Active matrix LCD

Fig. 4. (Left) Schematic representation of an active matrix LCD display, showing single transistors driving capacitive pixel elements. (Right) OLED displays, on the other hand, require current-based driving, and therefore, multi-transistor pixel architectures are more common.

A. The LCD watch was the precursor technology to active matrix LCD screens. 

Figure 4.7 Set-up of a typical active matrix LCD Exploded" view of twelve pixels (left) and cross-section of three sub-pixels in the basic colors (right) (PI = polyimide, TFT = thin film transistor). The spacers are used to adjust the cell gap to typically 5-6 pm.

Scheme 4.11 Examples of typical super-fluorinated materials (SFM) used in the current generation of active matrix LCD. The liquid crystals 7-13 have positive dielectric anisotropy, compounds 14 and 15 have negative dielectric anisotropy. The approximate orientation of the molecular dipole moment for the two classes of material is indicated by arrows.

Some of the reasons for the preference for fluorinated liquid crystals date back to the beginnings of LCD technology, others have gained paramount importance since the introduction of active matrix LCD in 1989. 

Shortly after the commercial introduction of the first active matrix LCD in 1989 it became clear that cyano-based liquid crystals of the second generation (e.g. 3-6) cannot be used for this application. Even after extensive purification, the voltage holding ratio of this type of material is too low. The stringent reliability requirements could, on the other hand, be easily fulfilled with super-fluorinated materials (SFM), such as those depicted in Scheme 4.11. For this reason - with in-plane switching (IPS) technology as the only exception - only SFM are currently used in AM-LCD. 

Active Matrix LCDs and Electrophoretic Displays Using Organic TFTs... 

In display technology flat-panel-displays are today in production. Advanced products such as large screen plasma displays and miniature thin-film-transistor-liquid-crystal display (LCDs) are commercially available. Recently the quality and reliablility have improved considerably. So, for instance, the conical viewing angle of liquid crystal displays has been extended to nearly 160° with in-plane-switching active-matrix LCDs. 

Supertwlsted nematic displays may be passive-matrix LCDs, containing no active (switching) electronic components. Nowadays much higher performance, especially for colour displays. Is obtained from active-matrix LCDs. In this construction, a thin-film transistor Is added to each pixel to ensure an adequate and constant drive is maintained between refresh cycles. This [Pg.466]

By far the most important application area of liquid crystals per se is of course in display devices. Who can fail to be amazed at the technological triumph of an active matrix LCD flat projection panel capable of displaying 1024 x 768 pixels X 256 colors Perhaps the insights derived from PLCs will have some impact on displays, or even modulators, nonlinear optical devices or data storage devices. 

Amorphous silicon TFTs are the more developed of the two, research on them having started earlier. Impressive active matrix LCDs have been fabricated using this technology, although longer term polysilicon offers a number... 

Y. Yamaguchi, T. Miyashita, and T. Uchida, Wide-viewing-angle display mode for the active-matrix LCD using bend-alignment liquid crystal cell, SID Tech. Digest 24, 277 (1993). 

One way in which the sharpness of an LCD can be increased is to fabricate the display with an active device such as a thin film transistor at each pixel. This is shown in Figure 13.12, where it can be seen that the voltages are applied to the electrodes and transistor in such a way that the response is determined mostly by the transistor rather than the display itself A properly designed transistor can increase the sharpness of a LCD a great deal, allowing for highly multiplexed displays. Such an active matrix LCD is used in laptop computers (see Plate 8). 

On the other hand, since 1995, active matrix displays have been predominant over passive matrix displaj in the market. Notebook computers with displays from 10 to 13 inches have been the main application for active matrix LCDs. For this application, TN mode displaj have been used and consequently demand for other modes has been limited. FLCs or AFLCs have little chance of being used for this application. 

Characteristics of poljdmide liquid crystal alignment films for active matrix-LCD use, Figure 9, M. Nishikawa,Y. Matsuki, N. Bessho, Y. limura and S. Kobayashi, Journal of Photopolymer Science and Technology, 8, p. 233 (1995). Reproduced by permission of Technical Association of Photopolymers, Japan. 

Active-Matrix LCDs. Increase in FPD size along with demand for video response equivalent to the CRT made it necessary to avoid the high level of cross talk between adjacent pixels in passive displays. The nonlinear response of the liquid crystals was no longer suffieient and it became apparent that a switch was needed at each pixel. In principle, several switching technologies could be utilized since they all could be fabricated with films and photolithography. Metal-insulator-metal (MIM) devices, diodes, and transistors have all been tried. Thin-fihn transistors (TFTs) have performed the best and as a result have been adopted for most active-matrix applications. 

An electrically addressed LCSLM is identical in construction to an active matrix LCD. The construction of an optically addressed LCSLM is shown in Figure 9.8. 

The drive electronics was invented at the beginning of LCD development. The main drive electronics was followed from the passive matrix LCD (PM LCD) to active matrix LCD (AM LCD). Even if the main products moved to active matrix LCDs, the rms-respending characteristics peculiar to nematic liquid crystals remained as the drive electronics progressed. 

From the first days of the LCD, there was the dream to make a flat panel LC TV, but the development of practical use active matrix LCD progressed too slowly. Development of the PM LCD became popular in the 1970s, and the necessary drive methods were developed. The basis in TN-LCDs, put to practical use in the early 1970s, was the root mean square value dependence (rms-respending) characteristics, and the drive methods were developed accordingly [5, 6]. 

Nonaka, T. J. Li, A. Ogawa, B. Homung, W. Schmidt, R. Wingen, and H.-R. Dubai. 1999. Material characteristics of an active matrix LCD based upon chiral smectics. Liq. Cryst. 26(11) 1599-1602. 

---