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[The history of liquid crystal display (LCD)]

Before the appearance of LCD, it is the Cathode Ray Tube (CRT) kingdom. The CRT monitors dominate almost all the display applications. The CRT, invented by Karl Ferdinand Braun, is also called Braun tube. Cathode rays exist in the form of streams of high speed electrons emitted from the heating of cathode inside a vacuum tube at its rear end. The released electrons form a beam within the tube due to the voltage difference applied across the two electrodes, and the direction of this beam is then altered either by a magnetic or electric field to trace over the inside surface of the phosphorescent screen (anode), covered by phosphorescent material (often transition metals or rare earths). Light is emitted by that material at the instant that electrons hit it. An illustration is shown in figure 1.

 

Figure 1. CRT interior: 1. Electron guns; 2. Electron beams; 3. Mask for separating beams for red, green , and blue part of displayed image; 4. Phosphor layer with red, green, and blue zones; 5. Close-up of the phosphor-coated inner side of the screen. (Created by Søren Peo Pedersen)

Despites its merits like good image quality, no problem with response time and viewing angle etc, it is bulky and high power consumption. Display engineers tried looking for alternative technologies like flat panel display. In 1990s, technology breakthrough brought the birth of active matrix LCD, along with the plasma display, both of which become the main stream of the flat display markets, replacing the CRT.

Although LC was discovered in the 19th century, its display application potential was realized until the first LCD based on dynamic scattering effect invented at RCA's David Sarnoff Research Center led by George Heilmeier in 1968, shown in figure 2.

Figure 2. The world first LCD based on dynamic scattering.

In 1970 Nunzio Luce, Louis Zanom, George Graham, and Joel Goldmacher left RCA and joined Optel Corporation, where they developed the first LCD display for commercial purposes, including the digital watch display. In 1970s, the Japanese entered into the LCD market, Seiko-Epson used in digital watchs, Casio and Sharp used in calculators. In 1971, M. Schadt and W. Helfrich, and James Fergason invented twisted nematic (TN) mode at almost the same time independently, the TN mode became a very important mode for display applications, even LCDs in today's market.

However, the direct-driving mode put limitations on the number of segments, i.e. resolution. As a solution, super twisted nematic (STN) mode was developed by T. Scheff and J. Nehring, with the capability of multiplexing. However STN still could satisfy the need for high information content display. The electro-optic performance is not as good as that of direct drive display. Therefore, the active matrix concept was raised. Lechner et al, and Marlowe and Nester introduced the idea of placing a switch at each pixel in a matrix. P. Brody invented CdSe thin film transistor (TFT) in 1973 as the switch to independently control the voltage at each pixel. However the CdSe TFT is not compatible with traditional microelectronics industry, where silicon is the main element. Soon after, silicon based TFT achieved great success in the LCD. Nowadays, different Si-TFT are being used according to different requirements, the most common are polycrystalline and amorphous Si-TFT. The following table shows a summary for the different types of switching devices for active matrix LCD.

 

Switching devices Mobility* (cm2/Vsec) Highest process temperature Major applications
a-Si TFT 0.3-1 ~ 3000C (glass) Laptop screen, PC monitor, LCD TV
High-T poly-Si TFT 100-300 ~ 10000C (quartz) Projection light valves, viewfinders
Low-T poly-Si TFT 10-200 ~ 5000C (glass) PDA and laptop screens, Projection light valves, viewfinders
Crystalline Si MOSFET 400 ~ 11000C (C-Si) Projection light valves, viewfinders
Thin film diode   < 3000C (glass) Handheld devices' screen

Table. 1 different types of switching devices for active matrix LCD.

 *Higher mobility enable higher integration of peripheral electronics

Along with the development of LCD's driving infrastructure, different LCD modes were introduced to improve the image quality. In-Plane-Switching (IPS) gives an excellent viewing angle performance and was commercialized by Hitachi; Samsung's Patterned Vertical Alignment (PVA) mode provide high contrast ratio, plus good viewing angle performance. Others like, Optical Compensated Bend (OCB), Multi-domain Vertical Alignment etc, stood out with its own advantages.

In parallel to the development of the direct-view displays, microdisplay for projection system has been improved since the late 1970s. Liquid Crystal on Silicon (LCoS) based rear-projection, high-definition TV is sharing the LCD market with direct-view displays. Microdisplay can also be used in personal viewer such as viewfinder in digital cameras and camcorders.

As the technology being more and more mature, price being lower and lower, the LCDs are increasingly used not only in the conventional fields like PC monitor and TV, but also others like medical display, retails, 3-D vision etc. After all, it is the cooperation of scientists and engineers from different fields, that leads to the great success of liquid crystal display.


Further Readings and References:

J. A. Castellano, "Liquid Gold: The Story Of Liquid Crystal Displays and the Creation of an Industry", World Scientific Publishing (2005).

 

Last update: April, 2006
Questions? Contact author.