29 August 2020
3603
TFT Display is a thin film transistor type liquid crystal display. TFT liquid crystal is equipped with a semiconductor switch for each pixel, and each pixel can be directly controlled by dot pulses. So each node is relatively independent and can be continuously controlled, which not only improves the response speed of the display screen, but also allows precise control of displaying color gradation, so the color of TFT liquid crystal is truer.
Catalog
I TFT VS. LCD
In the fierce competition among many flat panel displays(FPD), why can TFT display stand out and become a new generation of mainstream displays? This is by no means accidental, but owing to the inevitable development of human technology and thinking mode.
The liquid crystal has avoided the difficult light-emitting problem, and the light-emitting display device is broken down into two parts by using the excellent characteristics of the liquid crystal as a light valve, namely the light source and the control of the light source.
As a light source, brilliant results have been achieved in terms of luminous efficiency, full color, and service life, and they are still being deepened.
Since the invention of the LCD, the backlight has been continuously improved, from monochrome to color, from thick to thin, from side fluorescent lamp type to flat fluorescent lamp type. The latest achievements in luminous light sources will provide new backlight sources for LCDs. With the advancement of light source technology, newer and better light sources will appear and be applied to LCDs.
The rest is the control of the light source. The technology and process of semiconductor large-scale integrated circuits are transplanted to develop the thin film transistor (TFT) production process, which realizes the matrix addressing control of the liquid crystal light valve and the cooperation of the light valve and the controller of the liquid crystal display, realizing the advantages of the liquid crystal display.
II TFT Display Working Principle
TFT is the abbreviation of "Thin Film Transistor", generally refers to thin-film liquid crystal displays, but actually refers that thin-film transistors (matrix)-can "actively" control each independent pixel on the screen, which is the origin of the so-called active matrix TFT.
So how exactly is the image produced? The basic principle is very simple: the TFT display screen is composed of many pixels that can emit light of any color, and the purpose can be achieved by controlling each pixel to display the corresponding color.
In the TFT display, backlight technology is generally used. In order to accurately control the color and brightness of each pixel, it is necessary to install a shutter-like switch after each pixel. When the "shutter" is opened, light can pass through. Conversely, light cannot pass through. Of course, technically, it is not as simple as this. LCD utilizes the characteristics of liquid crystals (become liquid when heated, and crystallized into solid when cooled). Generally, liquid crystals have three forms:
● Smectic liquid crystal similar to clay
● Nematic liquid crystal similar to fine matchstick
● Cholestatic liquid crystal similar to cholesterol
Figure 1. The Arrangement of Molecules in the Nematic, Smectic, and Cholesteric Liquid Crystal Phases
The liquid crystal display uses filaments, and when the external environment changes, its molecular structure will also change. Thus, it will have different physical properties-achieving the purpose of letting light through or blocking light-which is just like the shutters.
Everyone knows the three primary colors, so each pixel on the display screen needs three similar basic components described above to control the three colors of red, green, and blue respectively.
The most commonly used one is the twisted nematic TFT display. The following figure explains the working principle of this type of TFT display.
There are grooves on the upper and lower layers. The grooves on the upper layer are arranged longitudinally and the grooves on the lower layer are arranged horizontally. When no voltage is applied to the liquid crystal in its natural state, the light emitted from the light-emitting layer of the twisted nematic TFT display will be twisted by 90 degrees after passing through the interlayer, so that it can pass through the lower layer smoothly.
When a voltage is applied between the two layers, an electric field is generated. At this time, the liquid crystals are aligned vertically, so the light will not be twisted－the result is that the light cannot pass through the lower layer.
III TFT Pixel Architecture
Color filters are divided into red, green, and blue according to their colors. They are arranged on the glass substrate to form a group of dot pitches corresponding to a pixel. Each monochromatic filter is called a sub-pixel. In other words, if a TFT display supports a maximum resolution of 1280×1024, then at least 1280×3×1024 sub-pixels and transistors are required.
For a 15-inch TFT display (1024×768), then a pixel is about 0.0188 inches (equivalent to 0.30mm), and for an 18.1 inch TFT display (1280×1024), it is 0.011 inches (equivalent to 0.28mm) .
As we all know, pixels are decisive for the display. The smaller each pixel is, the larger the maximum resolution that the display can achieve. However, due to the limitation of the physical characteristics of the transistors, the size of each pixel of the TFT at this stage is basically 0.0117 inches (0.297mm), so for a 15-inch display, the maximum resolution is only 1280×1024.
IV Main Features
The TFT LCD display is characterized by good brightness, high contrast, strong layering, and bright colors, but it also has the disadvantages of high power consumption and high cost. TFT display technology has accelerated the development of the color cell phone. Many of the new generations of color-screen mobile phones support 65536 TFT color displays, and some even support 160,000 color displays. Therefore, the advantages of TFT's high contrast and rich colors become very important.
1.
Technical Characteristics
TFT technology was developed in the 1990s. It uses new materials and new processes to manufacture large-scale semiconductor integrated circuits, which is the basis of liquid crystal (LC), inorganic and organic thin-film EL, and OEL flat panel displays.
TFT is the films formed on the non-single wafer such as a glass or plastic substrate (of course, it can also be on a wafer) through sputtering, chemical deposition processes, which is necessary for manufacturing circuits. Through film processing, large-scale semiconductor integrated circuits (LSIC) are manufactured.
Figure 2. Sputtering Operations
The use of non-single-crystal substrates can greatly reduce costs and is an extension of traditional large-scale integrated circuits to large-area, multi-function, and low-cost directions. Manufacturing TFTs that control the switching performance of pixel (LC or OLED) on a large-area glass or plastic substrate is more technically difficult than manufacturing large-scale ICs on silicon. The requirements for the production environment (purification degree is 100), the requirements for the purity of raw materials (the purity of electronic special gas is 99.999985%), and the requirements for production equipment and production technology exceed that of the large-scale integration of semiconductors, which is the top of modern large-scale production technology. Its main features are:
(1) Large Area
The first generation of large-area glass substrate (300mm×400mm) TFT display production line was put into production in the early 1990s. By the first half of 2000, the area of the glass substrate had been expanded to 680mm×880mm, and 950mm×1200mm glass substrate will also be put into operation. In principle, there is no area limitation.
(2) High Integration
The 1.3-inch TFT chip used for liquid crystal projection has a resolution of XGA containing millions of pixels. The film thickness of the SXGA (1280×1024) 16.1-inch TFT array amorphous silicon is only 50nm, with TAB ON GLASS and SYSTEM ON GLASS technologies. The integration of its IC, the requirements for equipment and supply technology, and the technical difficulty all exceed the traditional LSI.
(3) Powerful Function
TFT was first used as a matrix location circuit to improve the light valve characteristics of liquid crystal. For high-resolution displays, through voltage adjustment in the range of 0-6V (typically 0.2 to 4V), it can precisely control the object, making it possible for LCD to achieve high-quality and high-resolution displays.
TFT display is the first flat panel display in human history that exceeds CRT in display quality. And people begin to integrate the driver IC on the glass substrate, so the function of the entire TFT will be more powerful, which is unmatched by traditional large-scale semiconductor integrated circuits.
(4) Low Cost
Glass substrates and plastic substrates fundamentally solve the cost problem of large-scale semiconductor integrated circuits, opening up a broad application space for the application of large-scale semiconductor integrated circuits.
(5) Flexible Process
In addition to sputtering, CVD (Chemical Vapor Deposition), MCVD (Molecular Chemical Vapor Deposition), and other traditional processes for film formation, laser annealing technology has also begun to be applied. This technology can produce amorphous films, polycrystalline films, and single-crystal films. Besides, it can not only produce silicon films, but also semiconductor films from other groups as II-VI and III-V.
(6) Wide Range of Application Fields
Liquid crystal flat panel displays based on TFT technology are the pillar industry of the information society. The technology can also be applied to the rapidly growing thin film transistor organic electroluminescence (TFT-OLED) flat panel displays.
2.
Other Main Features
With the maturity of TFT technology in the early 1990s, color LCD flat-panel displays developed rapidly. In less than 10 years, TFT display has rapidly grown into mainstream displays, which is inseparable from its advantages. The main features are:
(1) Good Operating Characteristics :
● Low voltage application, low driving voltage, and improved safety and reliability of solid use;
● Flat-panel, light and thin, saving a lot of raw materials and used space;
● Low power consumption, about one-tenth of CRT display, and that of reflective TFT display is even only about one-percent of CRT, saving a lot of energy;
● TFT display products also have specifications, models, and sizes in series, with many varieties. They're convenient and flexible to use and easy for maintenance and update.
● The display range covers the application range of all displays from 1 inch to 40 inches and large projection planes, which is a full-size display terminal;
● The display quality ranges from the simplest monochrome character graphics to various specifications and models of video displays with high resolution, high color fidelity, high brightness, high contrast, and high response speed;
● Display methods include direct view, projection, see-through, and reflective type.
(2) Good Environmental Protection Features no radiation, no flicker, no harm to the health of users. Especially the emergence of TFT display e-books will bring mankind into the era of paperless office and paperless printing, triggering a revolution in human learning, transmitting, and recording of civilization.
(3) The application range is wide, and it can be used normally in the temperature range from -20℃ to +50℃. The low temperature working temperature of the TFT display after temperature strengthening can reach minus 80℃. It can be used as a mobile terminal display, desktop terminal display, and large-screen projection TV, which is a full-size video display terminal with excellent performance.
(4) High automation degree of manufacturing technology, and good large-scale industrial production characteristics. The TFT display industry technology is mature, and the mass production yield rate has reached more than 90%.
(5) TFT display is easy to integrate and upgrade. It is a perfect combination of large-scale semiconductor integrated circuit technology and light source technology and has great potential for continued development. At present, there are amorphous, polycrystalline, and monocrystalline silicon TFT displays. In the future, there will be TFTs of other materials, including glass substrates and plastic substrates.
V TFT Display Types
1.
TN
TN+film (Twisted Nematic + film) is the most common type, mainly due to the low price and variety of products. On modern TN-type panels, the pixel response time is fast enough to greatly reduce image retention. Even in terms of specifications, the response time is already fast, but this traditional response time is a standard established by ISO, which only defines the conversion time from black to full white, rather than between gray levels. The conversion time between grayscales (which is actually a more frequent conversion of ordinary liquid crystals) takes longer than that defined by ISO.
The RTC-OD (Response Time Compensation-Overdrive) technology currently used allows manufacturers to effectively reduce the conversion time between different gray levels (G2G). However, the response time defined by ISO has not actually changed. The response time is now represented by G2G (Gray To Gray) numbers, such as 4ms and 2ms, which are commonplace in TN+Film products.
TN-type displays have viewing angle limitations, especially in the vertical direction, and most of them cannot display the 16.7 million colors (24-bit true colors) output by the current graphics card. Through a special method, the RGB three colors use 6 bits as 8 bits. It uses a reduction method that combines adjacent pixels to approach the 24-bits color to simulate the desired grayscale. Some people use FRC (Frame Rate Control)
For liquid crystal displays, the actual transmittance of the pixel generally does not change linearly with the applied voltage.
In addition, B-TN (Best TN) was developed by Samsung Electronics, which improves TN color and response time.
2.
STN
STN display is the abbreviation of super twisted nematic liquid crystal. After TN liquid crystal was invented, people naturally thought of matrixing TN liquid crystal to display complex graphics. TN liquid crystal twists 90 degrees, while STN liquid crystal twists 180 degrees to 270 degrees.
Color STN liquid crystal came out in the early 1990s. One pixel of this liquid crystal is composed of three liquid crystal cells, covered with a layer of the color filter, and the brightness of the liquid crystal cells is controlled by the voltage to produce colors.
3.
VA
(1) CPA (Continuous Pinwheel Alignment) was developed by SHARP. High color reproduction, low output, and high price.
Figure 3. Sharp TFT LCD Display
(2) MVA (Multi-domain Vertical Alignment) was developed by Fujitsu in 1998 to serve as a compromise between TN and IPS. At the time, it had a fast pixel response, wide viewing angle, and high contrast, but it sacrificed brightness and color reproducibility.
Analysts predict that MVA technology will dominate the entire mainstream market, but TN has this advantage. The main reason is the higher cost and slower pixel response of MVA (it will increase greatly when the brightness changes are small).
● P-MVA (Premium MVA) was developed by AUO to improve the viewing angle and response time of MVA.
● A-MVA (Advanced MVA) was developed by AUO.
● S-MVA (Super MVA) was developed by Chimei Electronics.
● PVA (Patterned Vertical Alignment) was developed by Samsung Electronics. Although the company said it has the best comparison technology, it also has the same problems as MVA.
● S-PVA (Super PVA) was developed by Samsung Electronics to improve the viewing angle and response time of PVA.
● C-PVA was developed by Samsung Electronics.
4.
IPS
IPS (In-Plane Switching) was developed by Hitachi in 1996 to modify the poor viewing angle and color reproducibility of TN-type panels. This kind of improvement has increased the response time, and the initial level is 50ms. The cost of an IPS-type panel is also extremely expensive.
S-IPS (Super IPS) not only has the advantages of IPS technology but also improves the pixel update time. The color reproducibility is closer to that of CRTs, and the price is lower. However, the contrast is still very poor. At present, S-IPS is only used on larger displays for professional purposes.
5 . Super PLS
PLS (Plane to Line Switching) is developed by Samsung Electronics. In addition to the amazing viewing angle, it can also improve the brightness of the display by 10%. The manufacturing cost is 15% less than that of IPS. The current resolution provided is up to WXGA (1280×800). MacBook Pro with Retina display also partly uses this kind of display produced by Samsung (resolution up to 2880×1800), and the rest still use IPS display. The main objects will be focused on the small TFT display on mobile phones and tablet PCs and were mass-produced in 2011.
6. ASV
SHARP developed ASV (Advanced Super-V) technology to improve the viewing angle of TFT.
7.
FFS
Modern electronics adopts FFS (Fringe Field Switching) technology. FFS technology is an advanced extension of IPS (In-Plane Switching) wide viewing angle technology, which has low power consumption and high brightness. FFS can be extended to AFFS+ (Advanced FFS+) and HFFS (High aperture FFS) technology. AFFS+ has a visual function in the sun.
8.
OCB
OCB (Optical Compensated Birefringence) is a technology of Matsushita Electric.
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