Display device and method of driving the same

• US 6,697,057 B2
• Filed: 10/26/2001
• Issued: 02/24/2004
• Est. Priority Date: 10/27/2000
• Status: Expired due to Term

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Kimura, Hajime, Koyama, Jun
• Primary Examiner(s): Mengistu, Amare
• Application Number:
  US10/040,084
• Publication Number:
  US 20030107534A1
• Time in Patent Office: 851 Days
• Field of Search: 345/76, 345/77, 345/204, 345/211, 345/212, 345/214, 315/169.3, 315/169.2, 315/169.1
• US Class Current: 345/204
• CPC Class Codes: G09G 2300/0439 Pixel structures
• CPC Class Codes: G09G 2300/0842 forming a memory circuit, e.g. a dynamic memory with one capacitor
• CPC Class Codes: G09G 2310/0248 Precharge or discharge of column electrodes before or after applying exact column voltages
• CPC Class Codes: G09G 2310/0251 Precharge or discharge of pixel before applying new pixel voltage
• CPC Class Codes: G09G 2310/0272 Details of drivers for data electrodes, the drivers communicating data to the pixels by means of a current
• CPC Class Codes: G09G 2310/0275 Details of drivers for data electrodes, other than drivers for liquid crystal, plasma or OLED displays, not related to handling digital grey scale data or to communication of data to the pixels by means of a current
• CPC Class Codes: G09G 2310/065 Waveforms comprising zero voltage phase or pause
• CPC Class Codes: G09G 2320/0233 Improving the luminance or brightness uniformity across the screen
• CPC Class Codes: G09G 2320/029 by monitoring one or more pixels in the display panel, e.g. by monitoring a fixed reference pixel
• CPC Class Codes: G09G 2320/0295 by monitoring each display pixel
• CPC Class Codes: G09G 2320/043 Preventing or counteracting the effects of ageing
• CPC Class Codes: G09G 3/3233 with pixel circuitry controlling the current through the light-emitting element
• CPC Class Codes: G09G 3/3241 the current through the light-emitting element being set using a data current provided by the data driver, e.g. by using a two-transistor current mirror
• CPC Class Codes: G09G 3/3283 in which the data driver supplies a variable data current for setting the current through, or the voltage across, the light-emitting elements
• CPC Class Codes: H01L 27/12 the substrate being other than a semiconductor body, e.g. an insulating body
• CPC Class Codes: H10K 59/12 Active-matrix OLED [AMOLED] displays

Abstract

In a conventional EL display device, a general video signal (analog signal voltage) is sampled to be outputted to a source signal line and then inputted to a gate electrode of a driving TFT provided in each pixel. The drain current of the TFT is controlled to control the current flowing into an EL element, thereby adjusting the luminance. However, accurate gray scales cannot be obtained unless the video signal is converted before it is sampled and inputted to the source signal line since the drain current of the driving TFT does not correspond to the gate voltage of the driving TFT linearly. The invention solve this problem by converting an inputted analog signal voltage into a current, converting the current into the gate-source voltage of a correction TFT that has the same polarity as a driving TFT, supplying the gate-source voltage as a source line signal to a source signal line, and applying the source line signal to a gate electrode of the driving TFT. Thus an EL element can emit light at a luminance linearly corresponding to the inputted analog signal voltage.

Claims

1. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT;

a second TFT having the same polarity as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other; and

a power supply line connected to the first electrode of the EL element through the first TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the second TFT; and

means for equalizing the gate voltage of the first TFT with the gate voltage of the second TFT.

11. A device according to any one of claims 1 to 8, wherein the display device is incorporated into an electronic equipment selected from the group consisting of a video camera, a digital camera, a head-mountable display, a game machine, a car navigation, a personal computer, and a mobile information terminal.

2. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT;

a second TFT having the same polarity and the same threshold voltage as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other; and

a power supply line connected to the first electrode of the EL element through the first TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the second TFT; and

means for equalizing the gate voltage of the first TFT with the gate voltage of the second TFT.

3. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT;

a signal line;

a switching element for selecting the connection between the signal line and a gate electrode of the first TFT;

a second TFT having the same polarity as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other; and

a power supply line connected to the first electrode of the EL element through the first TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the second TFT;

means for equalizing the electric potential of a source region of the first TFT with the electric potential of a source region of the second TFT; and

means for equalizing the electric potential of the signal line with the electric potential of the gate electrode of the second TFT.

9. A display device according to any one of claims 3, 5, 6 and 8, wherein the first TFT and the second TFT have substantially same threshold voltage.

4. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT;

a second TFT;

a signal line connected to a gate electrode of the first TFT through the second TFT;

a third TFT having the same polarity as the first TFT, the third TFT including a gate electrode and a drain region that are connected to each other; and

a power supply line connected to the first electrode of the EL element through the first TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the third TFT;

means for equalizing the electric potential of a source region of the first TFT with the electric potential of a source region of the third TFT; and

means for equalizing the electric potential of the signal line with the electric potential of the gate electrode of the third TFT.

10. A display device according to any one of claims 4 and 7, wherein the first TFT and the third TFT have substantially same threshold voltage.

5. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT having a drain region that is connected to the first electrode of the EL element;

a second TFT having the same polarity as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other;

a power supply line connected to a source region of the first TFT; and

an operation amplifier having a non-inversion input terminal, an inversion input terminal, and an output terminal, the non-inversion input terminal being kept at an electric potential equal to the electric potential of the power supply line, the inversion input terminal being connected to a source region of the second TFT, and the output terminal being connected to the drain region of the second TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the second TFT; and

means for equalizing the electric potential of a gate electrode of the first TFT with the electric potential of the gate electrode of the second TFT.

6. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT having a drain region that is connected to the first electrode of the EL element;

a signal line;

a switching element for selecting the connection between the signal line and a gate electrode of the first TFT;

a second TFT having the same polarity as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other;

a power supply line connected to a source region of the first TFT; and

an operation amplifier having a non-inversion input terminal, an inversion input terminal, and an output terminal, the non-inversion input terminal being kept at an electric potential equal to the electric potential of the power supply line, the inversion input terminal being connected to a source region of the second TFT, and the output terminal being connected to the drain region of the second TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the second TFT; and

means for equalizing the electric potential of the signal line with the electric potential of the gate electrode of the second TFT.

7. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT having a drain region that is connected to the first electrode of the EL element;

a second TFT;

a signal line connected to a gate electrode of the first TFT through the second TFT;

a third TFT having the same polarity as the first TFT, the third TFT including a gate electrode and a drain region that are connected to each other; and

a power supply line connected to a source region of the first TFT; and

an operation amplifier having a non-inversion input terminal, an inversion input terminal, and an output terminal, the non-inversion input terminal being kept at an electric potential equal to the electric potential of the power supply line, the inversion input terminal being connected to a source region of the third TFT, and the output terminal being connected to the drain region of the third TFT, wherein the display device further comprises;

means for setting a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal as a drain current of the third TFT; and

means for equalizing the electric potential of the signal line with the electric potential of the gate electrode of the third TFT.

8. A display device comprising:

an EL element having a first electrode, a second electrode, and an EL layer that is interposed between the first electrode and the second electrode;

a first TFT having a drain region that is connected to the first electrode of the EL element;

a second TFT having the same polarity as the first TFT, the second TFT including a gate electrode and a drain region that are connected to each other;

a power supply line connected to a source region of the first TFT;

an operation amplifier having a non-inversion input terminal, an inversion input terminal, and an output terminal, the non-inversion input terminal being kept at an electric potential equal to the electric potential of the power supply line, the inversion input terminal being connected to a source region of the second TFT, and the output terminal being connected to the drain region of the second TFT; and

a resistor having a first terminal to which the electric potential of a video signal is inputted and a second terminal connected to the inversion input terminal of the operation amplifier,

wherein the display device further comprises means for equalizing the electric potential of a gate electrode of the first TFT with the electric potential of the gate electrode of the second TFT.

12. A method of driving a display device comprising the steps of:

causing a drain current of a first TFT to flow between a first electrode and a second electrode of an EL element, the EL element consisting of the first electrode, the second electrode, and an EL layer that is interposed between the first electrode and the second electrode; and

equalizing the electric potential of a source region of the first TFT with the electric potential of a power supply line, wherein;

a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal is set as a drain current of a second TFT, the second TFT having the same polarity as the first TFT;

the second TFT has a gate electrode and a drain region that have the same electric potential; and

the electric potential of the power supply line shifts by an amount corresponding to the gate voltage of the second TFT and the electric potential including the shift is inputted to a gate electrode of the first TFT.

17. A method of driving a display device according to any one of claims 12 to 16,

wherein the first TFT is a p-channel TFT, and

wherein the electric potential of the output terminal of the operation amplifier is kept lower than the electric potential of the non-inversion input terminal of the operation amplifier.

18. A method of driving a display device according to any one of claims 12 to 16,

wherein the first TFT is an n-channel TFT, and

wherein the electric potential of the output terminal of the operation amplifier is kept higher than the electric potential of the non-inversion input terminal of the operation amplifier.

19. A method of driving a display device according to any one of claims 12 to 16, wherein the display device is incorporated into an electronic equipment selected from the group consisting of a video camera, a digital camera, a head-mountable display, a game machine, a car navigation, a personal computer, and a mobile information terminal.

13. A method of driving a display device comprising the steps of:

causing a drain current of a first TFT to flow between a first electrode and a second electrode of an EL element, the EL element consisting of the first electrode, the second electrode, and an EL layer that is interposed between the first electrode and the second electrode; and

equalizing the electric potential of a source region of the first TFT with the electric potential of a power supply line, wherein;

a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal is set as a drain current of a second TFT, the second TFT having the same polarity and the same threshold voltage as the first TFT;

the second TFT has a gate electrode and a drain region that have the same electric potential; and

the electric potential of the power supply line shifts by an amount corresponding to the gate voltage of the second TFT and the electric potential including the shift is inputted to a gate electrode of the first TFT.

14. A method of driving a display device comprising the steps of:

causing a drain current of a first TFT to flow between a first electrode and a second electrode of an EL element, the EL element consisting of the first electrode, the second electrode, and an EL layer that is interposed between the first electrode and the second electrode; and

equalizing the electric potential of a source region of the first TFT with the electric potential of a power supply line, wherein;

a current linearly corresponding to the electric potential difference between the electric potential of the power supply line and the electric potential of a video signal is set as a drain current of a second TFT, the second TFT having the same polarity as the first TFT;

the voltage between an inversion input terminal and an output terminal of an operation amplifier is the same as the voltage between a source region and a drain region of the second TFT, the operation amplifier having a non-inversion input terminal that is kept to an electric potential equal to the electric potential of the power supply line;

a gate electrode of the second TFT and the drain region thereof have the same electric potential; and

the electric potential of the output terminal of the operation amplifier is inputted to a gate electrode of the first TFT.

16. A method of driving a display device according to any one of claims 14 and 15, wherein the first TFT and the second TFT have the same threshold voltage.

15. A method of driving a display device comprising the steps of:

causing a drain current of a first TFT to flow between a first electrode and a second electrode of an EL element, the EL element consisting of the first electrode, the second electrode, and an EL layer that is interposed between the first electrode and the second electrode; and

equalizing the electric potential of a source region of the first TFT with the electric potential of a power supply line, wherein;

a current flowing through a resistor is set as a drain current of a second TFT that has the same polarity as the first TFT, the resistor having a first terminal to which the electric potential of a video signal is inputted and a second terminal connected to an inversion input terminal of an operation amplifier;

the voltage between the inversion input terminal of the operation amplifier and an output terminal thereof is the same as the voltage between a source region and a drain region of the second TFT, the operation amplifier having a non-inversion input terminal that is kept to an electric potential equal to the electric potential of the power supply line;

a gate electrode of the second TFT and the drain region thereof have the same electric potential; and

the electric potential of the output terminal of the operation amplifier is inputted to a gate electrode of the first TFT.