[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).
