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TFT LCD, TFT LCD Panel, TFT LCD Monitor, TFT LCD Display, TFT LCD Monitors
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| TFT LCD TV - What is TFT LCD? |
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History of TFT LCD
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). |
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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. |
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| 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. |
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| 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. |
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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. |
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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: |
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| 1. LCD Panel - TFT-Array Substrate - Color Filter Substrate
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2.Driving Circuit Unit - LCD Driver IC (LDI) Chips -
Multi-layer PCBs - Driving Circuits |
3.Backlight & Chassis Unit - Backlight Unit - Chassis
Assembly |
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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. |
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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. |
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| A block diagram showing the driving of an LCD panel. |
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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. |
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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 |
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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. |
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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 |
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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. |
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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.
The content shown above was provided by Samsung Electronics. |
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