JPS597344A - Liquid crystal dispaly device of active matrix type - Google Patents

Abstract

PURPOSE:To eliminate the adverse influence of the parasitic capacity of the thin film transistors provided at the points of intersection between gate lines and data lines and to obtain a liquid crystal display device having high reliablity, by maintaining said parasitic capacity at a specified ratio or below to the sum of the capacity values of a capacitor for charge holding and a liquid crystal. CONSTITUTION:A gate line 416, a data line 419, a common electrode 417, a liquid crystal driving electrode 420, thin film transistors (TRs) 412-414, contact holes 421-422, and a capacitor 423 for charge holding shown by obliquelines are formed on a liquid crystal display device of a matrix type. The parasitic capacity of the TRs 412-414 is maintained at a prescribed ratio or below to the sum of the capacitor for charge holding and the liquid crystal in order to prohibit lighting of the liquid crystal when the data signal is zero. The liquid crystal display device having reliability is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an active matrix liquid crystal display device in which an active matrix substrate is constructed using thin film transistors made of silicon thin films.

FIG. 1b shows a block diagram of an active matrix IJ type liquid crystal display device. In the same figure, 101 to 104
is gate line, 105-107 rj: data line, 108
~110 etc. are pixels. Further, 111 is a gate line driving device, and 112 is a data line driving device. Figure 1 1b+
is a diagram showing the configuration of pixels 108 to 110, etc.; 1st
In FIG. 1b1, ] 13 is a gate line, 114 is a data line, 115 is a switching transistor, 118, 1
19 is the parasitic capacitance of the transistor 115, 116 is the liquid crystal,
120 is a liquid crystal drive electrode, and 117 is a common electrode for the liquid crystal 116. When displaying an image using the active matrix liquid crystal display device shown in Fig. 1 Cα), the game (line 1
01, 102, 103°. . (l-1: Figure 1 1cl
Signals such as 121, 122, 123 are applied to data lines 105, 106°, and so on. has data signal 12
4 is time-divisionally sampled and applied according to each pixel position. That is, each pixel is driven point-sequentially. Figure 1 f
In cl, 125 is the common electrode potential of the pixel, and at this time, the liquid crystal display cell 116 is being driven with alternating current.

The (11 active polymer) type liquid crystal display device in FIG.
(It is formed by 'I 8 transistor). Second
FIG. 1b+ is a diagram showing the structure of a pixel formed on a conventional quasi-crystalline silicon substrate. 113 in Figure 2 (α)
ν gate line, 114 data line, J16 liquid crystal, 11
7 is a common electrode, 201 is a single crystal MO8) transistor, 2
02 is a charge holding capacitor, 203 is a single crystal silicon substrate, and 204 and 205 are parasitic capacitances of M'OS transistors.

CLC the capacitance value of liquid crystal 116! , the capacitance value of the charge holding capacitor 202 is CH116, the capacitance value of the raw capacitance 205 is C81r, and the amplitude of the gate line drive signal is Va, then when the gate line drive signal falls from high to low, as shown in FIG.
(The potential of the L+ liquid crystal drive electrode 206 is lowered by .

In the accessory cord whose area is 20,000 to 30,000 μ7222, LC! , C; The value of Hl is C
L(! = 0.1~Sail 2PF.

CH1=4~IQPF, single crystal silicon M (l
8 The driving voltage Va for the transistor is Vo =
l (1 to +5 V. Therefore, in a conventional active matrix liquid crystal display device formed on a silicon substrate, even if the parasitic capacitance C81 is equal to or more than the liquid crystal capacitance 1dcLc, ΔV can be kept sufficiently small. By the way, active matrix liquid crystal display devices formed on silicon substrates are difficult to increase in size due to the limited area of the silicon substrate. It is.

(Since the unit price of the 2'I silicon substrate is high, the manufacturing cost of the resulting display device increases. Furthermore, there is little possibility that the manufacturing process will be simplified in the future, and no reduction in manufacturing cost can be expected.

Since the 13+ silicon substrate is opaque, it is impossible to make a transmissive liquid crystal display device, and the display contrast is poor.

It has the following disadvantages. Recently, in order to improve these drawbacks and obtain an IJ type liquid crystal display device that is inexpensive, large in size, and has excellent display performance, thin film transistors have been used as switching transistors and IITs have been fabricated on transparent substrates. Development of a matrix-type liquid crystal display device σ that forms an i-element is progressing. The second 4771 diagram of a pixel of a matrix type liquid crystal display device using thin film transistors is shown below.
Shown in Figure 1b+. 2112 In Figure Cb1, ], I
3 is a gate line, 114 is a data line, 116 is a raw liquid crystal,
117 is a common terminal, 211 is a photo-film transistor, 212 is a charge holding capacitor, 213 is an L common terminal; a terminal connected to a pole or a gate line of an adjacent pixel;
14 and j15 are the parasitic capacitance i of the thin film transistor 211,
216 & 2. The electrodes that drive the liquid crystal are shaved and shown. In FIG. 2 1b+, let CLO be the capacitance value of the liquid crystal 116, an be the capacitance value of the charge holding capacitor 212, an, 2 be the capacitance value of the raw capacitor 215, and let VG be the amplitude of the gate line drive signal. , the potential drop tit of the liquid crystal drive electrode 216 in FIG. 2 1b+ at the falling edge of the gate line drive signal.
,” (:, C2 △V2−−−−−−−−−−−−−−−・vGoo,
(21CIhC4-CH24-C,C2.

Forming an N-film transistor array on a transparent substrate 1. Since the matrix type liquid crystal display device is assumed to be used in a transmission type, it is desirable that the area of the charge retention capacitor made using a silicon thin film that does not transmit light is as small as possible. Therefore, in equation 12+, the value of CH2 is usually set equal to or less than I:LO. The adverse effect of ΔV2 on a liquid crystal display and a liquid crystal display body will be explained using FIG. 3 Cal and Ibl f. In FIG. 3 1711, 301 is 1711 in FIG.
The data signal applied to 14 is zero. ) 302 is a gate line drive signal applied to 113 in fal in FIG. 4, and its period is T.
, the amplitude is vO. At this time, the potential of the liquid crystal driving electrode 216 in FIG. 2b+ changes as shown in 303 in FIG. 3 fcxl. That is, at the same time as the gate line drive signal 302 falls, the potential of the gate line 216 changes from the common electrode potential 304 to △V2.
Then, the gate line drive signal becomes 7.a and is held at the point where data writing is performed. However, during this time, the potential of the liquid crystal 216 changes to approach the common electrode potential 304 due to leakage and flow of the liquid crystal 116. In the end, a constant effective voltage V[ is applied to the liquid crystal 116, V
If E is equal to or higher than the threshold voltage of the liquid crystal, the pixel will turn on even though the data is zero. VLC
The value of , becomes larger as △v2 becomes larger. Since △V2 is determined by the formula +21, the larger the parasitic capacitance C82 is, the larger it becomes. Another example is shown in FIG. 2 1b+. Figure 2 1b+
, 305 is a data signal applied to the data line 114, which is an AC signal that is vertically symmetrical with respect to the common electrode potential 306. A gate line drive signal 307 has a period of T and an amplitude of VG. 308 indicates a change in the liquid crystal drive electrode potential, and 309 is a common electrode potential. When the gate line drive signal 307 becomes ).a, the data signal at that time is written into the liquid crystal drive electrode 216Vc. After that, at the same time as the -gate line drive signal 307 falls, the potential of the liquid crystal side electrodynamic wL216 decreases by ΔV2,
The potential is held until the gate line drive signal becomes high again and data is written. However, during this holding period, the leakage current of the liquid crystal 116 and the thin film transistor 2]
Due to the current, the charges stored in the liquid crystal drive electrode 216 are gradually discharged. In FIG. 3, 1b+, the waveform 308 has an unequal area above and below the common electrode 309, and an equal area above and below the common electrode 310, which is lower than the common electrode potential by vO.
is not a complete AC waveform, but a waveform in which a DC component is superimposed by −vO. For this reason, the liquid crystal 1 in FIG. 2 1b+
Since the DC voltage vO continues to be applied to the liquid crystal 16, the life of the liquid crystal is significantly shortened.

The present invention aims to improve the above-mentioned drawbacks, and its purpose is to improve the gate-source and gate-source connections of thin film transistors.
It is an object of the present invention to provide an active matrix type liquid crystal display device having high reliability by eliminating the adverse effect that parasitic capacitance existing between drains has on the display performance of the liquid crystal display device. The gist of the present invention is that in FIG. 2 1711, the capacitance value of the parasitic capacitances 214- and 215 is the sum of the capacitance values of the charge holding capacitor 212 and the liquid crystal 116, that is, the pixel capacitance for one pixel lt1. ．． The objective is to manufacture an active matrix type liquid crystal display device so that the value is below a certain ratio.

The present invention will be explained in detail below. 4th ICa, +,
An example of the present invention is shown in 1/11. FIG. 4 1al is a top view of one pixel of a matrix type liquid crystal display device in which a layer film transistor is formed on a transparent substrate. In the same figure, 416 is Gateyacho! , 41.9 phantom, data line,
417 is a common electrode or a data line of an adjacent pixel, 42
onrg Akira λ1AilijJ'Fiek, 4 1
2 to 4 1 4 1d'lW blade transistor, 421 to 422 if contact hole. 42
A charge holding capacitor is formed in the shaded area 3. Figure 4 1b+ is a sectional view of Figure 4 1 (L+, and the same symbols indicate the same things. In Figure 2 1b+, 411
412 to 414 are silicon thin film layers; 415 is a gate oxide film; 416 and 41 are transparent substrates (glass substrates, etc.);
7 is a second silicon thin film layer, 418 is an interlayer insulating film, 41
9 is a wiring layer made of aluminum alloy, etc.; 420 is an I.T.
Transparent lightning due to n, etc., shows the pole part 1. Also in the same figure, 412 and 414 are the source and drain of the %9' film transistor, 416 is the gate, and 419 is the data line.
A holding capacitor is formed. In FIG. 3 141, the relationship between the effective value VE of the voltage waveform 303 applied to the liquid crystal and ΔV2 is expressed by the following equation.

VFJ=@△V2...(3) However, 0(
K(]. The condition under which the liquid crystal does not turn on when the data signal is zero is given by the following equation, where the threshold voltage of the liquid crystal is VT)I.

V E (VTH * * - (41 Equations +21, +31, 141) The conditions under which the liquid crystal does not light up when the data signal is zero are as follows.

However, Cs2. CLC and CI (2 are the capacitance values of the parasitic capacitance 215, liquid crystal 116, and charge holding capacitor 212 in FIG. 2 1/+1, respectively. FIG. The horizontal 11i1+ is the effective voltage applied to the liquid crystal, and the vertical 1QI+ is the contrast.The contrast is
J indicates black, and 0 indicates white. In the figure, 501, at which the contrast is 0.1, is the threshold voltage of the liquid crystal. By the way, MCl5) between the gate and source of transistor W)
, 1. , CG S+Gate-drain capacity: COD
is known to be expressed by the following equation as a function of the gate-source voltage VG,g and the gate-drain voltage VOD. (RL A R1! ?J Z l! 7
71 eVOL r 321ke CA1971] However, Cow is the threshold voltage of the oxide film under the / - gate electrode of the MOB) transistor.
It is considered that a relational expression roughly similar to il, +71 holds true. At f/+l in FIG. 2, at the point in time when data has been written to the liquid crystal drive electrode through the entire thin film transistor 211, the source-drain voltage becomes zero and the relationship VGs==VGD * * d81 is established. are doing. Therefore, 2F61, +71,
+81, in this case both the gate-source parasitic capacitance and the gate-drain parasitic capacitance are T. Therefore, the conditional expression for the LCD not to light up when the data signal is zero +5
1 can be rewritten as follows.

However, j, Cox is the gate oxide film capacitance of the thin film transistor, gate length 'f: L, gate width f: W, oxide film thickness is TQC, dielectric constant of the oxide film is ε, and vacuum electric constant is C
If it is set to 0, it is given by the following formula.

Threshold voltage VTH of the liquid crystal used when manufacturing an active matrix type liquid crystal display device using thin film transistors
, when the amplitude VG of the gate line drive signal is given, the formula (
Cox, CLO, CH2 to satisfy the conditions of 91
An active marker with excellent display performance can be created by setting
- An IJ box type liquid crystal display device is obtained.

Further, in the active matrix type liquid crystal display device manufactured in this manner, a direct current voltage that would degrade the liquid crystal is not applied to the liquid crystal during operation. Let's take a concrete example. Now, the amplitude VG of the gate line drive signal is 2.
0V, the threshold voltage VTR of the liquid crystal is 2V, and the value K determined from the time constant unique to the liquid crystal is 0.7. LCD capacity eLo sails 2P
F, charge retention capacity +#CH.

is 002PF'', then the gate oxide film weight of the thin film transistor, Cox 1l-1, Cox (0
,0667PF'. The dielectric constant ε of the gate oxide film is 3.9, and the g'a ratio ε0 of the :N vacancy is 8.
86
ttm. At this time, Coz=LXWX6.92X
IIl (from O00667 L X W < 386
If the dimensions of the thin film transistor and W are determined to be μm2, the parasitic capacitance of the thin film transistor will not cause the problem that the liquid crystal turns on when the data signal is zero.

Furthermore, a direct current voltage that would degrade the liquid crystal is not applied to the liquid crystal.

As described above, by making the active matrix liquid crystal display device satisfy conditional expression 19), an active matrix liquid crystal display device with excellent display quality and high reliability can be obtained.

[Brief Description of the Drawings] FIG. 1 IQ, +, fb+, lC1 is a diagram for explaining the outline of an active matrix liquid crystal display device. Figure 2 (czl is a diagram showing the pixels of an active matrix liquid crystal display device provided on a transparent substrate). Figure 2 1b1 is a diagram showing the active matrix liquid crystal display device provided on a transparent substrate. Figure 3 (αl, fb+ is a diagram for explaining the shortcomings of the conventional active matrix liquid crystal display device. Figure 1 is a 7i model of the one-note IS of a type liquid crystal display device. Figure 5 is a diagram showing the relationship between voltage and contrast of a liquid crystal. / θ13' (Illusion (Kyu) 4+1) (b7 Figure 4) Trust -2 (

Claims (1)

[Claims] A first substrate comprising a plurality of gate lines and a plurality of data lines orthogonal to the gate lines, a thin film transistor being formed at each intersection of the gate lines, and a transparent crystalline driving electrode connected to each of the thin film transistors. and a second substrate facing the second substrate, and in an active matrix liquid crystal display device, when a data signal of zero amplitude is read into the liquid crystal drive electrode, the liquid crystal is An active matrix type, characterized in that the parasitic capacitance of the thin film transistor, the capacity for charge retention, and the capacitance value of the liquid crystal capacitor are limited so that the effective value of the applied voltage is less than the threshold voltage of the liquid crystal. LCD display device.