Active matrix OLED display with normally-on thin-film transistors
A pixel circuit includes a first transistor, a second transistor connected to a first source/drain of the first transistor, a circuit element connected to a gate of the first transistor and ground and configured to receive a select input and maintain the select input less than or equal to a potential of the ground, and a resistive element connected to an organic light emitting diode (OLED) and a first source/drain of the second transistor.
1. A pixel circuit, comprising:
a first transistor;
a second transistor connected to a first source/drain of the first transistor;
a circuit element connected to a gate of the first transistor and ground and configured to receive a select input and maintain the select input less than or equal to a potential of the ground; and
a resistive element connected to an organic light emitting diode (OLED) and a first source/drain of the second transistor, the OLED having a threshold voltage greater than an absolute value of a pinch off voltage of the second transistor.
2. The pixel circuit as recited in claim 1 , wherein the circuit element is further configured to downshift the select input in accordance with a pinch off voltage of the first transistor.
3. 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.
4. The pixel circuit as recited in claim 1 , wherein the resistive element includes a transparent conductor that connects a source/drain of the second transistor to an anode of the OLED.
5. The pixel circuit as recited in claim 4 , wherein the transparent conductor includes a bilayer including a high resistance layer formed using a dilution ratio of formation gases.
6. The pixel circuit as recited in claim 4 , wherein the transparent conductor is formed on a liner in a contact hole corresponding with the source of the second transistor.
7. The pixel circuit as recited in claim 6 , wherein the liner includes a vertical stack of materials.
8. The pixel circuit as recited in claim 4 , wherein the transparent conductor is formed with a lateral break and a bridge is formed in the lateral break to provide the resistive element.
9. The pixel circuit as recited in claim 1 , wherein the ground includes a global ground.
10. The pixel circuit as recited in claim 9 , wherein the global ground is blanket deposited over an array of pixel circuits.
11. The pixel circuit as recited in claim 1 , wherein the first and second transistors include low temperature polysilicon (LTPS) material.
12. The pixel circuit as recited in claim 1 , further comprising a data line coupled to a second source/drain of the first transistor, wherein the second transistor further includes a second source/drain connected to a supply voltage.
13. A pixel circuit, comprising:
a first transistor;
a second transistor connected to a first source/drain of the first transistor;
a circuit element connected to a gate of the first transistor and ground and configured to receive a select input and downshift the select input in accordance with a pinch off voltage of the first transistor; and
a resistive element connected to an organic light emitting diode (OLED) and a first source/drain of the second transistor, the OLED having a threshold voltage greater than an absolute value of a pinch off voltage of the second transistor.
14. The pixel circuit as recited in claim 13 , wherein the circuit element is further configured to maintain the select input less than or equal to a potential of the ground.
15. The pixel circuit as recited in claim 13 , further comprising a data line coupled to a second source/drain of the first transistor, wherein the second transistor further includes a second source/drain connected to a supply voltage.
16. The pixel circuit as recited in claim 13 , wherein the resistive element includes a transparent conductor that connects the source of the second transistor to an anode of the OLED.
17. The pixel circuit as recited in claim 16 , wherein the transparent conductor is selected from the group consisting of: a bilayer, a transparent conductor formed on a liner in a contact hole, a transparent conductor including a vertical stack of materials and a transparent conductor formed with a bridge formed in a lateral break to provide the resistive element.
18. A pixel circuit, comprising:
a first transistor;
a second transistor connected to a first source/drain of the first transistor;
a circuit element connected to a gate of the first transistor and ground and configured to receive a select input and maintain the select input less than or equal to a potential of the ground; and
a resistive element connected to an organic light emitting diode (OLED) and a first source/drain of the second transistor;
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.