VARIABLE CAPACITOR AND LIQUID CRYSTAL DISPLAY DEVICE

• US 20120113341A1
• Filed: 10/31/2011
• Published: 05/10/2012
• Est. Priority Date: 11/05/2010
• Status: Active Grant

• Current Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Original Assignee: Semiconductor Energy Laboratory Co., Ltd.
• Inventors: Uochi, Hideki
• Granted Patent: US 8,957,468 B2
• Time in Patent Office: Days
• US Class Current: 349/38
• CPC Class Codes: G02F 1/136213 Storage capacitors associated with the pixel electrode
• CPC Class Codes: H01L 27/1225 with semiconductor materials not belonging to the group IV of the periodic table, e.g. InGaZnO
• CPC Class Codes: H01L 27/1255 integrated with passive devices, e.g. auxiliary capacitors
• CPC Class Codes: H01L 29/41733 for thin film transistors with insulated gate
• CPC Class Codes: H01L 29/93 Variable capacitance diodes, e.g. varactors

Abstract

A variable capacitor with high controllability and stable operation is provided. A liquid crystal display device with low power consumption and excellent display quality is provided. A variable capacitor is formed using two overlapping electrodes of different areas and a substantially intrinsic semiconductor layer formed in contact with one of the electrodes. According to the voltage applied to the electrodes, the semiconductor layer can be considered as a dielectric or a conductor, thereby allowing varying the capacitance of the variable capacitor. The variable capacitor is applied to pixels of a liquid crystal display device configured to switch between a low capacitance and a high capacitance of the variable capacitor in accordance with a moving image display mode and a still image display mode, respectively, whereby a liquid crystal display device with low power consumption and excellent display quality can be realized.

Claims

1. A variable capacitor comprising:

a first electrode;

a second electrode;

an insulating layer and an oxide semiconductor layer both interposed between the first electrode and the second electrode, the oxide semiconductor layer being electrically connected to one of the first electrode and the second electrode.

2. A variable capacitor according to claim 1,

wherein a work function of the one of the first electrode and the second electrode is less than or equal to an electron affinity of the oxide semiconductor layer.

3. A variable capacitor according to claim 1,

wherein the oxide semiconductor layer is in direct contact with the one of the first electrode and the second electrode.

4. A variable capacitor according to claim 1,

wherein the oxide semiconductor layer comprises a C-Axis Aligned Crystal.

5. An electronic device including the variable capacitor according to claim 1.

6. A variable capacitor comprising:

a first electrode;

a second electrode;

an insulating layer and an oxide semiconductor layer both interposed between the first electrode and the second electrode, the oxide semiconductor layer being electrically connected to the second electrode,

wherein an area where the first electrode and the oxide semiconductor layer overlap with each other is larger than an area where the first electrode, the oxide semiconductor layer, and the second electrode overlap with one another.

7. A variable capacitor according to claim 6,

wherein a work function of the second electrode is less than or equal to an electron affinity of the oxide semiconductor layer.

8. A variable capacitor according to claim 6,

wherein the oxide semiconductor layer is in direct contact with the second electrode.

9. A variable capacitor according to claim 6,

wherein the oxide semiconductor layer comprises a C-Axis Aligned Crystal.

10. An electronic device including the variable capacitor according to claim 6.

11. A liquid crystal display device comprising:

a pixel comprising;

a transistor comprising a gate, a source and a drain;

a variable capacitor comprising;

a first electrode and a second electrode, the first electrode being electrically connected to one of the source and the drain of the transistor;

an insulating layer and an oxide semiconductor layer both interposed between the first electrode and the second electrode, the oxide semiconductor layer being electrically connected to one of the first electrode and the second electrode; and

a liquid crystal element comprising an electrode electrically connected to the second electrode of the variable capacitor;

a first driver circuit;

a first wiring electrically connected to the first driver circuit and to the gate of the transistor;

a second driver circuit;

a second wiring electrically connected to the second driver circuit and to the other of the source and the drain of the transistor;

a third driver circuit; and

a third wiring electrically connected to the third driver circuit and to the first electrode of the variable capacitor.

12. A liquid crystal display device according to claim 11,

wherein the oxide semiconductor layer of the variable capacitor is electrically connected to the second electrode;

and

wherein the gate of the transistor and the first electrode of the variable capacitor are formed from a same conductive layer.

13. A liquid crystal display device according to claim 11,

wherein a work function of the one of the first electrode and the second electrode is less than or equal to an electron affinity of the oxide semiconductor layer.

14. A liquid crystal display device according to claim 11,

wherein the oxide semiconductor layer comprises a C-Axis Aligned Crystal.

15. An electronic device including the liquid crystal display device according to claim 11.

16. A liquid crystal display device comprising:

a pixel comprising;

a transistor comprising a gate, a source and a drain;

a variable capacitor comprising;

a first electrode and a second electrode, the first electrode being electrically connected to one of the source and the drain of the transistor;

an insulating layer and an oxide semiconductor layer both interposed between the first electrode and the second electrode, the oxide semiconductor layer being electrically connected to one of the first electrode and the second electrode; and

a liquid crystal element comprising an electrode electrically connected to the second electrode of the variable capacitor;

a memory circuit configured to store image signals corresponding to successive frame periods; and

a comparison circuit configured to compare the image signals corresponding to the successive frame periods;

wherein the liquid crystal display device is configured to apply a voltage to the first electrode of the variable capacitor in accordance with a result of a comparison between the image signals corresponding to the successive frame periods.

17. A liquid crystal display device according to claim 16,

wherein the oxide semiconductor layer of the variable capacitor is electrically connected to the second electrode;

and

wherein the gate of the transistor and the first electrode of the variable capacitor are formed from a same conductive layer.

18. A liquid crystal display device according to claim 16,

wherein a work function of the one of the first electrode and the second electrode is less than or equal to an electron affinity of the oxide semiconductor layer.

19. A liquid crystal display device according to claim 16,

wherein the oxide semiconductor layer comprises a C-Axis Aligned Crystal.

20. An electronic device including the liquid crystal display device according to claim 16.