and source of the first transistor, the gate-source voltage corresponding to a desired source-drain current for driving the display element;

removing the first voltage from the display element; and

driving the desired source-drain current through the electroluminescent display element.

10. The method as claimed in claim 9 , wherein the desired source-drain current is driven through the second transistor by applying a first power supply voltage to the second transistor.

11. The method as claimed in claim 10 , wherein the first power supply voltage is not applied to the second transistor while the first voltage across the display is held.

12. The method as claimed in claim 11 , wherein the first power supply voltage and the holding voltage are provided by a shared power supply line.

13. The method as claimed in any one of claims 9 to 12 , wherein storing a desired gate-source voltage on a storage capacitor comprises coupling data from a data signal line to the storage capacitor through an address transistor.

14. An active matrix electroluminescent display device comprising an array of display pixels, each pixel comprising:

an electroluminescent display element;

an amorphous silicon or microcrystalline silicon first drive NMOS transistor having a source directly connected to the anode of the display element and having a drain connected to a power supply line;

a storage capacitor between the anode of the display element and the gate of the first drive transistor; and

an amorphous silicon or microcrystalline silicon second drive NMOS transistor having a source directly connected to the anode of the display element and having a drain directly connected to a holding voltage for supplying the holding voltage, minus a drain to source voltage drop across the second drive transistor, to the anode of the display element, wherein the gate of the first drive transistor is coupled to a data signal line through an address transistor, and wherein the first and second drive transistors comprise microcrystalline silicon TFTs comprising silicon crystallites of size 40 nm-140 nm in an amorphous silicon matrix,

wherein when the holding voltage is supplied to the anode of the display element, a gate-source voltage, for the first drive transistor, is stored on the storage capacitor.

15. The device as claimed in claim 1 , wherein a cathode of the electroluminescent display element is directly connected to ground.

16. The method as claimed in claim 9 , wherein a cathode of the electroluminescent display element is directly connected to ground.