A liquid crystal display device

Abstract: A liquid crystal display device has a plurality of pixel elements, the optical transmissivity of which is varied by suitable electrical signals. The liquid crystal display device has electrodes which apply electric fields (7) to a liquid crystal layer (50), the electric fields (7) having components in a direction generally parallel to the liquid crystal layer (50). Each pixel element has at least one pixel electrode (1) which extends in a common direction as signal electrodes (12) and common electrodes (2) which extend over several pixel elements. The common electrodes (2) may be on the same side of the liquid crystal layer (50) as the pixel and signal electrodes (1,2), or they may be on opposite sides. Each pixel may have two pixel electrodes (1) with the signal electrode (12) therebetween and there are then a pair of common electrodes (2) with the pixel electrodes (1) therebetween. The common electrodes (2) may be common to adjacent pixel elements. The pixel electrodes (1) and the common electrodes (2) may be separated by an insulating film.

Description: The present invention relates to a liquid crystal display device having at least one, preferably a plurality, of pixel elements. In standard liquid crystal display device, the pixel element is a liquid crystal layer (normally common to the pixel elements) extending in a plane, and there is at least one polarizing means parallel to the plane of the liquid crystal layer. By applying electrical signals to the liquid crystal layer using suitable electrodes, it is possible to vary the angle of polarization of polarized light passing through the liquid crystal layer. Thus, by changing those electrical signals, it is possible to vary the optical transmissivity of a liquid crystal display device by varying the change in polarization relative to the at least one polarizing means. Normally, in such a liquid crystal display device, the polarizing means is formed by two polarizing plates, one on each side of the liquid crystal layer, but is also possible to provide an arrangement with a single polarizing plate on one side of the liquid crystal layer, and a reflective element on the other side of the liquid crystal element. In standard liquid crystal display devices, electrical fields are generated by the electrodes perpendicular to the plane of the liquid crystal layer. Therefore, if the change in the liquid crystal layer due to the electric fields is to be visible, the extent of those electrodes needs to be large, and therefore it is necessary to use transparent electrodes. Furthermore, at least two layers are normally needed between the transparent electrodes on each side of the liquid crystal layer and the liquid crystal layer itself. One layer forms an orientation layer for the liquid crystal layer, but a further insulating layer is then needed between the orientation layer and the transparent electrode. In International Patent Application No. PCT WO91/10936, a liquid crystal display device was disclosed in which electrical signals were applied to the liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. Such parallel field components cause reorientation of the molecules of the liquid crystal layer, thereby varying the optical transmissivity of the liquid crystal display device. In PCT WO91/10936, it was proposed that the electrodes for applying such field were, for each pixel element, in the form of combs, the teeth of the comb formed by one electrode extending into the spaces between the teeth of the comb formed by the other electrode. The teeth of each electrode were electrically connected in common, and a voltage was applied between the electrodes. JP-B-63-21907(1988) also disclosed a liquid crystal display device in which electrical signals were applied to the liquid crystal layer so as to generate electric fields having components in a direction parallel to the plane of the liquid crystal layer. As in PCT WO91/10936, the electrodes for applying such fields were, for each pixel element, in the form of combs. Use of comb-shaped electrodes was also disclosed in US Patent Specification No. 4,345,249; the preamble of claim 1 is known from this document. In each of these known arrangements, each pixel element thus has first and second electrodes of comb shape, with the teeth of one comb extending between the teeth of the other comb. Voltages are then applied to the electrodes by a suitable control circuit. It is important to note that the teeth of the comb-shaped electrodes are not electrically independent, so that size of the pixel is determined by the size of the comb-shaped electrode. The principles of operation of such devices, with comb shaped electrodes, is also discussed in an article entitled "Field Effects In Nematic Liquid Crystals Obtained With Interdigital Electrodes by R.A. Soref in the Journal of Applied Physics, pages 5466 to 5468, vol. 45, no. 12 (December 1974), and in article entitled "Interdigital Twisted-Nematic-Displays" by R.A. Soref published in the Proceedings of the IEEE, pages 1710 to 1711 (December 1974). In the standard liquid crystal display devices discussed above, it is necessary to use transparent electrodes, which are formed on facing surfaces of two substrates. However in order to form such transparent electrodes, it is necessary to use a vacuum manufacturing operation, such as sputtering, and thus the cost of manufacture of such standard liquid crystal display devices is high. Furthermore, it has been found that such transparent electrodes have vertical geometrical irregularities, of the order of several tens of nanometers, this prevents precise manufacture of active devices, such as thin film transistors needed to control the signals to the electrodes. Also, it has been found that parts of such transparent electrodes may come detached, to cause point or line defects. Thus, it has proved difficult to manufacture both reliably and cheaply liquid crystal devices. Such conventional liquid crystal display devices also have disadvantages in terms of picture quality. The problem of vertical geometrical irregularities in the transparent electrodes has been mentioned above, but similar irregularities around the controlling transistors may result in orientation failure domains being formed, requiring light shielding film to cover such transistor devices, using the light utilisation efficiency of the liquid crystal device. Also, such conventional liquid crystal display devices have disadvantage that their significant change in brightness when the visual angle is changed, and reversion of some gradation levels can occur in a half-term display, at some view angles. Although the use of comb shaped electrodes, such as previously discussed, prove the need for transparent electrodes, further problems have been found. While the use of such comb-shaped electrodes offers theoretical advantages, those are limited by practical consideration which have to be taken into account when the comb-shaped electrodes are used. If the teeth of such comb-shaped electrodes have a width of 1 to 2 micrometers, satisfactory practical operation can be achieved. However, it is extremely difficult to form such fine teeth over a large substrate without defects. Thus, in practice, the aperture factor of the liquid crystal display device is reduced, because of the need to provide relatively wide electrode teeth. There is thus a trade-off between aperture factor and production yield, which is undesirable. According to the present invention there is provided a liquid crystal display apparatus comprising: a pair of substrates, an orientation film arranged on one side of respective of said pair of substrates, a liquid crystal layer interposed between said orientation films arranged on said pair of substrates, the orientation films and the liquid crystal layer being such that the liquid crystal molecules are in a homogeneous alignment state; wherein a substrate of said pair of substrates is provided with plural scanning signal electrodes; plural signal electrodes arranged to intersect said plural scanning electrodes in a matrix form; and a plurality of thin film transistors formed at locations corresponding to respective crossing points of said plural scanning electrodes and said plural signal electrodes; a respective pixel is composed in respective regions surrounded by said plural scanning electrodes and said plural signal electrodes; a respective pixel comprises a pixel electrode and a common electrode; wherein at least a portion of said pixel electrode is overlapped with a portion of said common electrode with an insulating film interposed therebetween to form a capacitor at said overlapped portion. In a development of the present invention the angles between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at opposite surfaces of the liquid crystal layer are the same, and the product of the thickness of the liquid crystal layer and the refractive index anisotropy of the liquid crystal layer is between 0.21µm and 0.36µm. Alternatively, the absolute value of the difference between the angles between components of electric fields in a direction parallel, to the plane of said liquid crystal layer and the direction of orientation of molecules at opposite surfaces of the liquid crystal layer is not less than 80° and not greater than 100°, and product of the thickness of the liquid crystal layer and the refractive index anisotropy of the liquid crystal layer is between 0.4µm and 0.6µm. The dielectric constant anisotropy of the liquid crystal layer may be positive, and the absolute value of the angle between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer is less than 90° but not less than 45°. The dielectric constant anisotropy of the liquid crystal layer may be negative and the absolute value of the angle between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer is than greater than 0° but not greater than 45°. The dielectric constant anisotropy of the liquid crystal layer may be positive, and the value of the difference between: i) the angle between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer; and ii) the angle of the polarization axis of said at least one polarizing plate and said components of electric fields in a direction parallel to the plane of said liquid crystal layer, is 3° to 15°. The dielectric constant anisotropy of the liquid crystal layer may be negative, and the value of the difference between i) the angle of the polarization axis of said at least one polarizing plate and said components of electric fields in a direction parallel to the plane of said liquid crystal layer, and ii) the angle between components of electric fields in a direction parallel to the plane of said liquid crystal layer and the direction of orientation of molecules at the surface of the liquid crystal layer is 3° to 15°. The direction of orientation of molecules of said liquid crystal layer at a surface of said liquid crystal layer parallel to the plane of said liquid crystal layer and said surface may be not more than 4°. Embodiments of the present invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: Figs. 1(a) to 1(d) are schematic diagrams illustrating the behaviour of liquid crystal molecules in a liquid crystal display device according to the present invention; Figs. 2(a and 2(b) are plan and sectional views showing the structure of a thin film transistor which may be used in the present invention; Figs. 3(a) and 3(b) are graphs illustrating electro-optical characteristics (visual angle dependance), Fig. 3(a) corresponding to the present invention and Fig. 3(b) corresponding to a comparative example. Fig. 4 is a diagram illustrating the angles formed by the long axis orientation direction of the liquid crystal molecules at the interface the polarizing axis of the polarizing plate, and the phase advance axis of the phase difference plate, with respect to the direction of the electric field; Fig. 5 is a graph illustrating the electro-optical characteristics of various embodiments with different orientation directions of the long axis of the liquid crystal molecules at the interface when the dielectric constant anisotropy is positive Fig. 6 illustrates schematically a liquid crystal display driving circuit system which may be used in the present invention; Fig. 7 illustrates an example of the present invention applied to a liquid crystal display transmission type optical system; Fig. 8 illustrates an example of the present invention applied to a liquid crystal display reflection type optical system; Fig. 9 is a graph illustrating the electro-optical characteristics of various embodiments with different orientation directions of the long axis of the liquid crystal molecules at the interface when the dielectric constant anisotropy is negative; Figs. 10(a) and 10(b) are graphs illustrating the electro-optical characteristics of various embodiments with different thicknesses d of the liquid crystal layer when the dielectric constant anisotropy is negative; Figs. 11(a) and 11(b) are graphs illustrating the electro-optical characteristics of embodiments in which polarizing plates are arranged so that the dark state can be obtained by applying a small non-zero electric field; Fig. 12 is a graph illustrating the characteristics of a normally open type device and the characteristics when its residual phase difference at the interface is compensated; Fig. 13(a) is a plan view and Figs. 13(b) and 13(c) a sectional views of a first embodiment in which the capacitive device is formed between a common electrode and the pixel electrode on the respective facing interface of the substrates; Fig. 14 is a schematic plan view of a pixel when no electric field is applied, for a second embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer; Fig. 15 is a schematic cross sectional view of a pixel when no electric field is applied in the second embodiment in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer; Fig. 16 is a schematic plan view of a pixel when no electric field is applied in a third embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer, the pixel electrode is a closed loop and the common electrode is cruciform. Fig. 17 is a schematic plan view of a pixel when no electric field is applied in a fourth embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer, the pixel electrode has the shape of the letter I, and the common electrode is a closed loop; Fig. 18 is a schematic cross sectional view of part of a pixel when no electric field is applied in an fifth embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer, and there is an insulating layer between the scanning electrode and the common electrode; Fig. 19 is a schematic cross sectional view of part of a pixel when no electric field is applied in a sixth embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer, and the common electrode is formed on the protection insulating film; and Fig. 20 is a schematic cross sectional view of part of a pixel when no electric field is applied in a seventh embodiment of the present invention in which the pixel electrode and the common electrode are arranged at different layers separated by an insulating layer, and both the scanning electrode and the common electrode are made from aluminum coated with self-oxidized film. Before describing embodiments of the present invention, the general principles underlying the present invention will be explained. First, suppose that for the angle of a polarized light transmission axis of the polarized plate with respect to the direction of the electric field is  P , the angle of the longitudinal axis (optical axis) of liquid crystal molecules near the interface with respect to the electric field is  LC , and the angle of the phase advance axis of the phase difference plate inserted between the pair of polarizing plates with respect to the electric field is  R , as shown in Fig. 4. Because there are normally pairs of polarizing plates and liquid crystal interfaces, i.e. upper and lower plates and interface, these are represented as  P1 ,  P2 ,  LC1 , and  LC2 . Fig. 4 corresponds to a top view of Fig. 1, to be described later. Next, the operation of embodiments of the present invention will be described with reference to Figs. 1(a) to 1(d). Figs. 1(a) and 1(b) are side cross sections showing the operation of a liquid crystal display device according to the present invention. Fig.s 1(c) and 1(d) are front views of the liquid crystal display device. In Figs. 1(a) to 1(d), the thin film transistor device is omitted. In this invention, stripes of electrodes are used to form a plurality of pixels, but the Figs. 1(a) to 1(d) show the structure of only one pixel (one cell). The side cross section of the cell when no voltage is applied is shown in Fig. 1(a), and the top view is shown in Fig. 1(c). Linear electrodes 1, 2, are formed on the inner sides of paired transparent substrates 3, over which an orientation control film 4 is applied and subjected to an orientation processing. A liquid crystal is held between the paired substrates 3. Bar shaped liquid crystal molecules 5 of the liquid crystal are oriented so that they are at a slight angle to the longitudinal direction of the electrodes 1, 2, i.e. 45 degrees ≤  LC < 90 degrees, when no electric field is applied. This discussion assumes that the orientation directions of the liquid crystal molecules on the upper and lower interfaces are parallel to each other, i.e.  LC1 =  LC2 . It is also assumed that the dielectric constant anisotropy of the liquid crystal is positive. When an electric field 7 is applied, the liquid crystal molecules 5 change their directions and are aligned with the direction of the electric field 7, as shown in Figs. 1(b) and 1(d). By arranging the polarizing plates 6 at a specified angle 9, it is possible to change the light transmission factor when the electric field is applied. Thus, it is possible to provide a display having a contrast without using transparent electrodes. In Fig. 1 (b), the angle between the substrate surface and the direction of the electric field looks large and it seems not to be parallel to the substrates. In reality this is the result of magnifying the thickness direction in Fig. 1(b) and the angle is actually less than 20 degrees. In the following description, the electric fields whose inclination is less than 20 degrees are generally referred to as lateral electric fields. Figs. 1(a) and 1(b) show an arrangement in which the electrodes 1, 2, are provided separately on the upper and lower substrates respectively. However, it is possible to arrange them on one substrate and produce the same effect. Because the wiring pattern is very fine and may therefore deform due to heat and/or external forces, arranging the electrodes on a single substrate is preferable because it permits more precise alignment. Although the dielectric constant anisotropy of the liquid crystal is assumed to be positive, it may be negative. In that case, the initial orientation of the liquid crystal is set so that the liquid crystal molecules have a slight angle | LC | (i.e. 0 degree < | LC | ≤ 45 degrees) with respect to a direction perpendicular to the longitudinal direction of the electrodes 1, 2, (the direction of the electric field 7). Advantages which may thus be achieved with the present invention will now be explained. (1) The first advantage is that enhanced contrast may be achieved without using transparent electrodes. There are two types of structure for producing contrast. The one utilizes a mode in which the orientations of the liquid crystal molecules 5 on the upper and lower substrates 3 are almost parallel to each other (since color interface by birefringent phase difference is used, this mode is called a birefringent mode). The other structure has a mode in which the orientations of the liquid crystal molecules 5 on the upper and lower substrates 3 cross each other, twisting the molecular orientation in the cell (since light spiraling produced by rotation of the polarization plane in the liquid crystal composition layer is used, this mode is called a light spiraling mode). In the birefringent mode, when a voltage is applied, the molecular long axis 8 (optical axis) changes its direction in a plane almost parallel to the substrate interface, changing its angle with respect to the axis of a polarized plate (not shown in Figs. 1(a) and 1(b)) which is set at a specified angle. This results in a change in the light transmission factor. In the light spiraling mode, the application of a voltage similarly changes only the direction of the molecular long axis in the same plane. This mode, however, utilizes a change in the light spiraling as the spiral is unraveled. Next, a structure for making the display colorless and increasing the contrast ratio explained below hereinafter for two cases: one using the birefringent mode and the other using the light spiraling mode. II. Displaying in birefringent mode: generally, when a uniaxial birefringent medium is inserted between two orthogonal polarizing plates, the light transmission factor, T/T 0 , is expressed as follows. T/T 0 = sin 2 (2χ eff )·sin 2 (πd eff ·Δn/λ) In Equation 1, χ eff is the effective direction of light axis of liquid crystal composition layer (an angle between the light axis and the polarized light transmission axis), d eff is the effective thickness of the liquid crystal composition layer having birefringence, Δn is the refractive index anisotropy, and λ is the wave length of the light. In an actual cell the liquid crystal molecules are fixed at the interface and not all the liquid crystal molecules in the cell are parallel and uniformly oriented in one direction when an electric field is applied. Instead they are significantly deformed particularly near the interface. It is therefore convenient to assume an apparent uniform state as the average of these states. In Equation 1, an effective value is used for the light axis direction of the liquid crystal composition layer. To obtain a normally closed characteristic in which the display appears dark when a low voltage V L is applied and bright when a high voltage V H is applied, the polarizing plates should be arranged so that the light transmission axis (or absorption axis) of one of the polarizing plates is almost parallel to the orientation direction of the liquid crystal molecules (rubbing axis), i.e.  P2 ≒  LC1 ≒  LC2 . The light transmission axis of the other light polarizing plate is perpendicular to the first axis, i.e.  P2 =  P1 + 90°. When no electric field is applied, since χ eff in Equation 1 is zero, the light transmission factor T/T 0 is also zero. On the other hand, when an electric field is applied, the value of χ eff increases in dependence on the intensity of the electric field and becomes maximum at 45 degrees. Then, if the light wavelength is assumed to be 0.555 µm, in order to make the light colorless and the light transmission factor maximum, the effective value of d eff ·Δn should be set to half of the wavelength , i.e. 0.28 µm. Actually, however, there is a margin in the value, and values between 0.21 µm and 0.36 µm and suitable, with the values between 0.24 µm and 0.33 µm being preferred. On the other hand, in order to obtain a normally open characteristics in which the display appears bright when a low voltage V L is applied and dark when a high voltage V H is applied, the polarizing plates need be arranged so that χ eff in Equation 1 is almost 45° when no electric field is applied or an electric field of low intensity is applied. When an electric field is applied, the value χ eff decreases in dependence on the field intensity as opposed to the case of the normally closed characteristic. However, because there is a residual phase difference of the liquid crystal molecules fixed near the interface even when the χ eff is minimum (i.e. zero), a significant amount of light will leak under this condition. In an experiment conducted by the inventors of the present invention, in which the value of d•Δn was set between 0.27 and 0.37 and an effective voltage of 3 to 10 volts was applied, the residual phase difference on the interface was 0.02 to 0.06 µm. Hence, by inserting a phase difference plate having a birefringence phase difference of about 0.02 to 0.06 µm (this phase difference is represented as R f ) to compensate for the interface residual phase difference, the dark state becomes darker giving a high contrast ratio. The angle  R of the phase advance axis of the phase difference plate is set parallel to the effective light axis χ eff of the liquid crystal composition layer when the voltage is applied. To make the dark state be as black as possible, the angle of the phase advance axis should be aligned precisely with the residual phase difference that occurred when a voltage for displaying a dark state is applied. Therefore, to make the dark state compatible with the increased level of transmission factor and lightness of the bright state, the following relationship must be fulfilled. 0.21 µm < (d•Δn-R f ) < 0.36 µm Or more preferably, 0.23 µm < (d•Δn-R f ) < 0.33 µm II. Displaying in light spiraling mode: In a conventional twisted nematic (TN) system, when the value of d•Δn is set to around 0.50 µm, a first minimum condition, a high transmission factor and colorless light may be obtained. It has been found preferable to set the value in a range from 0.40 to 0.60 µm. The polarizing plates are arranged such that the transmission axis (or absorbing axis) of one of the polarizing plates is set almost parallel to the orientation direction (rubbing axis) of the liquid crystal molecules on the interface, i.e.  LC1 ≒  LC2 . For realize a normally closed type device, the transmission axis of the other polarizing plate is set parallel to the orientation direction of the liquid crystal molecules, and for a normally open type, the transmission axis of the polarizing plate is set perpendicular to the orientation direction. To eliminate light spiraling it is necessary to set the orientation direction of the liquid crystal molecules near the upper and the lower substrate interfaces so that they are almost parallel to each other. If a 90° TN mode is assumed, the liquid crystal molecules on one of the substrates must be turned nearly 90°. However, in displaying in the birefringence mode, the liquid crystal molecules need only be turned about 45°. Furthermore, the birefringence mode has a lower threshold value. (2) The second advantage is that the visual angle characteristics may be improved In the display mode, the long axes of the liquid crystal molecules are almost parallel to the substrate at all times and do not become perpendicular to the substrate, so that there is only a small change in brightness when the visual angle is changed. This display mode gives a dark state, not by making the birefringence phase difference almost zero by applying voltage as in the case of a conventional display device, but by changing the angle between the long axes of liquid crystal molecules and the axis (absorbing or transmission axis) of the polarizing plate. Thus, the display mode of the present invention differs fundamentally from that of the conventional device. In a conventional TN type of liquid crystal display deice in which the long axes of the liquid crystal molecules are perpendicular to the substrate interface, the birefringence phase difference of zero is obtained only in a visual direction perpendicular to the front or the substrate interface. Any inclination from this direction results in a birefringence phase difference, which means leakage of light in the case of the normally open type, causing reduction of contrast and reversal of tone levels. (3) The third advantage is that there is improved freedom in the selection of the materials of the orientation film and/or the liquid crystal, and margin for the related process may thus be increased. Since the liquid crystal molecules do not become erect, an orientation film for providing a large inclination angle (the angle between the long axis of the liquid crystal molecule and the interface of the substrate), which was used in a conventional device, is no longer necessary. In a conventional liquid crystal display device, when the inclination angle becomes insufficient, two states with different inclination directions and domains bordering the two states occur, giving a possibility to be a poor display. Instead of having such an inclination angle, the display system of the present invention may have the long axis direction of the liquid crystal molecule (rubbing direction) on the substrate interface to be set in a specified direction different from 0° or 90° with respect to the electric field direction. For example, when the dielectric constant anisotropy of the liquid crystal is negative, the angle between the electric field direction and the long axis direction of the liquid crystal molecule on the substrate interface  LC ( LC >0) need to exceed 0° normally more than 0.5°, preferably more than 2°. If the angle is to be set exactly 0° degree, two kinds of deformations with different directions, and domains of two different states and their bordering are generated, and a possibility of deterioration in display quality occurs. If the angle is set to more than 0.5°, the apparent long axis direction of the liquid crystal molecule ( LC (V)) increases uniformly with increasing intensity of electric field , and there is no possibility of the long axis being inclined in the reverse direction, i.e.  LC (V)<0. With this system, since no domains occur even if the angle (inclination angle) between the interface and the liquid crystal molecule is small, it is possible to set the angle to have a small value. The smaller the inclination angle, the greater the process margin for rubbing will be improving, the uniformity of the liquid crystal molecule orientation. Hence, when the present process in which an electric field is supplied in parallel to the interface combines with a low inclination technique, the orientation of the liquid crystal molecule becomes more uniform, and display variations can be reduced much better than in a conventional system even if there are variations of the same magnitude in the manufacturing process. Generally, there are fewer kinds of orientation films that produce high inclination angle than those giving small inclination angle. The present system increases freedom in the selection of the orientation film material. For example, when an organic polymer is used for the flattening film over the color filters and for the protective film over the thin film transistors and is directly subjected to the surface orientation processing such as rubbing, the organic film can be used with ease as the orientation film simultaneously because there is no need to provide an inclination angle. Hence, it becomes possible to simplify the process and to decrease the cost. In order to eliminate display irregularities due to variations in the manufacturing process, the inclination angle preferably is set below 4° more preferably below 2°. Furthermore, freedom in the selection of the liquid crystal material can be increased, as will be explained below. In the present invention, the pixel electrodes and the common electrodes may have a structure in which an electric field generally parallel to the interface of the substrate is applied to the liquid crystal composition layer. The distance between the electrodes can be chosen to be longer than the distance between the mutually facing transparent electrodes of the conventional vertical electric field active matrix type liquid crystal display device. The equivalent cross section area of the electrode can be made smaller than that of the conventional arrangement. Hence, the electric resistance between the paired pixel electrodes of the present invention can be significantly larger than that of the mutually facing transparent electrodes of the conventional active matrix liquid crystal display device. Furthermore, the electrostatic capacitance between the pixel electrode and the common electrode of the present invention can be connected in parallel with capacitive devices, and the capacitive device having a high electric resistance can be achieved. Therefore, the electric charge accumulated in the pixel electrode can be held with ease, and sufficient holding characteristics can be achieved even if the area of the capacitive device is decreased. This area reduces the aperture factor and therefore needs to be small, if possible. Conventionally the liquid crystal composition has an extremely high specific resistance, for instance 10 12 Ωcm. However, in accordance with the present invention, it is possible to use a liquid crystal composition having a lower specific resistance than the conventional one, without causing any problems. That means not only increased freedom for selection of the liquid crystal material, but also increases of the margin for the processing. In other words, a defect in display quality rarely occurs even if the liquid crystal is contaminated during the processing. Thus, the margin for the previously described variation at the interface with the orientation film increases, and defects caused at the interfaces are rare. Consequently, processes such as inspection and ageing can be significantly simplified, and the present invention can contribute significantly to a lowering in the cost of thin film transistor type liquid crystal display devices. Because the present invention permits the pixel electrode to have a more simple shape than the known comb shaped electrode, the efficiency of utilization of the light is increased. It is not necessary to sacrifice some of the aperture factor, as in conventional methods for obtaining a capacitive device which can accumulate sufficient amount of electric charge. Replacement of the insulator for protecting the thin film transistors with an organic composition enables the dielectric constant to be lower than that when an inorganic composition is used, making it possible to suppress the electric field component generated in the vicinity of the pixel electrode in a vertical direction to the interface of the substrate smaller than the electric field in a lateral direction. This enables the liquid crystal to operate in a wider region. It also contributes to an enhancement of brightness. When the common electrode is used in common as the electrode for the adjacent pixel electrodes, it operates in the same way as the common electrode in the conventional active matrix type liquid crystal display device, but its structure can be simplified as compared with more than conventional arrangements, and the aperture factor can be further increased. As there is increased freedom in the selection of materials for the liquid crystal, the orientation film, and the insulator, it becomes possible to select insulating materials of the capacitive devices so that the product of their specific resistance and dielectric constant is larger than that of the material of the liquid crystal. Then, one vertical scanning period in the driving signal output from the scanning wiring driving means can be set shorter than the time constant expressed by a product of the specific resistance and the dielectric constant of the 5 insulator of the capacitive devices. Hence, voltage variation at the pixel electrode can be reduced to sufficiently small value. (4) The fourth advantage is that a simple thin film transistor structure having a high aperture factor can be achieved, permitting enhancement of brightness. Consider the structure of a pixel including a thin film transistor, when comb shaped electrodes are used as disclosed in JP-B-63-21907 (1988). There is then the problem that the aperture factor decreases significantly and brightness is lowered. For mass productivity the necessary width of one tooth of the comb shaped electrode is about 8 µm, with a minimum of at least 4 µm. Hence, it is impossible to form a pixel of 0.3 × 0.1 mm 2 for a diagonal length 9.4 inches (23.9 cm) color VGA class with a structure having a total of 17 teeth as shown in Fig. 7 of JP-B-63-21907 (1988). The present invention permits a sufficient aperture factor to be maintained, without losing the described advantages (1) and (2) discussed above. Instead of structures such as comb shaped electrodes which inevitably reduce the aperture factor, the more simple structure of the electrode permits a practical high aperture factor to be achieved. In (JP-B-63-21907 (1988)), the directions of the signal wiring and the common electrode cross over at the right angles to each other in order to form the comb shaped electrodes. That is, the signal wiring extends in a first direction (Y direction), and the common electrode extends in a direction perpendicular to the first direction (X direction). On the other hand, the present invention permits the avoidance of a complex structure such as comb shaped electrodes by having the signal wiring, the pixel electrode, and the common electrode all extending in a common direction. In order to reduce the threshold voltage of the liquid crystal and to shorten the response time, it is preferable to make the interval between the pixel electrode and the common electrode narrow, locating the pixel electrode and the signal wiring electrode between a pair of common electrodes is also effective and it is not necessary to use a complex structure such as a comb shaped electrode. The present invention also permits the structure to be simplified and the aperture factor increased by providing the pixel electrode and the common electrode in different layers separated from each other by an insulating layer. This aspect of the present invention differs substantially from JP-B-63-21907 (1988) in that the pixel electrode and the common electrode are provided in separated layers. One advantage of this is that the region for the additional capacitive device which has been reduced by use of the lateral electric field system can be further reduced. Thus, overlapping of the pixel electrode and the common electrode separated by an insulating film becomes possible because they are in separate layers, and a load capacitance can be formed in the region of overlap. The overlapping parts can be used as a part of the wiring for the common electrodes. Hence, it is not necessary to sacrifice a part of the display in order to form a capacitive device. Accordingly, the aperture factor for the pixel can be further increased. Depending on a design of each pixel, a plurality of capacitive devices can be formed. Hence, the voltage holding characteristics may be significantly improved, and deterioration of the display quality rarely occurs even if severe contamination of the liquid crystal and lowering of off-resistance of the thin film transistor are generated. Furthermore, the insulating film formed between the pixel electrode and the common electrode can be commonly used in common as a gate insulating film of the scanning wiring (gate line) in the same layer. Therefore, there is no need to form a further film, and a process step for providing separate layers is not necessary. There are other advantages of the separate layers formed by insertion of the insulating film between the pixel electrode and the common electrode for example the probability of short-circuit failures between the pixel and common electrodes can be reduced significantly due to the existence of the insulating film therebetween, and accordingly, the probability of pixel defects can be reduced. The common electrode and/or the pixel electrode, - preferably have shapes making a pattern which makes the aperture factor as large as possible. The pixel electrode or the common electrode has any of a flat shape selected from the group of shapes of a ring, a cross, a letter T, a letter II, a letter I, and a ladder. By suitably combining the selected shapes the aperture factor can be increased significantly, as compared with the case using comb shaped electrodes. Because the common electrode and the pixel electrode are in separate layers with an insulating film therebetween, it becomes possible to provide electrodes having shapes which overlap each other. Hence, the present invention permits an increase in the aperture factor. When the common electrode is composed of a metallic electrode of which surface is coated with self-oxidized film or self-nitrized film, short-circuit failure between the common electrode and the pixel electrode can be prevented even if the two electrodes mutually overlap, so that the high aperture factor and the prevention of the pixel defects are compatible. Arrangements which are not embodiments of the invention, but are useful in understanding the invention will now be described. Arrangement 1 In the arrangement shown in Fig. 2(a) and 2(b) two glass substrates (not shown in Fig. 2(a) or 2(b) but as shown in Figs. 1(a) to 1(d)) were used which were polished on the surfaces and 1.1 mm thick. Between these substrates was interposed a nematic liquid crystal which had a positive dielectric constant anisotropy Δ∈ of 4.5 and birefringence Δn of 0.072 (589 nm, 20 C°). A polyimide orientation control film applied over the substrate surface was subjected to rubbing processing to produce a pretilt angle of 3.5 degrees. The rubbing directions of the upper and the lower interfaces were almost parallel and at an angle of 85 degrees ( LC1 =  LC2 = 85°) with respect to the direction of the applied electric field. A gap d was formed by dispersing spherical polymer beads between the substrates so that the gap became 4.5 µm when the liquid crystal was sealed. Hence, Δn·d was 0.324 µm. The resulting structure was clamped by two polarizing plates (not shown) (manufactured by e.g. Nitto Denko, with reference G1220DU). One of the polarizing plates had its polarized light transmission axis set in almost parallel to the rubbing direction, i.e.  P1 = 85°. The polarized light transmission axis of the other polarizing plate was set perpendicular to the former, i.e.  P2 = -5°. As a result, a normally closed characteristics was obtained. The structure of the thin film transistor and various electrodes for one pixel element are as shown in Fig. 2 (a) and Fig. 2 (b), such that the thin film transistor device (hatched portion in Fig. 2(a) has a pixel electrode (source electrode) 1, a signal electrode (drain electrode) 12, and a scanning electrode (gate electrode) 10. The pixel electrode 1 extends in a first direction (the vertical direction in Fig. 2), the signal electrode 12 and the common electrodes 2 extend in the first direction, and extend so as to cross over a plurality of pixels (the pixels being arranged vertically in Fig. 2), and the thin film transistor device is located between the common electrodes 2. Signal waves having information are supplied to the signal electrode 12, and scanning waves are supplied to the scanning electrode 10 synchronously. A channel layer 16 composed of amorphous silicon (a-Si) and the thin film transistor composed from an insulating protective film 15 of silicon nitride (SiN) are arranged between adjacent common electrodes. Information signals are transmitted from the signal electrode 12 to the pixel electrode 1 through the thin film transistor, and a voltage is generated between the common electrode 2 and the liquid crystal 50. In the present embodiment, the common electrodes 2 are arranged at the facing interface side of the substrate and is enlarged in the thickness direction in the illustration in Fig. 2 (b). Therefore, although the electric field direction 7 shown in Fig. 2(b) appears to be inclined relative to the horizontal, the thickness of the liquid crystal layer 5 is actually about 6 µm as compared with the width of 48 µm, so that the inclination is very small and the supplied electric field direction is almost parallel to the interface of the substrate. A capacitive device 4 was formed in a structure in which the protruded pixel electrode 1 and the scanning wiring 10 held a gate insulating film 13 therebetween as shown in Fig. 1 (c). The electrostatic capacitance of the capacitive device 11 was about 21 fF. Each of the lines of the scanning wiring 10 and the signal wiring 11 were connected to a scanning wiring driving LSI and a signal wiring driving LSI respectively. Electric charge accumulates in the pixel electrode 1 to about 24 fF, which is a capacitance of parallel connection of the electrostatic capacitance between the pixel electrode 1 and the common electrode 2 and that of the capacitive device 11. Therefore, even if the specific resistance of the liquid crystal 50 is 5 x 10 10 Ωcm, the voltage variation at the pixel electrode 1 could be suppressed, and deterioration of the display quality was prevented. In this arrangement the number of the pixels were 40 (x3) x 30, and pixel pitch was 80 µm in a lateral direction (i.e. between the common electrodes) and 240 µm in a vertical direction (i.e. between the scanning electrodes). A high aperture factor of e.g. 50 % was obtained, with the scanning electrode 10 being of 12 µm wide and the interval between the adjacent scanning electrodes being 68 µm. Three stripe shaped color filters 17 being respective red (R), green (G) and blue (B) were provided on the substrate facing the substrate supporting the thin film transistors. On the color filter. A transparent resin 14 was laminated on the color filters 17 for surface flattening. The material of the transparent resin 14 was preferably, epoxy resin. An orientation control film 4 e.g. of polyimide group, was applied to the transparent resin. A driving circuit was connected to the panel. The structure of the driving circuit system in this arrangement is shown in Fig. 6. A signal electrode 23 and a common electrode 31 extend to the end of the display portion. Figs. 7 and 8 show the structure of the optical systems needed, Fig. 7 for being for a transmission type device and Fig. 8 being for a reflection type device with a reflector 30. As this arrangement does not need any transparent electrodes, the manufacturing process becomes simple, the production yield increases, and the manufacturing cost can be significantly reduced. In particular, there is no need for extremely expensive facilities having vacuum furnaces for forming the transparent electrodes and there may thus be a significant reduction in investment in the manufacturing facilities, permitting and or accompanying cost reduction. The electro-optical characteristics showing the relationship between the effective voltage applied to the pixels and the brightness in the present embodiment is shown in Fig. 3(a). The contrast ratios exceeded 150 when driven by voltages of e.g. 7 V. The difference between the characterising when the visual angle was changed laterally or vertically were significantly smaller than in conventional system (to be discussed in comparative example 1), and the display characteristics was not changed significantly even if the visual angle was changed. Orientation character of the liquid crystal was preferable, and domains of orientation defects are not generated. The aperture factor maintained a sufficiently high value, e.g. 50 % by simplifying the structure of the thin film transistor and the electrodes, and a bright display was achieved. The average transmission factor for the whole panel was 8.4 %. It should, be noted that the term brightness is defined as the brightness of transmission when the two polarizing plates are arranged in parallel. The material of the liquid crystal 50 had a dielectric constant of 6.7 and a specific resistance of 5 x 10 10 Ωcm, and silicon nitride was used for the insulator of the capacitive device 11 had a dielectric constant of 6.7 and a specific resistance of 5 x 10 16 Ωcm. That means that the specific resistances of both the liquid crystal composition and the insulator of the capacitive device 11 were over 10 10 Ωcm, and the product of the dielectric constant and the specific resistance of the silicon nitride, about 3 x 10 4 seconds, was larger than the product of the dielectric constant and the specific resistance of the liquid crystal composition about 0.03 seconds. One vertical scanning period for the driving signal output from the scanning wiring driving LSI was about 16.6 ms with an ordinary liquid display device, and the value satisfied the condition that the scanning period should be far less than about 3 x 10 4 seconds. Therefore, it was possible to derive the time constant for accumulated charge leaking from the pixel electrode 1. This facilitates the suppression of voltage variations at the pixel electrode 1, and consequently a satisfactory display quality can be obtained. The value of 5 x 10 10 Ωcm for the specific resistance of the liquid crystal is lower than that for the liquid crystal used for the conventional vertical electric field thin film transistor liquid display device, which is about 10 12 Ωcm. However, defects in the display quality were not generated. Comparison example The comparison example referred to above was based on a conventional twisted nematic (TN) type of liquid crystal display device. Since this example had a transparent electrode, the structure was complex and the manufacturing process was long compared with the first embodiment. The nematic liquid crystal used in the comparison example had a dielectric constant anisotropy Δ∈ of positive 4.5 and a birefringence Δ n of 0.072 (589 nm, 20°C), the same as those of the Embodiment. The gap was set to 7.3 µm and the twist angle to 90 degrees. Thus, Δ n·d is 0.526 µm. The electro-optical characteristic of this comparison example is shown in Figure 3(b). The characteristic curves change greatly as the visual angle changes. Near a stepped portion adjacent to the thin-film transistor, an orientation failure domain occurs where the liquid crystal molecules are oriented in a direction different from that of the surrounding portion. Embodiment 1 The structure of the first embodiment is generally the same as the arrangement described above except as will be described below. Components of the first embodiment which correspond to the arrangement described above are indicated by the same reference numerals. Fig. 13(a) is a partial plan view of an active matrix type liquid crystal display device being the first embodiment. Fig. 13(b) is a cross sectional view taken on line A-A' in Fig. 13 (a), and FIG. 13 (c) is a cross sectional view taken on line B-B' in Fig. 13 (a). The capacitive device 11 which had a structure in which the gate insulating film composed from silicon nitride 13 was located between the pixel electrode 1 and the scanning wiring 10 in the first arrangement described above was changed to a structure in which the liquid crystal composition layer 50 extended between parts of the pixel electrode 1 and the common electrode 2 which faced each other, as shown in Fig. 13 (c). The first embodiment enables an electrostatic capacitance of the capacitive device 11 to be connected in parallel to an electrostatic capacitance between the pixel electrode 1 and the common electrode 2. Hence, any voltage variation at the signal wiring 10 does not affect the pixel electrode 1. Therefore, the voltage variation at the pixel electrode 1 could be reduced, reducing variations in the display. There was no deterioration in display quality with the active matrix type liquid crystal display device of this first the present embodiment, and the same advantages as the first embodiment were obtained. Embodiment 2 The structure of each of the second to seventh embodiments are genereally the same as the first arrangement, except as will be described below. Corresponding parts are indicated by the same reference numerals. Figs. 14 and 15 respectively show a plan view and a cross sectional view of an unit pixel of the second embodiment, in which the pixel electrode 1 and the common electrode 2, are located on the same side of the liquid crystal material and are separated by an insulating layer. A scanning electrode 10 and a common electrode 2 of chromium were formed on a glass substrate, and a gate insulating film 13 of silicon nitride (SiN) was formed so as to cover the above electrodes. An amorphous film (a-Si) 16 was formed on a part of the scanning electrode 10 with the gate insulating film 13 therebetween as an active layer of the transistor. A signal electrode 12 and a pixel electrode 1 of molybdenum were formed to overlap on a part of the pattern of the a-Si film 16, and a protection and insulating film 15 of SiN film was formed so as to cover the resulting structure. When the thin film transistor was operated by applying a voltage to the scanning electrode 13 of the thin film transistor, a voltage is applied to the pixel electrode 1. When an electric field is induced between the pixel electrode 1 and the common electrode 2 the liquid crystal molecules change their orientation in the direction of the electric field and the light transmission changes. In the second embodiment, the common electrode 2 was formed on the same layer as the scanning electrode 10, and the pixel electrode 1 and the signal electrode 12 were separated from the common electrode 2 by the gate insulating film 13. The device of the second embodiment thus differs from that of JP-B-63-21907 (1988), in that conventional comb shaped electrodes are not used, and the pixel electrode 1 and the common electrode 2 overlap with the gate insulating film 20 therebetween. By separating the pixel electrode 1 and the signal electrode 12 from the common electrode 2 by insulation, design freedom for the plan pattern of the pixel electrodes 1 and the common electrodes 2 is increased, and it becomes possible to increase the pixel aperture factor. The overlapping parts of the pixel electrode 1 and the common electrode 2 operate as an additional capacitance which is connected in parallel to the liquid crystal capacitance, and accordingly, it becomes possible to increase the holding ability of the liquid crystal charged voltage. This advantage cannot be achieved by conventional comb shaped electrode, and the advantages are achieved only by separating the pixel electrode 1 and the signal electrode 12 from the common electrode by insulation manner. As Fig. 14 reveals, it is not necessary to form a capacitive device by sacrificing a part of the display region, as in the case when the pixel electrode and the common electrode are formed on the same substrate, and all that is needed is to make an overlap in a part of the wiring for leading out the common electrode outside the display region. As described above, since the design freedom for the plan pattern increases by forming the pixel electrode 1 and the signal electrode 12 in a separate layer from the layer having the common electrode 2, various types and shapes for the electrodes can be adopted notwithstanding the present embodiment. Embodiment 3 Fig. 16 shows an plan view of a unit pixel in the third embodiment of the present invention in which the pixel electrode 1 and the common electrode 2 are located in different layers separated by an insulating layer. The cross sectional structure of the third embodiment is the same as that of the second embodiment (Fig. 15). In the third embodiment, the common electrode 2 is cruciform and the pixel electrode 1 forms a closed loop. The pixel electrode 1 and the common electrode 2 overlap at parts C1, C2, C3 and C4 in Fig. 16, so as to form additional capacitances. In the third embodiment, embodiment, the distance between the common electrode 2 and the scanning electrode 10 can be made wide, and thus failures due to short circuits between the common electrode 2 and the scanning electrode 10 can be prevented. Since the pixel electrode 1 is in the form of a closed loop, normal operation can be maintained as power is supplied to all parts of the pixel electrode even if a breaking occurs at an arbitrary portion of the pixel electrode, unless a break occurs at two or more parts. That means, that the structure of the third embodiment has a redundancy for breaking of the pixel electrode 1 and the production yield can therefore be increased. On account of the redundancy, it becomes possible to reduce the wiring gap of the closed loop electrode 1, to make that electrode 1 close to the wiring part of the scanning electrode 10 when they are arranged in different layers, so increasing the aperture factor. Embodiment 4 Fig. 17 shows a plan view of a unit pixel in a fourth embodiment of the present invention in which the pixel electrode 1 and the common electrode 2 are located in different layers separated by an insulating layer. The cross sectional structure of the fourth embodiment is the same as that of the second embodiment (Fig. 15). In the third embodiment, the pixel electrode 1 is in the form of a closed loop, and the common electrode is cruciform. In the fourth embodiment, the aperture factor can be improved as in the third embodiment, and the additional capacitance can be increased because the overlapping of the pixel electrode 1 and the common electrode 2 can be increased. Embodiment 5 Fig. 18 shows a plan view of a unit pixel in an fifth embodiment of the present invention in which the pixel electrode 1 and the common electrode 2 are located in different layers separated by an insulating layer. In the fifth embodiment, the common electrode 2 is mounted on the substrate 3 and separated from the scanning electrode 10 by a bedding layer insulating film 23. Thus, the common electrode 2 is located on the substrate 3, and is in a different layer separate from the layers forming the scanning electrode 10, the pixel electrode 1, and the signal electrode 12. Therefore, in accordance with the fifth embodiment, it becomes possible for the common electrode 2 to extend not only parallel to the scanning electrode 10 but also a perpendicular to. the scanning electrode 10 to form a network structure. Therefore, the resistance of the common electrode 2 can be decreased, and accordingly, reductions of the wave distortion in the common voltage and prevention of smear generation can be achieved. Embodiment 6 Fig. 19 shows a plan view of unit pixel in a sixth embodiment of the present invention in which the pixel electrode 1 and the common electrode 2 are located in different layers separated by an insulating layer. In the sixth embodiment, the common electrode 2 is mounted on a protecting-insulating film 15. In accordance with the present embodiment, the common electrode 2 is located in a different layer separate from the layers forming all the scanning electrode 10, the pixel electrode 1, and the signal electrode 12, as the eighth embodiment. Therefore, it is possible for the common electrode 2 to extend not only parallel to the scanning electrode 10 but also perpendicular direction to the scanning electrode 10 to form a network structure. Accordingly, the resistance of the common electrode 2 can be reduced, wave distortion in the common voltage can be decreased, and smear generation can be prevented. Embodiment 7 Fig. 20 shows a plan view of a unit pixel in a seventh embodiment of the present invention in which the pixel electrode 1 and the common electrode 2 are located in different layers separated by an insulating layer. In the seventh embodiment, the scanning electrode 10 and the common electrode 2 are made from aluminum (Al), and surfaces of those electrodes 2,10 are covered by alumina (AL 2 O 3 ) 32 which is a self-oxidized film of aluminum. The adoption of such a structure having double insulating layers decrease failure of the insulation between the common electrode 2, the signal electrode 12, and the pixel electrode 1, and accordingly, pixel failure can be decreased. Embodiment 8 The structure of the eighth embodiment is generally the same as the first arrangement except as will be described below. Corresponding parts are indicated by the same reference numerals. In the eighth embodiment a flattening film 14 (Fig. 2(b)) made from a transparent polymer was laminated on the color filters 27 as an organic insulating layer, and the surface of the film was subjected to a direct rubbing treatment without forming other films such as an orientation control film on the flattening film 14. Epoxy resin was used for the material of the transparent film. The epoxy resin has two functions namely of flattening and orientation control of the liquid crystal molecules. The liquid crystal composition layer contacted directly with the epoxy resin, and the inclination angle of the interface was 0.5 degrees. This structure eliminates the need to provide orientation film, and makes the manufacturing easier and shorter. Generally, in the conventional twisted nematic (TN) type, a variety of characteristics are required for the orientation film, and it was necessary to satisfy all of those above requirements. Therefore, the material for the orientation film was limited to a small variety of materials such as polyimide etc. The inclination angle is the most important characteristic. However, as explained previously, the present invention does not require a large inclination angle and thus the range of material selection is significantly increased. Measurement of electro-optical characteristics of the eigth embodiment revealed that, as in the first arrangement, the change in the characteristic curves was extremely small when the visual angle was changed laterally and vertically, and the display characteristics hardly changed. Although the inclination angle was as small as 0.5 degrees, the liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiment 9 In the nineth embodiment, the transparent resin forming the flattening film 14 in the eigth embodiment was changed from epoxy resin to polyimide resin. The surface of the polyimide resin was directly rubbed so that it had both the function of flattening and the function of orientation control of the liquid crystal molecules. The inclination angle on the interface was 2 degrees. In comparison with other embodiments, the display characteristics hardly changed. The liquid crystal orientation was satisfactory and no orientation failure domain was generated. Embodiment 10 The structure of the tenth embodiment was the same as the first arrangement except as will be described below. Corresponding parts are indicated by the same numerals. The protection film 15 (Fig. 2(b)) of silicon nitride for protecting the thin film transistor was in the first embodiment replaced with an organic insulating layer made from epoxy resin. The surface of the epoxy resin was directly treated by rubbing so that it functions both as a flattening film and an orientation control film for the liquid crystal molecules. The inclination angle was 0.5 degrees. Measurement of the electro-optical characteristics of the tenth embodiment revealed that the display characteristics were hardly changed in comparison with the first arrangement. Although the inclination angle was as small as 0.5 degrees, iin the eigth embodiment, the liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiment 11 In the eleventh embodiment, the epoxy resin used as the protection film in the tenth embodiment was replaced with an organic insulating 5 layer made from polyimide resin. Measurement of the electro-optical characteristics in the eleventh embodiment revealed that the display characteristics were hardly changed in comparison with the first arrangement. The inclination slightly increased angle to 2.0 degrees in comparison with the tenth embodiment. The liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiments 12 to 16 The structure of the twelvth to sixteenth embodiments were the same as the eleventh embodiment except as will be described below. In the twelvth embodiment, the directions of the long axes of the liquid crystal molecules on the upper and the lower interfaces (the rubbing direction) were almost in parallel to each other and set at 89.5 degrees ( LC1 =  LC2 = 89.5°) with respect to the applied electric field. The polarized light transmission axis of one of the polarizing plates was set almost in parallel to the rubbing direction ( P1 = 89.5°) and the polarized light transmission axis of the other polarizing plate was set perpendicular to the first axis ( P2 = -0.5°). Similarly, in the thirteenth embodiment,  LC1 =  LC2 =  P1 = 88°, and  P2 = -2.0°. Similarly, in the fourteenth embodiment,  LC1 =  LC2 =  P1 = 75°, and  P2 = -25°. Similarly, in the fifteenth embodiment,  LC1 =  LC2 =  P1 = 45°, and  P2 = -45°. Similarly, in the sixteenth embodiment,  LC1 =  LC2 =  P1 = 30°, and  P2 = -60°. The results of the measurement of the electro-optical characteristics for these embodiments are shown in a single diagram of Fig. 5. In these embodiments, the brightness was expressed by a normalized value such that the maximum brightness in a range of applied voltage from zero volt to 10 volts (effective value V rms ) was 100 % and the minimum brightness was 0 %. There was a tendency for the characteristics curves of threshold to become steep as the angle  LC was increased. In order to provide a large voltage margin for half-tone, the angle  LC must be reduced. However, there was a tendency that when the  LC was smaller than 45 degrees, brightness decreased. The optimum value of the angle  LC depends on the number of the half-tone levels to be displayed, the requirement for brightness, driving voltage, and whether or not the common electrode has a voltage applied thereto. A designer can control the threshold characteristics in a wide range by suitably selecting the angle  LC . When considering brightness, the angle is preferably  LC at least 45 degrees. An angle between 60 degrees and 89.5 degrees is more preferable. Measurement of the visual angle characteristics revealed that, in each case, the characteristics curve changed very slightly when the visual angle was changed laterally and vertically, and the display characteristics were hardly changed, as in the first embodiment 1. The liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiments 17 to 20 The greatest difference between the foregoing embodiments previously described and the seventeenth to twentieth embodiments was that the dielectric constant anisotropy of the liquid crystal composition layer was set to be negative and the rubbing direction was changed accordingly. A nematic liquid crystal (e.g. that known as ZLI-2806 of Merck Co.) with Δ∈ of -4.8, and Δn of 0.0437 (589 nm, 20 C°) was used. In the seventeenth to twentieth embodiments, the directions of the long axes of liquid crystal molecules on the upper and the lower interfaces (the rubbing directions, ( LC1 ,  LC2 ) were approximately parallel ( LC1 =  LC2 ) to each other and set at an angle,  LC1 , exceeding zero degree and less than 45 degrees with respect to the applied electric field. The polarized light transmission axis of one of the polarizing plates was set approximately parallel to the rubbing direction ( P1 ) and the polarized light transmission axis of the other polarizing plate was set perpendicular to the first axis ( P2 ). Thus, in the seventeenth embodiment,  LC1 =  LC2 =  P1 = 1.5°, and  P2 = -88.5°. In the eighteenth embodiment,  LC1 =  LC2 =  P1 = 15°, and  P2 = -75°. In the nineteenth embodiment,  LC1 =  LC2 =  P1 = 30°, and  P2 = -60°. In the twentieth embodiment,  LC1 =  LC2 =  P1 = 45°, and  P2 = -45°. The gap d was set to be 6.3 µm with the liquid crystal under sealed conditions and the Δn·d was set to be 0.275 µm. Other conditions such as the structure of the thin film transistor, and the electrodes were the same as the third embodiment. The results of the measurement of the electro-optical characteristics in these embodiments are shown in a single diagram of Fig. 9. Unlike to the case when the dielectric constant anisotropy was positive, there was a tendency for the characteristic curves of the threshold to become steep as the angle  LC was decreased. In order to provide a large voltage margin for half-tone, the angle  LC must be increased. However, there was a tendency that when the  LC was larger than 45 degrees, brightness decreased. As in the case when the dielectric constant anisotropy was positive, the optimum value of the angle  LC depends on the number of the half-tone levels to be displayed, the requirement for brightness, driving voltage, and whether or not the common electrode has a voltage applied. A designer can control the threshold characteristics in a wide range by suitably selecting the angle  LC . When considering the brightness, the angle  LC is almost 45 degrees. Measurement of the visual angle characteristics revealed that, in each case, the characteristics curve changed very slightly when the visual angle was changed laterally and vertically, and the display characteristics were hardly changed, as in the first embodiment. No reversions of levels in the half-tone display (8 steps) were observed in a range ±50 degrees both laterally and vertically. The liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiments 21 to 23 In these embodiments, the directions of the long axes of liquid crystal molecules and the arrangement of the polarizing plates were set to be the same as the twenty-first embodiment which produced the best characteristics among the embodiments 17 to 20 ( LC1 =  LC2 =  P1 = 15°, and  P2 = -75°). Only the product, d·Δn, of the thickness of the liquid crystal composition layer d and refractive index anisotropy Δn was changed. In the twenty-first, twenty-second and twenty-third embodiments the thickness d of the liquid crystal composition layer were set to be 4.0, 4.9, and 7.2 µm, respectively. Thus, the product, d·Δn, was 0.1748, 0.2141, and 0.3146 µm, respectively. In these embodiments, the refractive index anisotropy Δn was a constant and only the thickness of the liquid crystal composition layer d was changed. However, as well as the other type of the liquid crystal display (such as 90 degrees twisted nematic type), the same result for the optimum brightness can be obtained even if the refractive index anisotropy Δn is changed. Moreover, the same result can be obtained even if the liquid crystal composition layer has a positive dielectric constant anisotropy. The results of the measurement in these embodiments are shown in Fig. 10. The abscissa in Fig. 10 (a) indicates applied voltages, and the abscissa in FIG. 10 (b) indicates d·Δn with the applied voltage being a constant 7 Volts. As Fig. 10 (b) reveals, the brightness depends strongly on d·Δn and an optimum value exists. In order to maintain a brightness of at least 30 %, which is preferable for practical use, the value of d·Δn should be between 0.21 and 0.36 µm, and further if the brightness exceeding 50 % is desired, the value of d·Δn between 0.23 and 0.33 µm must be selected. In consideration of sealing time for the liquid crystal, thickness control of the liquid crystal composition layer etc, and mass-productivity, 5 the value for d must be at least 5.0 µm, and Δn of almost 0.08 is preferable. Embodiments 24 to 26 As the results of the twenty-first to twenty-third embodiments reveal, the optimum value of d·Δn is between 0.21 and 0.36 µm, preferably between 0.23 and 0.33 µm. Since the thickness of the liquid crystal composition layer which is preferable for mass-production is at least 5.0 µm, the value of the refractive index anisotropy Δn must be almost 0.072, preferably almost 0.066. However, the kinds of liquid crystal compounds having such an extremely low value for Δn are very limited, and it is very difficult for them to be compatible with other characteristics required for practical use. Therefore, a new method was considered in which d and Δn of the liquid crystal composition layer are set rather higher than the optimum values, and an optically anisotropic medium having a lower value for phase difference Rf than the value for d·Δn of the liquid crystal composition layer is provided. This compensates for the departure from the optimum value by the phase difference of with the liquid crystal composition layer. As the result, the effective phase difference which is generated by the combination of the liquid crystal composition layer and the optically anisotropic medium is in the optimum range between 0.21 and 0.36 µm. In the twenty-fourth to twenty-sixth embodiments, the structure was the same as earlier embodiments except as will be described. The thickness of the liquid crystal composition layers were set to be as 5.0, 5.2, and 5.5 µm, respectively. A nematic liquid crystal composition having the refractive index anisotropy Δn of 0.072 (589 nm, 20 C°) was used, so that the value for d·Δn was 0.360, 0.3744, and 0.396 µm, respectively. As the brightness and color tone are higher than the preferable range between 0.21 to 0.36 µm, the liquid crystal cell is colored orange. An optically anisotropic medium 28 (see Fig.7) of an uniaxially stretched film made from polyvinyl alcohol was laminated over the liquid crystal cell so as to compensate for the birefringent phase difference of the liquid crystal at low voltage driving condition (in these embodiments, zero volts). Therefore,  R was selected to be 85 degrees, being the same as  LC1 (=  LC2 ). The phase difference Rf was 0.07, 0.08, and 0.10 µm, respectively, and the value for (d·Δn - Rf) was selected to be 0.29, 0.3044, and 0.296 µm, respectively, so as to be in the preferable range from 0.21 to 0.36 µm for the brightness and the color tone. As the results, a bright display having the brightness exceeding 50 % without coloring could be obtained. Embodiment 27 In the twenty-seventh embodiment, the liquid crystal composition layer in the twenty-fourth embodiment was replaced with a nematic liquid crystal composition (e.g. ZLI-4518 of Merck Co.) for which dielectric constant anisotropy Δ∈ was negative with a value of -2.5, and its Δn was 0.0712 (589 nm, 20 C°). The rest of the device was the same as the twenty-first embodiment except as will be described below. The thickness of the liquid crystal composition layers was set to be 5.5 µm. Thus, the value for d·Δn is 0.3916 µm. An optically anisotropic medium of an uniaxially stretched film made from polyvinyl alcohol having a phase difference Rf of 0.11 µm was laminated over the liquid crystal cell so that the value for (d·Δn - Rf) became 0.2816 µm within the preferable range from 0.21 to 0.36 µm for the brightness and the color tone. As the results, a bright display having the brightness exceeding 50 % without coloring could be obtained. Embodiment 28 The structure of the twenty-eighth embodiment was generally the same as the twelvth embodiment except as will be described below. In the twenty eighth embodiment Δn of the liquid crystal composition layer was 0.072, and the gap was 7.0 µm. Therefore, d·Δn is 0.504 µm.  LC1 was 89.5 degrees, the orientation direction of the liquid crystal molecules at the upper interface and the lower interface were set so as to cross over perpendicularly to each other, and the angle, | LC1 -  LC2 |, was 90 degrees. The polarizing plates were arranged perpendicularly to each other (| P2 -  P1 | = 90°), and the relationship with the orientation direction of the liquid crystal molecules was selected so that  LC1 =  P1 , so providing an optical rotatory mode. As the result, a normally open type of liquid crystal display device was obtained. Measurement of the electro-optical characteristics in the twenty-eighth embodiment revealed the result that the brightness exceeded 50 %, there was an extremely small difference in characteristics curve when the visual angle was changed laterally and vertically, and scarcely changed display characteristics could be obtained. The orientation of the liquid crystal was satisfactory and no orientation failure domain was generated. Embodiments 29 and 30 The structures of the twenty-ninth and thirtieth embodiments were generally the same as the first arrangement except as will be described below. In these embodiment, the polarizing plates were so arranged that the dark state was obtained when an electric field of low level, not zero, was applied. The value of | LC1 -  P1 | was set at 5 degrees in the twenty-ninth embodiment, and 15 degrees in the thirtieth embodiment, respectively, and the value of | P2 -  P1 | was set at 90 degrees for both the twenty ninth and thirtieth embodiments. A satisfactory display characteristic, for both brightness and visual angle was obtained, as in the other embodiments. Also, the liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiments 31 and 32 The structures of the thirty-first and thirty-second embodiments were generally the same as the eightenth embodiment except as will be described below. In the twenty-first and twenty-second embodiments, the polarizing plates were arranged so that the dark state was obtained when an electric field of low level, not zero, was applied. The value of | P1 -  LC1 | was set at 5 degrees in the thirty-first embodiment and 7 degrees in the thirty-second embodiment, respectively, and the value of | P2 -  P1 | was set as 90 degrees for both embodiments. The thickness d of the liquid crystal composition layer was 3 µm. Therefore, d·Δn was 0.275 µm. The results of measurement of the electro-optical characteristics measurement in these embodiments are shown in Fig. 11. For the thirty-fourth embodiment the voltage causing the dark state, V OFF , was 3.0 Volts and the voltage causing the maximum brightness, V ON , was 9.2 Volts. Therefore, sufficiently high contrast can be obtained if its operation is performed with the voltage between V OFF and V ON . Similarly, for the thirty-second embodiment, V OFF was 5.0 Volts and V ON was 9.0 Volts. When it was operated with a voltage between the V OFF and the V ON , suitable display characteristics for both brightness and visual angle were obtained, as in the other embodiments. The liquid crystal orientation was satisfactory and no orientation failure domain was generated. Embodiment 33 The structure of the thirty-third embodiment was the same as the thirty-first embodiment except as will be described below. In the thirty-third embodiment, image signals were supplied to the signal electrode and at the same time, an alternating current at 3.0 V was applied to the common electrode. As a result, there was a reduction of the voltage which needed to be supplied to the signal electrode (8.3 V ⇒ 6.2 V). As its operation was performed with the voltage between V OFF and V ON , a satisfactory display characteristics in both brightness and visual angle was obtained, as in the other embodiments. The liquid crystal orientation was satisfactory and no orientation failure domain was generated. Embodiment 34 The structure of the thirty-fourth embodiment was the same as the first arrangement except as will be described below. In the thirty-fourth embodiment, the polarizing plates were arranged so that the dark state was obtained when an electric field of low level, not zero, was applied. The value of | LC1 -  P1 | was set at 45 degrees and the value of | P2 -  P1 | was set at 90 degrees. Therefore, a bright state was obtained by applying a low voltage and dark state was obtained by applying a high voltage. The results of measurement of the voltage dependance of the brightness in this present embodiment is shown with a solid line in Fig. 12. Satisfactory display characteristics for both brightness and visual angle were obtained, as in the other embodiments. The contrast ratio was 35. The, liquid crystal orientation was satisfactory, and no orientation failure domain was generated. Embodiment 35 With the structure of the thirty-fourth embodiment, a birefringent medium (uniaxially stretched polyvinyl alcohol film) was inserted between the two polarizing plates for compensating interface residual phase difference. The stretched direction of the film  R was -45 degrees, and the stretched direction crossed over perpendicularly to the transmission axis of the polarized plate. The phase difference R f was 15 nm. As shown by a dotted line in Fig. 12, the light leakage at high voltage was suppressed more than in the thirty-seventh embodiment, and the contrast ratio was improved to 150. In accordance with the present invention, it is possible to provide: i) firstly, a thin film transistor type liquid crystal display device having a high contrast without using a transparent electrode which can be mass-produced with high yields at low cost by using inexpensive manufacturing facilities; ii) secondly, a thin film transistor type liquid crystal display device having a satisfactory visual angle characteristics which can easily display multiple-tone images; iii) thirdly, a thin film transistor type liquid crystal display device having a large margin for the processes of the liquid crystal orientation and materials, such a device can have a large aperture factor, improved light transmission, and brighter images; iv) fourthly, a thin film transistor type liquid crystal display devices having a large aperture factor, improved light transmission, and brighter images by providing simple structures for the thin film transistor structure. These advantages may be achieved independently in combination depending on the structure of the device.

Claims: A liquid crystal display apparatus comprising: a pair of substrates (3), an orientation film (4) arranged on a major surface of each substrate, a liquid crystal layer (50) interposed between said orientation films (4) arranged on said pair of substrates, the orientation films (4) and the liquid crystal layer (50) being such that the liquid crystal molecules are in a homogeneous alignment state; plural scanning signal electrodes (10) provided on a major surface of one of said substrates; plural signal electrodes (12) arranged to intersect said plural scanning electrodes (10) in a matrix form; and a plurality of thin film transistors formed at locations corresponding to respective crossing points of said plural scanning electrodes (10) and said plural signal electrodes (12); wherein a respective pixel is composed in respective regions surrounded by said plural scanning electrodes (10) and said plural signal electrodes (12) and comprises a pixel electrode (1) and a common electrode (2); characterised in that at least a portion of said pixel electrode (1) is overlapped with a portion of said common electrode (2) with an insulating film (13, 15) interposed therebetween to form a capacitor (11) at said overlapped portion. A liquid crystal display apparatus as claimed in claim 1, wherein said pixel electrode (1) comprises: a portion extending in the same direction as a direction of extension of said signal electrodes (12); and a portion extending in the same direction as a direction of extension of said scanning electrodes (10); said common electrode comprises: a portion extending in the same direction as a direction of extension of said signal electrodes (12); and a portion extending in the same direction as a direction of extension of said scanning electrodes (10); and at least said portion of said pixel electrode (1) extending in the same direction as the direction of extension of said scanning electrodes (10) overlaps with said portion of said common electrode (2) extending to a same direction as a direction of extension of said scanning electrodes (10) with the insulating film (15) interposed therebetween to form the capacitor (11) at said overlapped portion. A liquid crystal display apparatus according to claim 2, wherein said portion of said common electrode (2) extending in the same direction as the direction of extension of said signal electrodes (12) is arranged closer to said signal electrode (12) than said portion of said pixel electrode (1) extending in the same direction as the direction of extension of said signal electrodes (12). A liquid crystal display apparatus according to any one of the preceding claims, wherein an electric field is generated in said liquid crystal layer by applying a voltage between said pixel electrode (1) and said common electrode. A liquid crystal display apparatus according to claim 4, wherein the angle LC1 between the longitudinal axis of the liquid crystal molecules and the electric field at one substrate side of said liquid crystal layer is approximately in parallel to the corresponding angle LC2 at the other substrate side of said liquid crystal layer (50), and the product d . Δn of the thickness d of said liquid crystal layer (50) and the anisotropy of refractive index Δn is the range from .021 µm to .036 µm. A liquid crystal display apparatus according to claim 4, wherein the angle LC1 between the longitudinal axis of the liquid crystal molecules and the electric field at one substrate side of said liquid crystal layer is inclined to the corresponding angle LC2 at the other substrate side of said liquid crystal layer (50) by an angle |LC1-LC2| in the range from 80 degrees to 100 degrees, and the product d . Δn of the thickness d of said liquid crystal layer (50) and the anisotropy of refractive index Δn is the range from .040 µm to .060 µm. A liquid crystal display apparatus according to any one of the preceding claims, wherein said pixel electrode (1) is connected to said thin film transistor. A liquid crystal display apparatus according to any one of the preceding claims, wherein said common electrode (2) comprises a metallic electrode, the surface of which is coated with an oxide film or a nitride film. A liquid crystal display apparatus according to any one of the preceding claims, further including: a polarizing means composed of at least one polarizer (6) arranged on a substrate of said pair of substrates (30) at a side opposite to the side where said orientation film is arranged; and a reflector (30) arranged on another substrate of said pair of substrates (3) at a side opposite to the side where said orientation film is arranged.

Inventors: ABE HIDETOSHI, KAWACHI GENSHIRO, KONDO KATSUMI, OHTA MASUYUKI, OHWADA JUNICHI, SASAKI TOHRU, SUZUKI KENKICHI, TERAO HIROMU, ABE, HIDETOSHI, KAWACHI, GENSHIRO, KONDO, KATSUMI, OHTA, MASUYUKI, OHWADA, JUNICHI, SASAKI, TOHRU, SUZUKI, KENKICHI, TERAO, HIROMU
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: G02F1
IPC codes: G02F1