Active matrix OLED display with normally-on thin-film transistors
A pixel circuit includes a circuit element connected to a gate of a first transistor and ground, which receives a select input. A data line is coupled to a first source/drain of the first transistor, and a second source/drain of the first transistor is coupled to a gate of a second transistor. The second transistor has a drain connected to a supply voltage and a source connected to a resistive element. The resistive element connects to an organic light emitting diode (OLED), which connects to the ground.
1. A pixel circuit, comprising:
a circuit element including a capacitive element and a diode element connected to a gate of a first transistor and ground, which receives a select input and maintains the select input less than or equal to a potential of the ground; and
a data line coupled to a first source/drain of the first transistor and a second source/drain of the first transistor is coupled to a gate of a second transistor, the second transistor having a drain connected to a supply voltage and a source connected to a resistive element, the resistive element connecting to an organic light emitting diode (OLED), which connects to the ground.
2. The pixel circuit as recited in claim 1 , wherein the circuit element downshifts the select input in accordance with a pinch off voltage of the first transistor.
3. The pixel circuit as recited in claim 1 , wherein a threshold voltage of the OLED is selected to be greater than an absolute value of a pinch-off voltage of the second transistor.
4. The pixel circuit as recited in claim 1 , wherein the second transistor is coupled to a supply voltage and a pixel current flows through the second transistor, the resistive element and the OLED, the resistive element limits the pixel current to a range, and the supply voltage biases the second transistor within a saturation regime so that supply voltage dependence of the pixel current is reduced or eliminated.
5. The pixel circuit as recited in claim 1 , wherein the resistive element includes a transparent conductor that connects the source of the second transistor to an anode of the OLED.
6. The pixel circuit as recited in claim 5 , wherein the transparent conductor includes a bilayer including a high resistance layer formed using a dilution ratio of formation gases.
7. The pixel circuit as recited in claim 5 , wherein the transparent conductor is formed on a liner in a contact hole corresponding with the source of the second transistor.
8. The pixel circuit as recited in claim 7 , wherein the liner includes a vertical stack of materials.
9. The pixel circuit as recited in claim 5 , wherein the transparent conductor is formed with a lateral break and a bridge is formed in the lateral break to provide the resistive element.
10. The pixel circuit as recited in claim 1 , wherein the ground includes a global ground.
11. The pixel circuit as recited in claim 10 , wherein the global ground is blanket deposited over an array of pixel circuits.
12. The pixel circuit as recited in claim 1 , wherein the first and second transistors include low temperature polysilicon (LTPS) material.
13. The pixel circuit as recited in claim 1 , wherein the first and second transistors include heterojunction field effect transistors.
14. A pixel circuit, comprising:
a circuit element including a capacitive element and a diode element connected to a gate of a first transistor and ground, which receives a select input and downshifts the select input in accordance with a pinch off voltage of the first transistor; and
a data line coupled to a first source/drain of the first transistor and a second source/drain of the first transistor is coupled to a gate of a second transistor, the second transistor having a drain connected to a supply voltage and a source connected to a resistive element, the resistive element connecting to an organic light emitting diode (OLED), which connects to the ground.
15. A system, comprising:
select lines running parallel to one another in a first direction;
data lines running parallel to one another in a second direction transverse to the first direction;
a row driver connected to the select lines to drive the select lines;
a column driver connected to the data lines to drive the data lines; and
pixel circuits connected to the select lines and data lines at cross-over positions, the pixel circuits employing a circuit element including a capacitive element and a diode element connected to a gate of a first transistor and ground, which receives a select input and maintains the select input less than or equal to a potential of the ground, and a data line coupled to a first source/drain of the first transistor and a second source/drain of the first transistor, which is coupled to a gate of a second transistor, the second transistor having a drain connected to a supply voltage and a source connected to a resistive element, the resistive element connecting to an organic light emitting diode (OLED), which connects to the ground.
16. The system as recited in claim 15 , wherein the circuit element maintains the select input less than or equal to a potential of the ground.
17. The system as recited in claim 15 , wherein a threshold voltage of the OLED is selected to be greater than an absolute value of a pinch-off voltage of the second transistor.
18. The system as recited in claim 15 , wherein the circuit element serves an entire row of pixel circuits.
19. The system as recited in claim 15 , wherein the resistive element includes a transparent conductor that connects the source of the second transistor to an anode of the OLED.
20. The system as recited in claim 15 , wherein the resistive element is selected from the group consisting of a bilayer, a liner in a contact hole, a vertical stack of materials and a bridge formed in a lateral break in a transparent conductor or metal which forms or is connected to an anode of the OLED.