Liquid
crystals were first discovered in 1888, by Austrian botanist Friedrich Reinitzer
[7]. Reinitzer observed that when he melted a
curious cholesterol-like substance (cholesteryl benzoate), it first became
a cloudy liquid and then cleared up as its temperature rose. Upon cooling,
the liquid turned blue before finally crystallizing. This experiment showed
the intermediate properties, between those of liquid and solids that a LC
possesses. Eighty years later, in 1968, the first experimental LCD was synthesized.
How can liquid crystals be used in displays?
The properties of liquid crystals which make them suitable
for use in displays are; their ability to affect the path of plane polarized
light and their reaction to changes of temperature. Thermotropic liquid crystals
are a type of
liquid crystal which reacts to changes in temperature. This class of liquid
crystals is subdivided into isotropic, where molecules have a random order
and nematic liquid crystals where all the axes through the centre of the molecules
are aligned. The nematic phase of liquid crystals is the one most often used
in LCD displays. The molecules are rod-shaped and can move with respect to
each other but with their molecular long axes, n, remaining aligned (Figure
2) [9].
Changes in this alignment of the nematic phase are seen when an external electric
field is applied. Twisted versions of these nematic phases are used in the
formation of pixels. A twisted nematic liquid crystal is trapped between two
parallel glass sheets with polarizers at 90° to each other placed on either
side of this sandwich. The nature of the liquid crystal is such that the orientation
of a beam of polarized light will be turned through 90° allowing it to
pass, unchanged, through the pixel as it will now be at the correct angle
to pass through the second polariser. This causes the device to be in an on
state and glowing. On application of an external electric field the liquid
crystal molecule will realign so that polarized light is not transmitted by
the molecule and as such does not pass through the second polarizer (Figure
3) [10] turning that pixel off. A series of transistors
is used to turn on and off pixels in order to make up an image.
An active matrix is a way of arranging transistors such that
each pixel can be accessed individually. A pixel gets voltage from transistors
within the matrix’s column turning it on. [When the voltage is turned
off other rows within the matrix are activated whilst charge remains on the
required pixel. After all the rows within a screen have been addressed the
display is refreshed. This process occurs very quickly and prevents picture
distortions as it reduces the "cross-talk" voltage that causes picture
interference. This is in contrast to a passive matrix where, as can be seen
in Figure 4, there are no transistors and cross-talk is greater.
The transistors used in active matrices are known as TFTs or thin film transistors.
TFTs consist of deposited layers of different materials making up a semiconductor
layer, an insulating layer and electrodes.Two terminals of the transistor
conduct current and the third switches the device on and off. They can be
deposited on virtually any surface including glass; this is favourable due
to its relatively low cost. The manufacturing of TFTs involves purifying a
glass layer to remove alkali metals then in a process pioneered by Corning
the molten glass is poured onto a very thin sheet and a semiconductor layer
deposited on top by a plasma process using silane at very low pressure. This
involves an electrical discharge ionizing the silane whose fragments condense
onto the glass as a hydrogen-rich network of silicon. The hydrogen is important
in tying up broken bonds that might otherwise trap electrons and disrupt the
semiconductor. P.K. Wemer of RCA first invented TFTs in 1962 but it was not
until 1974 that their use as switches in displays was demonstrated and even
then, manufacturing costs stood in the way of mass-production. Use of amorphous
silicon was pioneered in 1979 giving favourable and cheap results. The combining
of LCD and TFT technologies was pioneered in the mid 1980s by several companies,
mostly in Japan, and was by the end of the 80s producing excellent image quality
in colour displays of 10inch size or more diagonally. As costs fell and technology
advanced many companies produced larger and larger sized screens. The first
real LCD screens were in laptops but by the early 1990s LCDs became available
for desktop PCs and more recently televisions.
The number of pixels present in a screen is a factor in the relative expense
of TFT/ LCD monitors. For example in a 20 inch screen there are 2 million
pixels and it takes only a few malfunctioning cells to ruin the image. This
means there are a higher number of rejected monitors produced in a certain
batch, which in turn raises the price of those which pass the quality tests.
Liquid
crystals were first discovered in 1888, by Austrian botanist Friedrich Reinitzer
[7]. Reinitzer observed that when he melted a
curious cholesterol-like substance (cholesteryl benzoate), it first became
a cloudy liquid and then cleared up as its temperature rose. Upon cooling,
the liquid turned blue before finally crystallizing. This experiment showed
the intermediate properties, between those of liquid and solids that a LC
possesses. Eighty years later, in 1968, the first experimental LCD was synthesized.
liquid crystal which reacts to changes in temperature. This class of liquid
crystals is subdivided into isotropic, where molecules have a random order
and nematic liquid crystals where all the axes through the centre of the molecules
are aligned. The nematic phase of liquid crystals is the one most often used
in LCD displays. The molecules are rod-shaped and can move with respect to
each other but with their molecular long axes, n, remaining aligned (Figure
2) 
greater.