Department of Electronics Engineering Tsinghua University, Beijing, ChinaA Report On Thin Film Transistors December 28th 2012 By Tayyab Farooq (阿里) Student ID: 2012280158
What is TFT LCD?TFT stands for Thin Film Transistor, A TFT is actually a component of a LCD designed to improve thequality and control of the LCD display. It is basically a tiny transistor linked to each individual pixel on thescreen. In today’s marketplace, TFT technology provides the best resolution of all the flat-paneltechniques. TFT screens are sometimes called active-matrix LCDs.TFT LCD (Thin Film Transistor Liquid Crystal Display) has a sandwich-like structure with liquid crystalfilled between two glass plates. Fig: TFT Sandwich between two glassesTFT Glass has as many TFTs as thenumber of pixels displayed, while aColor Filter Glass has color filter whichgenerates color. Liquid crystals moveaccording to the difference in voltagebetween the Color Filter Glass and theTFT Glass. The amount of lightsupplied by Back Light is determined bythe amount of movement of the liquidcrystals in such a way as to generatecolor. Fig: A PixelThe TFTs in active-matrix LCD act as simple ON/OFF switches, at different speeds which depend on therefresh rate of the LCD, for example 60Hz. Figure below shows a simple structure of TFT, it consists ofthree terminals: the gate, the source and the drain. As seen in figure 1, the gate is insulated from thesemiconductor film by a gate insulation film; while the drain and source directly contact the semiconductorfilm.
A simple Thin-Film-Transistor (TFT) structureIn a simple TFT, for example N-channel TFT, a positive voltage is applied on the gate in order to switch itON; the insulation layer can be considered as the dielectric layer in a capacitor, hence negative chargesare induced on the semiconductor channel, which is the region between source and drain; these negativecharges create a electrons flow from source to drain to make the channel conductive. When a negativevoltage is applied on the gate,electrons are depleted in thechannel, hence almost no current ispresent. The ON current dependson different parameters, forexample channel width, channellength, gate voltage and thethreshold voltage of the TFT.When the TFT is switched ON, adata voltage is applied on thesource, the drain with the LC loadcapacitance will charge up to thevoltage with same amplitude, i.e.transferring the data voltage from the data line to the pixel electrode. When switched OFF, no current inthe channel, and data voltage cannot be transferred. A TFT substrateconsists of a matrix of pixels and a regioncalled ITO (a transparentelectricallyconductivefilm) eachhaving a TFT device. Thousandsormillions of thesepixelstogethercreateanimageonthescreen.Thediagram shows thestructure of a single pixel.
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 filterPolarizer Black MatrixAlignment Layer Pixel Electrode Layer (ITO) LCD Crystals TF Space Seal T rAlignment Layer Pixel ElectrodeBonding Pad Layer (ITO) Array Substrat ePolarizer Backlight
A TFT panel array contains a specific number of pixels, often known as subpixels. Thousands or millionsof these unit pixels together create an image on the display. This diagram shows the simple structure of asub-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 toturn each individual pixel on or off therebycontrolling the number of electrons thatflow into the ITO zone. As the number ofelectrons reaches the expected value,TFT turns off and these electrons can bekept within the ITO zone.Because each unit pixel is connectedthrough the matrix, each is individuallyaddressable from the bonding pads at theends of the rows and columns.The performance of the TFT LCD isrelated to the design parameters of theunit pixel, i.e., the channel width W andthe channel length L of the TFT, the overlap between TFT electrodes, the sizes of the storage capacitorand 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 linesalso 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 whichcontains a transistor, charge collector electrode and storage capacitor, all arranged on an amorphoussilicon substrate. Individual elements are connected by gate lines along rows (operating the TFT), bydrain lines along columns (connected tothe TFT output), and charge amplifiersconnected to the drain lines to receive thecharge from specific detector elements. Inoperation, local charge created by local X-ray absorption is stored at each detectorelement and actively read by turning rowgate lines on one at a time, allowingcharge to pass from the local storagecapacitor through the TFT, down the drainline to the charge amplifier. Eachtransistor is reset and ready for the nextexposure. The image is createdsequentially, row by row. For real-time (30frame per second) fluoroscopy, all of thedetector 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 fallupon inactive areas of the TFT matrix
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 FiltersFig: Illustration represents one pixel.
When power is applied to bend the liquid crystal, light passes through from the backlight into the colorfilter. How much light that passes through depends on the amount of power applied to the pixel. If therewere no color filter, the output would be in the form of a grayscale. The color filter is an RGB (red, greenand blue) stripe. One set of three subpixels makes up one unit pixel. The white light from the backlightpasses through the color filter and outputs all three colors; the intensity of which depends on how far theliquid crystal gets bent. The human eye cannot resolve each color from a tiny pixel; instead the brainmixes the 3 colors together to give the appearance of the combined color (such as mixing red and blue tomake purple).Difference between monochromatic and TFT:Monochromatic displays consist of a passive-matrix structure utilizing super-twisted nematic fluid with noswitching devices. Most of the monochromatic displays offer black and white images except for the colorSTN types which offers 16 colors only. Slow response time and less contrast are typical of passive-matrixaddressed LCDs. TFTs consist of an active matrix structure utilizing a layer of transistors for addressingeach pixel. TFT offers full color capability, high pixel resolution and good contrast
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 400Architecture 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 thatwith 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) its about 0.011 inch (or 0.28 mm).
Fig: Pixels of a TFT. The left upper corner of a cell incorporates a Thin Film Transistor. Color filters allow the cells to change theirRGB basic colors.The pixels are decisive and the smaller their spacing, the higher the maximum possibleresolution. However, TFTs are also subject to physical limitations due to the maximum displayarea. With a diagonal of 15 inch (or about 38 cm) and a dot pitch of 0.0117 inch (0.297 mm), itmakes little sense to have a resolution of 1280 x 1024. Part 4 of this report covers therelationship 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
depositedthinfilms,whicharethereforecalledthin-filmtransistors(TFTs). Themostcommonsemiconductinglayerismadeofamorphoussilicon(a- Si). a-SiTFTsareamenabletolarge-areafabricationusingglasssubstratesinalow- temperature(300°Cto400°C)process. AnalternativeTFTtechnology,polycrystallinesilicon-orpolysiliconorp-Si-iscostlyto produceandespeciallydifficulttofabricatewhenmanufacturinglarge- areadisplays. NearlyallTFTLCDsaremadefroma-Sibecauseofthetechnologyseconomyand maturity,buttheelectronmobilityofap-SiTFTisoneortwoordersofmagnitudegreater thanthatofana-SiTFT. Thismakesthep-SiTFTagoodcandidateforanTFTarraycontainingintegrateddrivers, whichislikelytobeanattractivechoiceforsmall,highdefinitiondisplayssuchasview findersandprojectiondisplays.Active addressing of a 3x3 matrix: Fig: Active Addressing of 3x3 matrixBy scanning the gate bus-linessequentially, and by applying signalvoltages to all source bus-lines in aspecified sequence, we can address allpixels. One result of all this is that theaddressing of an AMLCD is done lineby line.Virtually all AMLCDs are designed toproduce gray levels - intermediatebrightness levels between the brightest
white and the darkest black a unit pixel can generate. There can be either a discrete numbers oflevels - 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 appliedvoltage.An analog LDI is capable of producing a continuous voltage signal so that a continuous range ofgray levels can be displayed.The digital LDI produces discrete voltage amplitudes, which permits on a discrete numbers ofshades to be displayed. The number of gray levels is determined by the number of data bitsproduced by the digital driver.