Liquid crystal display device and electronic device configured to operate at two different refresh ratees
To provide a liquid crystal display device which can perform image display in both modes: a reflective mode where external light is used as an illumination light source; and a transmissive mode where a backlight is used. In one pixel, a region where incident light through a liquid crystal layer is reflected to perform display (reflective region) and a region through which light from the backlight passes to perform display (transmissive region) are provided, and image display can be performed in both modes: the reflective mode where external light is used as an illumination light source; and the transmissive mode where the backlight is used as an illumination light source. In addition, two transistors connected to respective pixel electrode layers are provided in one pixel, and the two transistors are separately operated, whereby display of the reflective region and display of the transmissive region can be controlled independently.
1. A display device comprising:
a transistor comprising an oxide semiconductor layer including a channel region; and
a pixel electrode electrically connected to the transistor,
wherein the oxide semiconductor layer comprises indium, zinc, and gallium,
wherein an off-state current density per micrometer in a channel width of the transistor at room temperature is less than or equal to 1×10 −17 A/μm,
wherein the display device is configured to operate at a first refresh rate in a first period and at a second refresh rate in a second period, and
wherein the second refresh rate is lower than the first refresh rate.
2. The display device according to claim 1 , wherein the display device is configured to display a moving image in the first period and a still image in the second period.
3. The display device according to claim 1 , wherein a cycle of one frame period in the second period is longer than or equal to one minute.
4. The display device according to claim 1 ,
wherein the pixel electrode is a reflective electrode.
5. The display device according to claim 1 ,
wherein the pixel electrode is a transparent electrode.
6. The display device according to claim 1 , further comprising a liquid crystal layer adjacent to the pixel electrode.
7. The display device according to claim 1 , wherein the oxide semiconductor layer comprises a crystal whose c-axis is perpendicular to a surface of the oxide semiconductor layer.
8. A display device comprising:
a display panel comprising a transistor and a pixel electrode electrically connected to the transistor, the transistor comprising an oxide semiconductor layer including a channel region; and
a processing circuit configured to determine whether the display panel is operated in a first operation mode at a first refresh rate or a second operation mode at a second refresh rate,
wherein the oxide semiconductor layer comprises indium, zinc, and gallium, and
wherein an off-state current density per micrometer in a channel width of the transistor at room temperature is less than or equal to 1×10 −17 A/μm.
9. The display device according to claim 8 ,
wherein the second refresh rate is lower than the first refresh rate.
10. The display device according to claim 8 ,
wherein the display device is configured to display a moving image in the first operation mode and a still image in the second operation mode.
11. The display device according to claim 8 , wherein the processing circuit is an image processing circuit comprising:
a memory circuit configured to store an image signal;
a comparison circuit configured to compare the image signal with image signals in a series of periods and to calculate a difference; and
a display control circuit configured to control operation of the display panel in accordance with the comparison result.
12. The display device according to claim 8 , wherein a cycle of one frame period in the second operation mode is longer than or equal to one minute.
13. The display device according to claim 8 ,
wherein the pixel electrode is a reflective electrode.
14. The display device according to claim 8 ,
wherein the pixel electrode is a transparent electrode.
15. The display device according to claim 8 , further comprising a liquid crystal layer adjacent to the pixel electrode.
16. The display device according to claim 8 , wherein the oxide semiconductor layer comprises a crystal whose c-axis is perpendicular to a surface of the oxide semiconductor layer.
17. A driving method of a display device comprising:
a transistor comprising an oxide semiconductor layer including a channel region; and
a pixel electrode electrically connected to the transistor,
the driving method comprising:
performing first display at a first refresh rate in a first period; and
performing second display at a second refresh rate in a second period, wherein the second refresh rate is lower than the first refresh rate,
wherein the oxide semiconductor layer comprises indium, zinc, and gallium, and
wherein an off-state current density per micrometer in a channel width of the transistor at room temperature is less than or equal to 1×10 −17 A/μm.
18. The driving method according to claim 17 ,
wherein the display device further comprises a liquid crystal layer adjacent to the pixel electrode.
19. The driving method according to claim 17 ,
wherein a moving image is displayed in the first period and a still image is displayed in the second period.
20. The driving method according to claim 17 , wherein a cycle of one frame period in the second period is longer than or equal to one minute.
21. The driving method according to claim 17 , wherein the oxide semiconductor layer comprises a crystal whose c-axis is perpendicular to a surface of the oxide semiconductor layer.