TFT DISPLAY Thin Film Transistors TFTPIXEL TFT FET How TFT PIXEL WORKS

1. Department of Electronics Engineering
Tsinghua University, Beijing, China
A Report On
Thin Film Transistors
December 28th 2012
By
Tayyab Farooq (阿里)
Student ID: 2012280158

2. What is TFT LCD?
TFT stands for Thin Film Transistor, A TFT is actually a component of a LCD designed to improve the
quality and control of the LCD display. It is basically a tiny transistor linked to each individual pixel on the
screen. In today’s marketplace, TFT technology provides the best resolution of all the flat-panel
techniques. TFT screens are sometimes called active-matrix LCDs.
TFT LCD (Thin Film Transistor Liquid Crystal Display) has a sandwich-like structure with liquid crystal
filled between two glass plates.
Fig: TFT Sandwich between two glasses
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.
Fig: A Pixel
The TFTs in active-matrix LCD act as simple ON/OFF switches, at different speeds which depend on the
refresh rate of the LCD, for example 60Hz. Figure below shows a simple structure of TFT, it consists of
three terminals: the gate, the source and the drain. As seen in figure 1, the gate is insulated from the
semiconductor film by a gate insulation film; while the drain and source directly contact the semiconductor
film.

3. A simple Thin-Film-Transistor (TFT) structure
In a simple TFT, for example N-channel TFT, a positive voltage is applied on the gate in order to switch it
ON; the insulation layer can be considered as the dielectric layer in a capacitor, hence negative charges
are induced on the semiconductor channel, which is the region between source and drain; these negative
charges create a electrons flow from source to drain to make the channel conductive. When a negative
voltage is applied on the gate,
electrons are depleted in the
channel, hence almost no current is
present. The ON current depends
on different parameters, for
example channel width, channel
length, gate voltage and the
threshold voltage of the TFT.
When the TFT is switched ON, a
data voltage is applied on the
source, the drain with the LC load
capacitance will charge up to the
voltage with same amplitude, i.e.
transferring the data voltage from the data line to the pixel electrode. When switched OFF, no current in
the channel, and data voltage cannot be transferred.
A TFT substrateconsists of a matrix of pixels and a regioncalled ITO (a transparentelectricallyconductive
film) eachhaving a TFT device. Thousandsormillions of thesepixelstogethercreateanimageonthescreen.
Thediagram shows thestructure of a single pixel.

4. The advantage of TFTs is that they are fast enough for video, provide a large and smooth color palette,
and are pixel addressable through an electronic two-dimensional control matrix. Most low-cost displays
use an amorphous silicon crystal layer deposited onto the glass through a plasma-enhanced chemical
vapor deposition.
Many versions of TFT technologies have led us to the
modern displays. Early complaints like poor viewing angles,
poor contrast, and poor backlighting have been addressed.
Better light sources, diffusers, and polarizers make many
displays very vivid, some even claiming to be daylight
readable. Modern day techniques like in-plane switching
improve viewing angles by making the crystals move in a
parallel direction to the display plane instead of vertically.
Better speeds and contrasts of modern display make them
high performance for a fairly low cost.
How TFT Works:
A TFT uses liquid crystal to control the passage of light. The basic structure of a TFT-LCD panel may be
thought of as two pieces of glass with a layer of liquid crystal between them. The front glass is fitted with
a color filter, while the back glass has transistors on it. When voltage is applied to a transistor, the liquid
crystal is bent, allowing light to pass through to form a pixel. A light source, in many cases an LED, is
located at the back of the panel and is what, makes up the backlight. The front glass is fitted with a color
filter, which gives each pixel its own color. The combination of these pixels in different colors forms the
image on the panel.
Color filter
Polarizer
Black Matrix
Alignment Layer Pixel Electrode
Layer (ITO)
LCD Crystals TF Space
Seal T r
Alignment Layer
Pixel
Electrode
Bonding Pad Layer (ITO)
Array
Substrat
e
Polarizer
Backlight

5. A TFT panel array contains a specific number of pixels, often known as subpixels. Thousands or millions
of these unit pixels together create an image on the display. This diagram shows the simple structure of a
sub-pixel. Each unit pixel contains a TFT, a pixel electrode or ITO and microscopic storage capacitors.
Each unit pixel is connected to one of the gate bus lines and one of the data bus lines in a matrix format.
This allows for easy individual pixel addressing. TFT devices are switching devices, which function to
turn each individual pixel on or off thereby
controlling the number of electrons that
flow into the ITO zone. As the number of
electrons reaches the expected value,
TFT turns off and these electrons can be
kept within the ITO zone.
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.
TFT ACTIVE MATRIX ARRAY:
The TFT active matrix array is composed of millions of individual detector elements, each of which
contains a transistor, charge collector electrode and storage capacitor, all arranged on an amorphous
silicon substrate. Individual elements are connected by gate lines along rows (operating the TFT), by
drain lines along columns (connected to
the TFT output), and charge amplifiers
connected to the drain lines to receive the
charge from specific detector elements. In
operation, local charge created by local X-
ray absorption is stored at each detector
element and actively read by turning row
gate lines on one at a time, allowing
charge to pass from the local storage
capacitor through the TFT, down the drain
line to the charge amplifier. Each
transistor is reset and ready for the next
exposure. The image is created
sequentially, row by row. For real-time (30
frame per second) fluoroscopy, all of the
detector rows must be read in 33 ms or less. The upper left detector element illustrates the concept of
‘fill-factor–with TFT arrays, caused by less than 100% geometric capture efficiency of X-rays that fall
upon inactive areas of the TFT matrix

6. Vertical Structure of 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.
How TFT Generates Color:-
Color Filters
Fig: Illustration represents one pixel.

7. When power is applied to bend the liquid crystal, light passes through from the backlight into the color
filter. How much light that passes through depends on the amount of power applied to the pixel. If there
were no color filter, the output would be in the form of a grayscale. The color filter is an RGB (red, green
and blue) stripe. One set of three subpixels makes up one unit pixel. The white light from the backlight
passes through the color filter and outputs all three colors; the intensity of which depends on how far the
liquid crystal gets bent. The human eye cannot resolve each color from a tiny pixel; instead the brain
mixes the 3 colors together to give the appearance of the combined color (such as mixing red and blue to
make purple).
Difference between monochromatic and TFT:
Monochromatic displays consist of a passive-matrix structure utilizing super-twisted nematic fluid with no
switching devices. Most of the monochromatic displays offer black and white images except for the color
STN types which offers 16 colors only. Slow response time and less contrast are typical of passive-matrix
addressed LCDs. TFTs consist of an active matrix structure utilizing a layer of transistors for addressing
each pixel. TFT offers full color capability, high pixel resolution and good contrast

8. Chart of Number of Pixels:
DisplayFormat Columns Rows Number of pixels
VGA 640 4 307.200
80
SVGA 800 6 480.000
00
XGA 1024 7 786.432
68
SXGA 1280 1 1310720
024
UXGA 1600 1 1920000
200
QXGA 2048 1 3145728
536
QSXGA 2560 2 5242800
048
QUXGA 3200 2 7680000
400
Architecture of A TFT Pixel:
The color filters for red, green and blue are integrated on to the glass substrate next to each other.
Each pixel (dot) is comprised of three of these color cells or sub-pixel elements. This means that
with a resolution of 1280 x 1024 pixels, exactly 3840 x 1024 transistors and pixel elements exist.
The dot or pixel pitch for a 15.1 inch TFT (1024 x 768 pixels) is about 0.0188 inch (or 0.30 mm)
and for an 18.1 inch TFT (1280 x 1024 pixels) it's about 0.011 inch (or 0.28 mm).

9. Fig: Pixels of a TFT. The left upper corner of a cell incorporates a Thin Film Transistor. Color filters allow the cells to change their
RGB basic colors.
The pixels are decisive and the smaller their spacing, the higher the maximum possible
resolution. However, TFTs are also subject to physical limitations due to the maximum display
area. With a diagonal of 15 inch (or about 38 cm) and a dot pitch of 0.0117 inch (0.297 mm), it
makes little sense to have a resolution of 1280 x 1024. Part 4 of this report covers the
relationship between dot pitch and diagonal dimensions in more detail.
Passive-matrixvs.active-matrixdrivingofLCDMonitors.
Inpassive-
matrixLCDs(
PMLCDs)ther
earenoswitch
ingdevices,an
deachpixelis
addressedfor
morethanone
frametime.Th
eeffectivevolt
ageappliedtot
heLCmust
averagethesi
gnalvoltagepulsesoverseveralframetimes,whichresultsinaslow
responsetimeofgreaterthan150msecandareductionofthemaximumcontrastratio.
TheaddressingofaPMLCDalsoproducesakindofcrosstalkthatproducesblurredimages
becausenon-selectedpixelsaredriventhroughasecondarysignal-voltagepath.Inactive-
matrixLCDs(AMLCDs),ontheotherhand,aswitchingdeviceandastoragecapacitorare
integratedattheeachcrosspointoftheelectrodes.
Theactiveaddressingremovesthemultiplexinglimitationsbyincorporatinganactive
switchingelement.Incontrasttopassive-matrixLCDs,AMLCDshavenoinherentlimitation
inthenumberofscanlines,andtheypresentfewercross-talkissues.Therearemany
kindsofAMLCD.Fortheirintegratedswitchingdevicesmostusetransistorsmadeof

10. depositedthinfilms,whicharethereforecalledthin-filmtransistors(TFTs).
Themostcommonsemiconductinglayerismadeofamorphoussilicon(a- Si).
a-SiTFTsareamenabletolarge-areafabricationusingglasssubstratesinalow-
temperature(300°Cto400°C)process.
AnalternativeTFTtechnology,polycrystallinesilicon-orpolysiliconorp-Si-iscostlyto
produceandespeciallydifficulttofabricatewhenmanufacturinglarge- areadisplays.
NearlyallTFTLCDsaremadefroma-Sibecauseofthetechnology'seconomyand
maturity,buttheelectronmobilityofap-SiTFTisoneortwoordersofmagnitudegreater
thanthatofana-SiTFT.
Thismakesthep-SiTFTagoodcandidateforanTFTarraycontainingintegrateddrivers,
whichislikelytobeanattractivechoiceforsmall,highdefinitiondisplayssuchasview
findersandprojectiondisplays.
Active addressing of a 3x3 matrix:
Fig: Active Addressing of 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

11. 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.