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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).
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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We would like to express our appreciation to Samsung
Electronics for the preceding
information. | 
