System and methods for extraction of threshold and mobility parameters in AMOLED displays
A system reads a desired circuit parameter from a pixel circuit that includes a light emitting device, a drive device to provide a programmable drive current to the light emitting device, a programming input, and a storage device to store a programming signal. One embodiment of the extraction system turns off the drive device and supplies a predetermined voltage from an external source to the light emitting device, discharges the light emitting device until the light emitting device turns off, and then reads the voltage on the light emitting device while that device is turned off. The voltages on the light emitting devices in a plurality of pixel circuits may be read via the same external line, at different times.
1. A display comprising:
a first signal line for outputting an output signal;
a plurality of pixel circuits, each pixel circuit comprising:
a light emitting device;
a drive transistor for controlling current supplied to the light emitting device, said drive transistor having a gate terminal, a source terminal and a drain terminal;
a storage device coupled between the gate terminal of the drive transistor and one of the source terminal and the drain terminal of the drive transistor, a first node of the pixel circuit located between the storage device and the drive transistor;
a first switching transistor controllably coupling the first signal line to a second node of the pixel circuit located between the storage device and the one of the source terminal and the drain terminal of the drive transistor;
and
a controller coupled to each pixel circuit and configured to supply controlling input signals to the pixel circuit in a predetermined sequence to produce the output signal which is a function of a parameter of the pixel circuit, the sequence including:
i) supplying a first initial voltage to the first node;
ii) turning off the first switching transistor, and controlling the drive transistor so that current flows through the light emitting device and the drive transistor, the magnitude of said current being controlled by a gate voltage applied to the gate terminal of the drive transistor discharged by the storage device; and
iii) turning on the first switching transistor and extracting the parameter of the pixel circuit by reading the output signal over the first signal line.
2. The display according to claim 1 , wherein the controller is configured to extract the parameter after the light emitting device turns off; wherein the output signal is a voltage signal which is a function of the on threshold voltage of the light emitting device.
3. The display according to claim 1 , wherein the controller is configured to extract the parameter after the drive transistor turns off; wherein the output signal is a voltage signal which is a function of the threshold voltage of the drive transistor.
4. The display according to claim 1 , wherein the controller is configured to supply a second initial voltage to the second node, and supply the first initial voltage, externally via the first signal line.
5. The display according to claim 1 , wherein the controller is configured to supply the first initial voltage to the first node via the first signal line, and to supply a second initial voltage to the second node via a second signal line coupleable to the second node.
6. The display according to claim 1 , wherein the controller is configured to supply controlling input signals to the first switching transistor and a second switching transistor coupled between a second signal line and the second node to turn off both the first and second switching transistors to reset the voltages at the first and second nodes.
7. The display according to claim 1 , further comprising a third switching transistor controllably coupling a supply voltage to the drive transistor;
wherein the first node is between the third switching transistor and the drive transistor; and
wherein the controller is configured to delay connecting the supply voltage to the drive transistor in step ii) using the third switching transistor.
8. The display according to claim 1 , wherein the controller is configured to:
turn on the drive transistor and measure current or voltage of the drive transistor over the first signal line while changing a driving voltage between the gate terminal and the one of the source terminal and the drain terminal of the drive transistor to operate the drive transistor in the linear regime during one time interval and in the saturated regime during a second time interval, and
extract the voltage of the light emitting device from the relationship of the currents or voltages measured with the drive transistor operating in the two regimes.
9. The display according to claim 1 , wherein the controller is configured to turn off the drive transistor during step ii); and
extract an off voltage of the light emitting device when the light emitting device turns off during step iii).
10. The display according to claim 1 , wherein the controller is configured to determine a parasitic capacitance by:
determining a first voltage or current on the first node during step i);
determining a second voltage or current on the first node during step iii); and
based on a pixel model, calculate the parasitic capacitance from the first and second voltages or currents.
11. A method of operating a display, the display comprising:
a first signal line for outputting an output signal;
a plurality of pixel circuits, each pixel circuit comprising:
a light emitting device;
a drive transistor for controlling current supplied to the light emitting device, said drive transistor having a gate terminal, a source terminal and a drain terminal;
a storage device coupled between the gate terminal of the drive transistor and one of the source terminal and the drain terminal of the drive transistor, a first node of the pixel circuit located between the storage device and the drive transistor;
a first switching transistor controllably coupling the first signal line to a second node of the pixel circuit located between the storage device and the one of the source terminal and the drain terminal of the drive transistor;
and
a controller coupled to each pixel circuit and capable of supplying controlling input signals to the pixel circuit in a predetermined sequence to produce the output signal which is a function of a parameter of the pixel circuit,
the method comprising:
i) supplying a first initial voltage to the first node;
ii) turning off the first switching transistor, and controlling the drive transistor so that current flows through the light emitting device and the drive transistor, the magnitude of said current being controlled by a gate voltage applied to the gate terminal of the drive transistor discharged by the storage device; and
iii) turning on the first switching transistor and extracting the parameter of the pixel circuit by reading the output signal over the first signal line.
12. The method according to claim 11 , wherein step iii) includes extracting the parameter after the light emitting device turns off; wherein the output signal is a voltage signal which is a function of the threshold voltage of the light emitting device.
13. The method according to claim 11 , wherein step iii) includes extracting the parameter after the drive transistor turns off; wherein the output signal is a voltage signal which is a function of the threshold voltage of the drive transistor.
14. The method according to claim 11 , wherein step i) includes supplying a second initial voltage to the second node, and supplying the first initial voltage, externally via the first signal line.
15. The method according to claim 11 , wherein step i) includes supplying the first initial voltage to the first node via the first signal line, and supplying a second initial voltage to the second node via a second signal line coupleable to the second node.
16. The method according to claim 11 , further comprising supplying controlling input signals to the first switching transistor and a second switching transistor coupled between a second signal line and the second node to turn off both the first and second switching transistors to reset the voltages at the first and second nodes.
17. The method according to claim 11 , wherein each pixel further comprises a third switching transistor controllably coupling a supply voltage to the drive transistor;
wherein the first node is between the third switching transistor and the drive transistor; and
wherein the controller is capable of delaying connecting the supply voltage to the drive transistor in step ii) using the third switching transistor.
18. The method according to claim 11 , wherein step iii) includes turning on the drive transistor and measuring current or voltage of the drive transistor over the first signal line while changing a driving voltage between the gate terminal and the one of the source terminal and the drain terminal of the drive transistor to operate the drive transistor in the linear regime during one time interval and in the saturated regime during a second time interval, and
extracting the voltage of the light emitting device from the relationship of the currents or voltages measured with the drive transistor operating in the two regimes.
19. The method according to claim 11 , further comprising:
turning off the drive transistor during step ii); and
extracting an off voltage of the light emitting device when the light emitting device turns off during step iii).
20. The method according to claim 11 , wherein step iii) includes determining a parasitic capacitance by:
determining a first voltage or current on the first node during step i);
determining a second voltage or current on the first node during step iii); and
based on a pixel model, calculate the parasitic capacitance from the first and second voltages or currents.