Thin film transistor

Contents

Ø Introduction

Ø How is TFT manufactured???

Ø What are the aplication of TFT’S ???

Ø TFT LCD

Ø Construction

Ø Types

Ø Electrical interface

Thin-film transistor

Intoduction

A thin film transistor comprises a semiconductor layer, a gate insulating layer and a gate electrode. Further, the semiconductor layer is typically a polysilicon layer whose electron mobility is 100 times higher than that of an amorphous silicon layer. Thin film transistors are used as switching devices in flat display panels such as organic light emitting diodes (OLEDs) or liquid crystal displays (LCDs). A liquid crystal display (LCD) is one of the most widely used flat panel displays since it is lightweight and occupies less space than conventional cathode ray tube (CRT) displays. The applications for liquid crystal displays are extensive, such as mobile phones, digital cameras, video cameras, notebooks, and monitors. LCD together with other flat panel type display such as plasma displays and electroluminance displays has become one of the most researched types of displays. A TFT LCD display is composed of a thin film transistor array substrate, a color filter array substrate and a liquid crystal layer, wherein the thin film crystal transistor array substrate is composed of many thin film transistors arranged in an array and pixel electrodes corresponding to each thin film transistor to form pixel structures. The thin film transistor array substrate includes gate lines and data lines, a thin film transistor formed as a switching device at every crossing of the gate lines and the data lines, a pixel electrode connected to the thin film transistor substantially defining a liquid crystal cell, and an alignment film applied to the substrate. A thin film transistor is usually fabricated as MOSFET (metal oxide semiconductor field effect transistor) including an electrically insulating substrate such as a glass substrate and a thin semiconductor layer formed on the substrate and acting as an active region. When a thin film transistor is used in an active matrix type liquid crystal display device, for instance, the thin film transistor is designed to be driven by a driver integrated circuit as a switching device for switching pixels.

How is a TFT Manufactured???????

TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon based TFT depend on the crystalline state. That is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be annealed into polysilicon.

Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide and metal oxides such as Zinc Oxide. TFT's have also been made using organic materials (referred to as an Organic TFT or OTFT).

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent.

Because the substrate cannot withstand the high annealing temperature, the deposition process has to be completed under relatively low temperature. Chemical vapor deposition, physical vapor deposition (usually sputtering) are applied. Also, the first solution processed transparent TFTs (TTFTs), based on zinc oxide were reported in 2003 by researchers at Oregon State University.

Meanwhile, Portuguese laboratory CENIMAT, Universidade Nova de Lisboa, discovered a way of producing TFT at room temperature, having produced the world’s first completely transparent TFT at room temperature. CENIMAT also developed the first paper transistor, which may lead to applications such as magazines and journal pages with moving images.

What are the Applications of TFT’S ???

The best known application of thin-film transistors is in TFT LCDs, an implementation of LCD technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability.

As of 2008^[update], many color LCD TVs and monitors use this technology.^{[broken citation]} TFT panels are heavily used in digital radiography applications in general radiography. It is used in both direct and indirect capture as a base for the image receptor in medical radiography.

The new AMOLED (Active Matrix OLED) screens also contain a TFT layer.

The most beneficial aspect of TFT technology is a separate transistor for each pixel on the display. As each transistor is small, the amount of charge needed to control it is also small. This allows for very fast re-drawing of the display.

Prior to TFT, passive matrix LCD displays could not keep up with fast moving images. A mouse dragged across the screen, for example, from point A to point B, would disappear between the two points. A TFT monitor can track the mouse, resulting in a display that can be used for video, gaming and all forms of multimedia.

TFT LCD

A thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin-film transistor (TFT) technology to improve image quality (e.g., addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones and computers, handheld video game systems, personal digital assistants, navigation systems, projectors, etc.

Construction

Small liquid crystal displays as used in calculators and other devices have direct driven image elements—a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of picture elements (pixels), since it would require millions of connections—top and bottom connections for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor prevents the voltage applied to the pixel from leaking away between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.

The circuit layout of a TFT-LCD is very similar to that of a DRAM memory. However, rather than fabricating the transistors from silicon formed into a crystalline wafer, they are made from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.

The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon (frequently LTPS, low-temperature poly-Si) is sometimes used in displays requiring higher TFT performance. Examples include high-resolution displays, high-frequency displays or displays where performing some data processing on the display itself is desirable. Amorphous silicon-based TFTs have the lowest performance, polycrystalline silicon TFTs have higher performance (notably mobility), and single-crystal silicon transistors are the best performers.

Types of TFT

Twisted nematic (TN)

TN display under a microscope, with the transistors visible at the bottom

The inexpensive twisted nematic display is the most common consumer

display type. The pixel response time on modern TN panels is sufficiently fast

to avoid the shadow-trail and ghosting artifacts of earlier production. The fast response time has been emphasised in advertising TN displays, although in most cases this number does not reflect performance across the entire range of possible color transitions. More recent use of RTC (Response Time Compensation—Overdrive) technologies has allowed manufacturers to significantly reduce grey-to-grey (G2G) transitions, without significantly improving the ISO response time. Response times are now quoted in G2G figures, with 4ms and 2ms now being commonplace for TN based models. The good response time and low cost has led to the dominance of TN in the consumer market.

TN displays suffer from limited viewing angles, especially in the vertical direction. Als

o, TN panels represent colors using only 6 bits per color, instead of 8, and thus are not able to display the 16.7 million color shades (24-bit truecolor) that are available from graphics cards. Instead, these panels display interpolated 24-bit color using a dithering method that combines adjacent pixels to simulate the desired shade. They can also use Frame Rate Control (FRC), which cycles pixels on and off to simulate a given shade. These color simulation methods are noticeable to many people and bothersome to some. FRC tends to be most noticeable in darker tones, while dithering appears to make the individual pixels of the LCD visible. Overall, color reproduction and linearity on TN panels is poor. Shortcomings in display color gamut (often referred to as a percentage of the NTSC 1953 color gamut) are also due to backlighting technology. It is not uncommon for displays with CCFL (Cold Cathode Fluorescent Lamps)-based lighting to range from 10% to 26% of the NTSC color gamut, whereas other kind of displays, utilizing RGB LED backlights, may extend past 100% of the NTSC color gamut—a difference quite perceivable by the human eye.

The transmittance of a pixel of an LCD panel typically does not change linearly with the applied voltage, and the sRGB standard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of the RGB value.

In-plane switching (IPS)

In-plane switching was developed by Hitachi Ltd. in 1996 to improve on the poor viewing angle and the poor colour reproduction of TN panels at that time. Its name comes from the main difference from TN panels, that the crystal molecules move parallel to the panel plane instead of perpendicular to it. This change reduces the amount of light scattering in the matrix, which gives IPS its characteristic wide viewing angles and good colour reproduction.

Initial iterations of IPS technology were plagued with slow response time and a low contrast ratio, but later evolutions have made marked improvements to these shortcomings. Because of its wide viewing angle and accurate colour reproduction it's widely employed in high-end monitors aimed at professional graphic artists, although with the recent fall in price it has been seen in the mainstream market too.

Multi-domain vertical alignment (MVA)

Multi-domain vertical alignment was originally developed in 1998 by Fujitsu as a compromise between TN and IPS. It achieved pixel response which was fast for its time, wide viewing angles, and high contrast at the cost of brightness and color reproduction. Modern MVA panels can offer wide viewing angles (second only to S-IPS technology), good black depth, good color reproduction and depth, and fast response times due to the use of RTC (Response Time Compensation) technologies. There are several "next-generation" technologies based on MVA, including AU Optronics' P-MVA and A-MVA, as well as Chi Mei Optoelectronics' S-MVA.

Analysts predicted that MVA would dominate the mainstream market, but the less expensive and slightly faster TN overtook it. The pixel response times of MVAs rise dramatically with small changes in brightness. Less expensive MVA panels can use dithering and FRC (Frame Rate Control).

Patterned vertical alignment (PVA)

Patterned vertical alignment and super patterned vertical alignment (S-PVA) are alternative versions of MVA technology offered by Samsung's and Sony's joint venture S-LCD. Developed independently, they offer similar features to MVA, but with higher contrast ratios of up to 3000:1. Less expensive PVA panels often use dithering and FRC, while S-PVA panels all use at least 8 bits per color component and do not use color simulation methods. S-PVA also largely eliminated off angle glowing of solid blacks and reduced the off angle gamma shift. Some newer S-PVA panels offered by Eizo offer 16-bit color internally, which enables gamma and other corrections with reduced color banding. Some high end Sony BRAVIA LCD-TV offer 10bit and xvYCC color support. PVA and S-PVA offer the best black depth of any LCD type along with wide viewing angles. S-PVA also offers fast response times using modern CRT technologies.

Advanced super view (ASV)

Advanced super view, also called axially symmetric vertical alignment was developed by Sharp. It is a VA mode where LC molecules orient perpendicular to the substrates in the off state. The bottom sub-pixel has continuously covered electrodes, while the upper one has a smaller area electrode in the center of the subpixel.

When the field is on, the LC molecules start to tilt towards the center of the sub-pixels because of the electric field; As a result, a continuous pinwheel alignment (CPA) is formed; the azimuthal angle rotates 360 degrees continuously resulting in an excellent viewing angle. The ASV mode is also called CPA mode.

Electrical interface

External consumer display devices like a TFT LCD mostly use an analog VGA connection, while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort. Inside external display devices there is a controller board that will convert CVBS, VGA, DVI, HDMI etc. into digital RGB at the native resolution of the display panel. In a laptop the graphics chip will directly produce a signal suitable for connection to the built-in TFT display. A control mechanism for the backlight is usually included on the same controller board.

The low level interface of STN, DSTN, or TFT display panels use either single ended TTL 5V signal for older displays or TTL 3.3V for slightly newer displays that transmits Pixel clock, Horizontal sync, Vertical sync, Digital red, Digital green, Digital blue in parallel. Some models also feature input/display enable, horizontal scan direction and vertical scan direction signals.

New and large (>15 in) TFT displays often use LVDS or TMDS signaling that transmits the same contents as the parallel interface (Hsync, Vsync, RGB) but will put control and RGB bits into a number of serial transmission lines synchronized to a clock at 1/3 of the data bitrate. Usually with 3 data signals and one clock line. Transmitting 3x7 bits for one clock cycle giving 18-bpp. An optional 4th signal enables 24-bpp.

Backlight intensity is usually controlled by varying a few volts DC, or generating a PWM signal, adjusting a potentiometer or simple fixed. This in turn controls a high-voltage (1.3 kV) DC-AC inverter or a matrix of LEDs.

The bare display panel will only accept a digital video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore colour LSB bits to present a consistent interface (8bit->6bit/colour).

The reason why laptop displays can't be reused directly with an ordinary computer graphics card or as a television, is mainly because it lacks a hardware rescaler (often using some discrete cosine transform) that can resize the image to fit the native resolution of the display panel. With analogue signals like VGA the display controller also needs to perform a highspeed analog to digital conversion. With digital input signals like DVI or HDMI some simple bit stuffing is needed before feeding it to the rescaler if input resolution doesn't match the display panel resolution. For CVBS (TV) usage a tuner and colour decode from a quadrature amplitude modulation (QAM) to Luminance (Y), Blue-Y (U), Red-Y (V) representation which in turn is transformed into Red, Green Blue is needed.