Liquid crystal was discovered
by the Austrian botanist Fredreich Rheinizer in 1888. "Liquid crystal"
is neither solid nor liquid (an example is soapy water).
In the mid-1960s, scientists
showed that liquid crystals when stimulated by an external electrical charge
could change the properties of light passing through the crystals.
The early prototypes (late
1960s) were too unstable for mass production. But all of that changed when
a British researcher proposed a stable, liquid crystal material (biphenyl).
Today's color LCD
TVs and LCD Monitors have
a sandwich-like structure (see figure below).
What
is TFT LCD?
TFT LCD (Thin Film Transistor
Liquid Crystal Display) has a sandwich-like structure with liquid crystal
filled between two glass plates.
TFT Glass has as many TFTs
as the number of pixels displayed, while a Color Filter Glass has color filter
which generates color. Liquid crystals move according to the difference in
voltage between the Color Filter Glass and the TFT Glass. The amount of light
supplied by Back Light is determined by the amount of movement of the liquid
crystals in such a way as to generate color.
TFT
LCD - Electronic Aspects of LCD TVs and LCD Monitors
Electronic Aspects of
AMLCDs
The most common liquid-crystal
displays (LCDs) in use today rely on picture elements, or pixels, formed by
liquid-crystal (LC) cells that change the polarization direction of light
passing through them in response to an electrical voltage.
As the polarization direction
changes, more or less of the light is able to pass through a polarizing layer
on the face of the display. Change the voltage, and the amount of light is
changed.
There are two ways to produce
a liquid-crystal image with such cells: the segment driving method and the
matrix driving method.
The segment driving method displays characters and pictures with cells defined
by patterned electrodes.
The matrix driving method
displays characters and pictures in sets of dots.
Direct vs. multiplex
driving of LCD TVs.
The segment drive method
is used for simple displays, such as those in calculators, while the dot-matrix
drive method is used for high-resolution displays, such as those in portable
computers and TFT monitors.
Two types of drive method
are used for matrix displays. In the static, or direct, drive method, each
pixel is individually wired to a driver. This is a simple driving method,
but, as the number of pixels is increased, the wiring becomes very complex.
An alternative method is the multiplex drive method, in which the pixels are
arranged and wired in a matrix format.
To drive the pixels of a
dot-matrix LCD, a voltage can be applied at the intersections of specific
vertical signal electrodes and specific horizontal scanning electrodes. This
method involves driving several pixels at the same time by time-division in
a pulse drive. Therefore, it is also called a multiplex, or dynamic, drive
method.
Passive and Active Matrix LCDs
There are two types of dot-matrix
LCDs.
Passive-matrix vs.
active-matrix driving of LCD Monitors.
In passive-matrix LCDs (PMLCDs)
there are no switching devices, and each pixel is addressed for more than
one frame time. The effective voltage applied to the LC must average the signal
voltage pulses over several frame times, which results in a slow response
time of greater than 150 msec and a reduction of the maximum contrast ratio.
The addressing of a PMLCD also produces a kind of crosstalk that produces
blurred images because non-selected pixels are driven through a secondary
signal-voltage path. In active-matrix LCDs (AMLCDs), on the other hand, a
switching device and a storage capacitor are integrated at the each cross
point of the electrodes.
The active addressing removes
the multiplexing limitations by incorporating an active switching element.
In contrast to passive-matrix LCDs, AMLCDs have no inherent limitation in
the number of scan lines, and they present fewer cross-talk issues. There
are many kinds of AMLCD. For their integrated switching devices most use transistors
made of deposited thin films, which are therefore called thin-film transistors
(TFTs).
The most common semiconducting
layer is made of amorphous silicon (a-Si).
a-Si TFTs are amenable to large-area fabrication using glass substrates in
a low-temperature (300°C to 400°C) process.
An alternative TFT technology,
polycrystalline silicon - or polysilicon or p-Si-is costly to produce and
especially difficult to fabricate when manufacturing large-area displays.
Nearly all TFT LCDs are
made from a-Si because of the technology's economy and maturity, but the electron
mobility of a p-Si TFT is one or two orders of magnitude greater than that
of an a-Si TFT.
This makes the p-Si TFT
a good candidate for an TFT array containing integrated drivers, which is
likely to be an attractive choice for small, high definition displays such
as view finders and projection displays.
Structure
of Color TFT LCD TVs and LCD Monitors
A TFT LCD module consists
of a TFT panel, driving-circuit unit, backlight system, and assembly unit.
Structure of a color
TFT LCD Panel:
LCD Panel
- TFT-Array Substrate
- Color Filter Substrate
Driving
Circuit Unit
- LCD Driver IC (LDI) Chips
- Multi-layer PCBs
- Driving Circuits
Backlight
& Chassis Unit
- Backlight Unit
- Chassis Assembly
It is commonly used to display
characters and graphic images when connected a host system.
The TFT LCD panel consists of a TFT-array substrate and a color-filter substrate.
The vertical structure
of a color TFT LCD panel.
The TFT-array substrate
contains the TFTs, storage capacitors, pixel electrodes, and interconnect
wiring. The color filter contains the black matrix and resin film containing
three primary-color - red, green, and blue - dyes or pigments. The two glass
substrates are assembled with a sealant, the gap between them is maintained
by spacers, and LC material is injected into the gap between the substrates.
Two sheets of polarizer film are attached to the outer faces of the sandwich
formed by the glass substrates. A set of bonding pads are fabricated on each
end of the gate and data-signal bus-lines to attach LCD Driver IC (LDI) chips
Driving
Circuit Unit
Driving an a-Si TFT LCD
requires a driving circuit unit consisting of a set of LCD driving IC (LDI)
chips and printed-circuit-boards (PCBs).
The assembly of LCD
driving circuits.
A block diagram showing
the driving of an LCD panel.
To reduce the footprint
of the LCD module, the drive circuit unit can be placed on the backside of
the LCD module by using bent Tape Carrier Packages (TCPs) and a tapered light-guide
panel (LGP).
How TFT LCD Pixels Work
A TFT LCD panel contains
a specific number of unit pixels often called subpixels.
Each unit pixel has a TFT, a pixel electrode (IT0), and a storage capacitor
(Cs).
For example, an SVGA color TFT LCD panel has total of 800x3x600, or 1,440,000,
unit pixels.
Each unit pixel is connected to one of the gate bus-lines and one of the data
bus-lines in a 3mxn matrix format. The matrix is 2400x600 for SVGA.
Structure of a color
TFT LCD panel.
Because each unit pixel
is connected through the matrix, each is individually addressable from the
bonding pads at the ends of the rows and columns.
The performance of the TFT LCD is related to the design parameters of the
unit pixel, i.e., the channel width W and the channel length L of the TFT,
the overlap between TFT electrodes, the sizes of the storage capacitor and
pixel electrode, and the space between these elements.
The design parameters associated with the black matrix, the bus-lines, and
the routing of the bus lines also set very important performance limits on
the LCD.
In a TFT LCD's unit pixel,
the liquid crystal layer on the ITO pixel electrode forms a capacitor whose
counter electrode is the common electrode on the color-filter substrate.
Vertical structure
of a unit pixel and its equivalent circuit
A storage capacitor (Cs)
and liquid-crystal capacitor (CLC) are connected as a load on the TFT.
Applying a positive pulse of about 20V peak-to-peak to a gate electrode through
a gate bus-line turns the TFT on. Clc and Cs are charged and the voltage level
on the pixel electrode rises to the signal voltage level (+8 V) applied to
the data bus-line.
The voltage on the pixel
electrode is subjected to a level shift of DV resulting from a parasitic capacitance
between the gate and drain electrodes when the gate voltage turns from the
ON to OFF state. After the level shift, this charged state can be maintained
as the gate voltage goes to -5 V, at which time the TFT turns off. The main
function of the Cs is to maintain the voltage on the pixel electrode until
the next signal voltage is applied.
Liquid crystal must be driven
with an alternating current to prevent any deterioration of image quality
resulting from dc stress.
This is usually implemented with a frame-reversal drive method, in which the
voltage applied to each pixel varies from frame to frame. If the LC voltage
changes unevenly between frames, the result would be a 30-Hz flicker.
(One frame period is normally 1/60 of a second.) Other drive methods are available
that prevent this flicker problem.
Polarity-inversion
driving methods.
In an active-matrix panel,
the gate and source electrodes are used on a shared basis, but each unit pixel
is individually addressable by selecting the appropriate two contact pads
at the ends of the rows and columns.
Active addressing
of a 3x3 matrix
By scanning the gate bus-lines
sequentially, and by applying signal voltages to all source bus-lines in a
specified sequence, we can address all pixels. One result of all this is that
the addressing of an AMLCD is done line by line.
Virtually all AMLCDs are
designed to produce gray levels - intermediate brightness levels between the
brightest white and the darkest black a unit pixel can generate. There can
be either a discrete numbers of levels - such as 8, 16, 64, or 256 - or a
continuous gradation of levels, depending on the LDI.
The optical transmittance
of a TN-mode LC changes continuously as a function of the applied voltage.
An analog LDI is capable of producing a continuous voltage signal so that
a continuous range of gray levels can be displayed.
The digital LDI produces discrete voltage amplitudes, which permits on a discrete
numbers of shades to be displayed. The number of gray levels is determined
by the number of data bits produced by the digital driver.
Generating Colors
The color filter of a TFT
LCD TV consists of three primary colors - red (R), green (G), and blue (B) -
which are included on the color-filter substrate.
How an LCD
Panel produces
colors.
The elements of this color
filter line up one-to-one with the unit pixels on the TFT-array substrate.
Each pixel in a color LCD is subdivided into three subpixels, where one set
of RGB subpixels is equal to one pixel.
(Each subpixel consists of what we've been calling a unit pixel up to this
point.)
Because the subpixels are
too small to distinguish independently, the RGB elements appear to the human
eye as a mixture of the three colors.
Any color, with some qualifications, can be produced by mixing these three
primary colors.
The total number of display
colors using an n-bit LDI is given by 23n, because each subpixel can generate
2n different transmittance levels.
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