EXTENT OF DEVELOPMENT OF LIQUID CRYSTAL DISPLAYS

All the leading forms of LCDs are under research and development in Japan. In all the major LCD areas, Japanese companies have made fully functional prototypes representing the most advanced product demonstrators to be found in the world. Table 1.4 lists the leading examples.

The predominant AMLCD technology is the amorphous silicon thin-film transistor (a-Si TFT). One low-mobility field-effect transistor is used at each addressable dot where the row line is connected to the gate electrode for synchronization and the column line is connected to the source electrode, and where the active area of the pixel is connected to the drain. Several forms of the TFT are used. Typically a storage capacitor is used at each pixel for improved performance.

Most manufacturers believe that the a-Si TFT LCD has become the approach of choice for the AMLCD. It has good gray shades and color, fast response, and a wide viewing angle. Manufacturing machinery has been developed to make displays 15 inches in diagonal.

The latest accomplishment has been to significantly improve the viewing angle. Two of the techniques for doing this are as follows:

  1. Control the optical retardation in all three display axes by adding a retardation film. The film is designed with reduced birefringence in the axis away from the normal to compensate for the increased optical path length of the display cell when viewing it away from the normal. The conical retardation films possessing these properties are manufactured by Nitto Denko.
  2. Divide the pixel electrode into subpixels for half-tone/gray-scale method of achieving a wide viewing angle. The voltage is varied between the subpixels by a capacitor divider circuit. Thus, the LC material is at different states of rotation at each subpixel and exhibits an effective wider viewing angle for the complete pixel. An AMLCD with this feature has been successfully built and demonstrated by Hosiden.

The second high-performing AMLCD uses poly-silicon (p-Si) TFT LCDs. Poly- silicon is very similar to a-Si except that it is deposited and annealed at a temperature above 600 degrees centigrade to give it quasi-crystalline structure and higher mobility. This technology is usually made on quartz substrate and fabricated on a metal-oxide semiconductor (MOS) line. Production machinery for large substrates has not yet been developed. The primary motivation for poly-Si TFTs is that they have the mobility and speed for the peripheral row and column drivers and shift registers; therefore, they can be made at the same time and on the same substrate as the pixel TFTs. Additionally, the smaller substrate of an MOS line allows for smaller design rules for the circuits and higher resolution displays. These two features--higher mobility and higher resolution--make p-Si LCDs most suitable for LC projector displays and LCD viewfinders for camcorders.

Table 1.4
Largest LCD Prototypes

Because of the high process temperature, the p-Si LCD has not gone into large- volume production for large displays (over five inches diagonal) in Japan. Seiko- Epson and others are developing a low-temperature (below 600 degrees centigrade) p-Si process. In the meantime, Seiko-Epson, Sony, and others continue to manufacture p-Si TFT LCDs for the higher performance, higher cost, smaller size applications.

A third AMLCD technology is metal-insulator-metal (MIM) diodes. At each pixel the MIM diodes are fabricated as a nonlinear device to prevent cross-coupling. This approach is less expensive than TFTs and gives better performance than the low-cost passive LCDs. Seiko-Epson and Toshiba have this LCD technology in production.

Several LCD passive technologies are either in production or are being developed for production. In Japan, the most successful display uses the STN technology; in 1991, over six million monochrome STN displays were made there for computers and word processors. It is the first HIC, low-cost display that could be made with acceptable performance. The speed is too slow for video, and the color is limited. However, the contrast and viewing angle is better than in its predecessor, the twisted nematic (TN) LCD. Almost all the Japanese displays manufacturers--led by Sharp, Toshiba, Hitachi, Sanyo, Seiko-Epson, Matsushita, and others--make STN LCDs. The performance of the STN has been improving, with innovations such as double cells and film, optically compensated displays for black-and-white images, with color filters as in AMLCDs for color, and further, with retardation films for wider viewing angles and better transmittance. One product is the Sharp triple-layer STN, which uses a retardation film on the top and the bottom of the STN LCD cell. An early (1988) high-brightness STN black-and- white display was the Toshiba M-ST LCD, which used one compensating film and one STN cell (Model TLX-1501-C3M).

Another passive LCD approach, called electrically controlled birefringence (ECB) was developed by Stanley using a French research concept in a joint developmental effort. The ECB LCD has the advantage of a wide viewing angle and the disadvantage of slow speed of response.

A third passive LCD approach using bistable ferroelectric LCDs (FLCDs) has been developed by Canon. It has the advantage of image storage and the disadvantage of slow addressing speed.

Several Japanese companies have started research projects using polymer-dispersed LC (PDLC) material. The PDLC requires an active matrix addressing technique. Here, the advantage is high transmittance, because the PDLC, unlike the other LCDs, does not use polarizers. It scatters light when at rest and transmits light when energized. Therefore it is best-suited to projector displays.

Magnitude of Research and Development

The Japanese electronics industry has a high interest in developing FPDs for new industrial and consumer products. Scientists at Nippon Telephone and Telegraph (NTT) estimated that more than ten industrial research laboratories and more than ten government, university, and utility company laboratories have research projects devoted to LCDs. These laboratories have engaged more than one thousand engineers and scientists to work on LCDs alone.


Published: June 1992; WTEC Hyper- Librarian