LCD device having relationship between spontaneous polarization and capacitance

• US 6,046,790 A
• Filed: 03/19/1999
• Issued: 04/04/2000
• Est. Priority Date: 03/20/1998
• Status: Expired due to Fees

• Current Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation)
• Inventors: Yamaguchi, Takeshi, Hara, Yujiro, Itoh, Goh, Okumura, Haruhiko, Akiyama, Masahiko, Fujiwara, Hisao
• Primary Examiner(s): Sikes, William L.
• Assistant Examiner(s): TON, MINH TOAN T
• Application Number:
  US09/272,335
• Time in Patent Office: 382 Days
• Field of Search: 349/38, 349/33, 349/139, 349/172, 349/179, 349/133, 395/97
• US Class Current: 349/172
• CPC Class Codes: G02F 1/136213 Storage capacitors associated with the pixel electrode
• CPC Class Codes: G02F 1/141 using ferroelectric liquid crystals
• CPC Class Codes: G09G 2310/06 Details of flat display driving waveforms
• CPC Class Codes: G09G 3/3648 using an active matrix G09G3/367 - G09G3/3696 take precedence
• CPC Class Codes: G09G 3/3651 using multistable liquid crystals, e.g. ferroelectric liquid crystals

Abstract

A liquid crystal display device is constituted so that liquid crystal having an inherent spontaneous polarization or a spontaneous polarization induced by applying an electric field is interposed between a pixel electrode disposed in matrix and a counter electrode, and a display signal is applied to the pixel electrode through a switching element. In the liquid crystal display device constituted mentioned above, when an electrostatic capacitance of one pixel is C_LC (F), a spontaneous polarization per unit electrode area of the liquid crystal is P_s (C/m²), a voltage applied between the pixel electrode and the counter electrode is E (V), a pixel electrode area of one pixel is A (m²), and a storage capacitance for one pixel is C_s (F), they satisfy the following equation;

P.sub.s ×A≦5×(C.sub.s +C.sub.LC)×E(1)

Thereby, a display of high contrast and high response speed can be obtained. Further, a display of low power consumption and excellent display uniformity, that is, excellent image quality can be obtained.

Claims

1. A liquid crystal display device comprising:

a first substrate having one major surface;

pixel electrodes arranged in matrix fashion on the major surface of the first substrate;

storage capacitors connected to the pixel electrodes;

switching elements connected to the pixel electrodes;

a liquid crystal layer provided close to the pixel electrodes arranged in matrix fashion, the liquid crystal having an inherent spontaneous polarization or a spontaneous polarization induced by application of an electric field;

a common electrode formed on the liquid crystal layer; and

a second substrate formed on the common electrode;

wherein an electrostatic capacitance of one pixel in a saturation alignment state of the liquid crystal is C_LC (F), a spontaneous polarization of the liquid crystal per unit area is P_s (C/m²), a voltage applied between the pixel electrode and the common electrode is E (V), a pixel electrode area of one pixel is A (m²), and a storage capacitance for one pixel is C_s (F), they satisfy the following equation;

space="preserve" listing-type="equation">P.sub.s ×

A≦

5×

(C.sub.s +C.sub.LC)×

E(1).

2. The liquid crystal display device as set forth in claim 1,

wherein, when P₁ =5×

10^-6 (C/m²), the following equation holds,

space="preserve" listing-type="equation">P.sub.s ≧

P.sub.1 (

     2).

3.

3. The liquid crystal display device as set forth in claim 1,

wherein the following equation is satisfied,

4. The liquid crystal display device as set forth in claim 1,

wherein the following equations are further satisfied,

5. The liquid crystal display device as set forth in claim 4,

wherein, when P₁ =5×

10^-6 (C/m²), the following equation is further satisfied,

space="preserve" listing-type="equation">P.sub.s ≧

P.sub.1 (

     5).

6. The liquid crystal display device as set forth in claim 1,

wherein the liquid crystal display device has a thin film transistor (TFT) as the switching element.

7. The liquid crystal display device as set forth in claim 6,

wherein the thin film transistor has a polycrystalline silicon layer.

8. The liquid crystal display device as set forth in claim 6,

wherein the thin film transistor has an amorphous silicon layer.

9. The liquid crystal display device as set forth in claim 6,

wherein the following equation is satisfied;

space="preserve" listing-type="equation">(P.sub.s ×

A+(C.sub.s +C.sub.LC)×

E)/(2×

T.sub.gon)≦

I.sub.on (

     6)

where T_gon (s) is in one frame time a time period during which the gate electrode of the thin film transistor is selected; and

I_on (A) is the maximum value of a current that flows between a source and drain electrodes of the thin film transistor while the gate electrode is selected.

10. The liquid crystal display device as set forth in claim 9,

wherein the following equations hold;

space="preserve" listing-type="equation">I.sub.on ≦

I.sub.max (

     7)

and

space="preserve" listing-type="equation">I.sub.max 3×

10.sup.-4 (A) (7'"'"')

where I_max is the maximum value of an electric current that the thin film transistor can flow.

11. The liquid crystal display device as set forth in claim 6, further comprising:

a frame memory for memorizing a potential being applied to a signal line to apply a potential to the liquid crystal;

wherein one frame time is consisting of a plurality of sub-frame times, and within the respective sub-frame times, a gate line is selected on-state once.

12. A liquid crystal display device comprising:

a first substrate having one major surface;

pixel electrodes arranged in matrix fashion on the major surface of the first substrate;

storage capacitors connected to the pixel electrodes;

switching elements connected to the pixel electrodes;

a liquid crystal layer provided close to the pixel electrodes arranged in matrix fashion, the liquid crystal having an inherent spontaneous polarization or a spontaneous polarization induced by application of an electric field;

a common electrode formed on the liquid crystal layer; and

a second substrate formed on the common electrode; and

wherein the following equations are satisfied;

space="preserve" listing-type="equation">E≧

V.sub.sat and F≦

1,

where V_sat (V) is a saturation voltage of a liquid crystal display element;

C_LC (F) is an electrostatic capacitance of one pixel in a saturation alignment of the liquid crystal;

P_s (C/m²) is a spontaneous polarization of the liquid crystal per unit area is;

E (V) is a voltage applied between the pixel electrode and the common electrode;

A (m²) is an pixel electrode area for one pixel;

C_s (F) is a storage capacitance for one pixel;

T_gon (s) is a time period during which the switching element is selected for addressing display signal to the pixel electrode; and

τ

(s) is a response time of a liquid crystal cell when a voltage E (V) is applied between the pixel electrode and the common electrode and the following relation is satisfied;

space="preserve" listing-type="equation">F=P.sub.s ×

A×

exp(-(T.sub.gon ×

log.sub.e 10/τ

))/((C.sub.s +C.sub.LC)×

(E-V.sub.sat)).

13. The liquid crystal display device as set forth in claim 12,

wherein, when P₁ =5×

10^-6 (C/m²), further the following equation holds;

space="preserve" listing-type="equation">P.sub.s ≧

P.sub.1.

14. The liquid crystal display device as set forth in claim 12,

wherein the liquid crystal display device has a thin film transistor as the switching element.

15. The liquid crystal display device as set forth in claim 14,

wherein the thin film transistor has a polycrystalline silicon layer.

16. The liquid crystal display device as set forth in claim 15,

wherein the following equation is satisfied,

17. The liquid crystal display device as set forth in claim 14,

wherein the thin film transistor has an amorphous silicon layer.

18. The liquid crystal display device as set forth in claim 17,

wherein the following equation is satisfied,

19. The liquid crystal display device as set forth in claim 17,

wherein the following equation is satisfied,

20. A liquid crystal display device comprising:

a first substrate having one major surface;

pixel electrodes arranged in matrix fashion on the major surface of the first substrate;

storage capacitors connected to the pixel electrodes;

switching elements connected to the pixel electrodes;

a liquid crystal layer provided close to the pixel electrodes arranged in matrix fashion, the liquid crystal having an inherent spontaneous polarization or a spontaneous polarization induced by application of an electric field;

a common electrode formed on the liquid crystal layer; and

a second substrate formed on the common electrode;

wherein the following equation is satisfied;

space="preserve" listing-type="equation">5×

C.sub.gs ×

Δ

V.sub.g ≦

(C.sub.s +C.sub.cell)×

V.sub.sat (

     11)

where C_cell (F) is a combined capacitance for one pixel consisting of a ferroelectric capacitance due to the spontaneous polarization of the liquid crystal and a capacitance of an alignment film;

C_gs (F)is a parasitic capacitance for one pixel between the gate electrode of the thin film transistor and the pixel electrode;

C_s (F) is the storage capacitance for one pixel;

Δ

V_g (V) is a potential difference of the gate electrode upon on/off; and

V_sat (V) is a saturation voltage of a liquid crystal display element.