JPH0350528A - Active matrix substrate for liquid crystal display device - Google Patents

Abstract

PURPOSE:To nearly eliminate a display flicker due to variance in liquid crystal cell thickness and variance in parasitic capacity by using a transfer gate of CMS constitution as an active element and equalizing the size ratio of an n channel transistor (TR) and a p channel TR. CONSTITUTION:On a c-Si wafer, the CMOS transfer gate is formed as the active element by combining an n channel TR 101 and a p channel TR 102 for each picture element by the CMOS technique of a normal silicon LSI process. The n channel TR 101 and p channel TR 102 of this CMOS transfer gate are equalized in size, i.e. channel length and channel width. When the channel length and channel width are equalized, it is equivalent that both the channel TRs 101 and 102 are equal in source-gate parasitic capacity. Consequently, the display flicker due to variance in liquid crystal cell thickness and variance in parasitic capacity is almost eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an active matrix substrate for a liquid crystal display device having active elements.

[Conventional technology]

In recent years, there has been active development of televisions and projectors that use liquid crystal panels as small, thin, and lightweight display devices to replace CRTs. In particular, there is active development of active matrix liquid crystal display devices that aim to achieve high image quality with liquid crystal panels in which each pixel is equipped with an active element using half-quartets such as switching transistors and switching diodes. Such a liquid crystal display device has an i-structure in which a liquid crystal is sandwiched between two substrates, one of which is an active matrix substrate in which the active elements are formed in a matrix, and the other is at least a glass substrate with transparent electrodes formed on the entire surface. It consists of a counter substrate consisting of a

A schematic plan view of such an active matrix substrate for a liquid crystal display device is shown in Figure 2. In this example, a switching transistor 201 is used as an active element.

The switching transistor 201 may be a MOS transistor formed on a single crystal silicon (c-Si) wafer, or an amorphous silicon (a-S) transistor using a thin film semiconductor formed on a transparent substrate such as a glass substrate or a quartz substrate.
i) and polysilicon (p-Si) thin film transistors (
In the case of a MOS} transistor that uses TPT, the substrate wafer is opaque c-Si, so the pixel electrode 104 is a reflective type such as an aluminum electrode, or a d-S
In the case of i- or p-SL TPT, a transparent substrate such as a glass substrate or a quartz substrate can be used, so the liquid crystal panel is generally constructed as a transmissive type in which the pixel electrode 104 is a transparent electrode of ITO. An outline of the driving method is that a video signal input from the outside into the horizontal drive circuit 105 shown in FIG. The switching transistor 201 is turned on and a signal is written to the scanned line. After that, it is sequentially scanned and one pixel is written in one frame. When the switching transistor 201 is off, the already written signals are accumulated in each pixel electrode 104 and will be rewritten in the next frame. Therefore, generally, the horizontal drive circuit 105 is composed of a sample and hold circuit, and the vertical drive circuit 106 is composed of a shift register circuit. In addition, such a circuit may be formed on the same substrate at the same time as the active element, or may be mounted in the form of a separate IC chip (not shown) in a hybrid manner on a mounting substrate, using wire bonding, TAB, etc. In some cases, it is connected with The potential written to each pixel electrode 104 will be explained using FIGS. 3 and 4. Figure 3 shows 1 when used as a liquid crystal panel.
FIG. 4 shows an equivalent circuit of a pixel, and FIG. 4 shows the potential at each point in FIG. When the gate potential VG is applied to the gate 301 of the switching transistor 201 and turns on at a high level, the signal potential applied to the drain 302 is applied to the source (
The liquid crystal is turned on and off depending on the magnitude of the potential difference between the pixel electrode 303 and the counter potential applied to the counter electrode 304, thereby turning the light on and off. When the gate potential VG returns to low level, the source pixel potential is maintained as it is.
In general, applying a DC voltage to a liquid crystal causes it to deteriorate, so as shown in Figure 4, a method is used in which the signal zero is applied in an AC manner to the opposing potential for each feed. Next, we will explain the details of the pixel potential. Ideally, the drain 302
Although the signal potential applied to the switching transistor 201 is applied as is, it is actually affected by the feedthrough through the source-to-gate parasitic capacitance Csa 305 of the switching transistor 201. This feedthrough is the switching transistor 2
It works when 01 is off, and in either case it pulls the pixel potential to the negative side. Therefore, the potential shape of the AC signal potential differs between odd and even frames.
At this time, the counter potential is adjusted to the negative side by the feedthrough amount so that the potential difference applied to the liquid crystal is the same in odd and even frames. If this is not done, 30Hz flicker will occur and the screen will flicker. The amount of feedthrough is expressed below by the amplitude of the margin gate potential. For example, considering a 100 μm square pixel, appropriate values gate potential amplitude 20 V, CLc = 50 fF, Cso
If =15fF is inserted, the feedthrough will be about 4V or more. [Problems to be Solved by the Invention] By the way, the thickness of the liquid crystal layer sandwiched between two substrates of a liquid crystal panel is generally as thin as 5 to 10 .mu.m, and therefore the thickness may vary depending on the location. This thickness unevenness causes a decrease in contrast, but if it is about ±10%, it is not that much of a problem, and the above value is acceptable from the viewpoint of productivity, yield, etc. However, there is another reason for this uneven thickness: LCD capacity JI.
This results in variations in CLC3 0 6, and therefore the feedthrough varies as described above. For example, if you calculate with the above value, it will be close to IV. Therefore, as shown in FIG. 4, variations in feedthrough result in variations in opposing potential, and if either one is matched, flicker will occur on the other. In other words, unevenness in the thickness of the liquid crystal layer directly leads to variations in screen flickering, which deteriorates image quality. Furthermore, the source-to-gate parasitic capacitance 05G305 of the switching transistor 201 is expected to vary within the active matrix substrate, further accelerating image quality deterioration. In particular, this variation in the source-to-gate odd capacitance Cse305 is a larger problem between panels than within the board, and therefore an increase in the number of steps required to adjust the opposing potential between panels is inevitably required. An object of the present invention is to provide a high-performance active matrix substrate for a liquid crystal display device that eliminates such conventional drawbacks. [Means for Solving the Problems] In order to achieve the above object, the active matrix substrate for a liquid crystal display device of the present invention includes semiconductor active elements arranged in a matrix on the substrate, and semiconductor active elements arranged one-on-one on the active elements. An active matrix substrate for a liquid crystal display device comprising at least a connected pixel electrode and a matrix wiring for controlling and applying a signal to the pixel electrode through the active element, wherein the semiconductor active element includes both an n-channel transistor and a p-channel transistor. Garanaru CM
It serves as a transfer gate for the OS configuration. [Example] An example of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic plan view of an active matrix substrate for a liquid crystal display device for explaining one embodiment of the present invention. For example, although not shown in FIG.
N-channel transistor 101 for each pixel using S technology
C by combining and p-channel transistor 102
A MOS transfer gate is formed as an active element. This CMOS process is not particularly limited, whether it is an n-well elliptical structure, a p-well elliptical structure, or a double-well structure. At the time of manufacturing this transfer gate, the CMOS inverter 103 is also designed for each pixel. The performance required of these transistors is one line scanning time, which is 16ms per frame for video.
The value is e c divided by the number of horizontal lines, for example, if 525 lines are compatible with NTSC, it is 1 6 m s e c / 5
2 5Σ30μsec slow. Therefore, the dimensions of the transistor can be made as small as possible within the limits of the manufacturing process. In the case of this example, since a MoS transistor using a c-Si wafer is explained, the liquid crystal panel will be treated as a reflective type. Therefore, by constructing the pixel electrode 104 with the same aluminum electrode as the video signal supply line from the horizontal drive circuit 105, an active matrix substrate for liquid crystal display is completed. At this time, in the present invention, the dimensions of the n-channel transistor 101 and the p-channel transistor 102 of the CMOS transfer gate, that is, the channel length and channel width, are configured to be the same. When the channel length and channel width are made equal, it is equivalent to making the source-to-gate parasitic capacitance of both channel transistors equal. Therefore, when the CMOS transfer gate is turned off, feedthrough of the pixel electrode 104 hardly occurs because the gate pulses applied to both gates cancel each other out because they are inverted. Therefore, the structure is such that not only the variation in feedthrough, which was a problem with conventional technology, but also the feedthrough itself does not occur. Usually CMO
S}-transfer gate is constructed by comparing the dimensions of the n-channel transistor 101, that is, the ratio of channel length to channel width, with that of the p-channel transistor 102 in order to match the performance of the n-channel transistor 101 and the p-channel transistor 102 and to increase the degree of integration. It is made smaller by the amount of mobility. However, as mentioned above, as an active element for a liquid crystal panel, the speed is not high, so the performance and degree of integration are almost unrelated to the size ratio of the n-channel transistor 101 and the p-channel transistor 102. Therefore, there is no particular problem with the configuration of the present invention.

In this example, an active matrix substrate for a reflective type liquid crystal display device using a c-Si wafer was explained, but the invention is not limited to this, and it is not limited to this, but it is also possible to use an a-Si or p - The effect is the same even in the case of SiTPT active elements. Further, in this embodiment, the inverter 103 is provided for each pixel, but if the vertical drive circuit 106 has this output terminal for each line, the inverter 103
3 is not necessary. However, in this case, the number of horizontal lines is doubled for controlling the n-channel transistor 101 and for controlling the p-channel transistor 102.

Note that the horizontal drive circuit and vertical drive circuit are the same as before, so detailed explanations will be omitted.

〔Effect of the invention〕

As explained above, according to the active matrix substrate for a liquid crystal display device of the present invention, a CMS elliptical transfer gate is used as an active element, and the size ratio of the n-channel transistor 101 and the p-channel transistor 102 is made the same. Since the LCD screen has a high temperature, it is possible to almost eliminate screen flickering caused by variations in liquid crystal cell thickness and parasitic capacitance, making it possible to obtain a good display screen. In addition, since feedthrough itself does not occur, there is no need to adjust the potential of the opposing electrode on each panel to prevent flickering, leading to cost reductions.

[Brief explanation of drawings]

FIG. 1 is a schematic plan view of an active matrix substrate for a liquid crystal display device for explaining one embodiment of the present invention.
Figure 2 is a schematic plan view of an active matrix substrate for a liquid crystal display device to explain a conventional example, Figure 3 is an equivalent circuit of one pixel to explain a conventional example, and Figure 4 shows each point in Figure 3. This is a schematic potential waveform diagram.

Claims (1)

[Claims] semiconductor active elements formed in a matrix on a substrate; pixel electrodes connected one-to-one to the active elements;
An active matrix substrate for a liquid crystal display device comprising at least matrix wiring for controlling and applying signals to the pixel electrode through the active element,
1. An active matrix substrate for a liquid crystal display device, wherein the semiconductor active element is a CMOS-structured transfer gate comprising both n-channel and p-channel transistors.