Thin film transistors [TFT]

Introduction

Flat-panel displays (FPDs) are becoming increasingly commonplace in today's commercial electronic devices. FPDs are finding widespread use in many new products, such as cellular phones, personal digital assistants (PDAs), camcorders, and laptop personal computers (PCs). This generation of handheld electronics places stringent demands on their displays. FPDs in these devices are expected to be lightweight, portable, rugged, low-power and high-resolution. Displays having all these attributes will enable a wide variety of commercial applications in the future.

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, personal digital assistants, navigation systems, projectors, etc.

Definitions of Terms:

• Liquid-Crystal Displays (LCD's):
  LCD's are currently the leading flat-panel display technology. Liquid crystals change orientation under an applied electric field and can thereby block or pass light.
  
• Active-Matrix Liquid-Crystal Displays (AMLCD's):
  LCD technology that incorporates an active-matrix, as opposed to a passive-matrix or "dual-scan" technology.
  
• High Mobilities:
  Mobility is the proportionality constant that relates the drift velocity to the electric field strength in a semiconductor. Mobility essentially gauges how easily current carriers (i.e. electrons, holes) can move through a piece of silicon. Electrons move most easily through single-crystalline silicon because of the uniform arrangement of the atoms. Unfortunately, single-crystalline films are difficult to deposit due to the low melting point of glass.
• Low Leakage Currents:
  Leakage current refers to the small amount of current that flows (or "leaks") through a transistor when it is "turned off." In an ideal transistor, leakage current would be zero, but in practice, leakage current always has a finite value. Leakage current causes the voltage in the pixel capacitor to drop between each frame refresh, and thus changes the pixel brightness. Leakage current most significantly affects the fineness of the display's grayscale. With a low leakage current, finer levels of grayscale can be achieved.
  
• Threshold Voltages:
  The voltage necessary to turn on a transistor. Threshold voltages should be low so that it takes lower voltages to charge and discharge the display's pixels (thereby turning them on and off).
  
• Integration of Driver Circuitry:
  A display needs row and column drivers to properly read the image data into the pixels. Most displays are "dumb" and have external IC drivers that require bonded connections to the rows and columns.
  
• Amorphous Silicon TFT's:
  TFTs that are made using a thin layer of amorphous silicon. Atoms in amorphous silicon have no short- or long-range order. When a film of silicon is deposited at low-temperature on glass or plastic, the atoms are normally arranged in this amorphous state. High temperatures are required if films are to crystallize into poly-Si.
  
  
• Gate-Dielectric Materials:
  In a transistor, we want the current to flow from the source to the drain, and not into the gate. Thus, we must put an insulating material between the gate and the channel of the transistor. The most common gate-dielectric material is silicon dioxide.
  
  
• High-Voltage Bias Stressing:
  This refers to a testing procedure in which newly fabricated electronics are tested for reliability by applying high voltages to them.
  
  
• Physically Based Model:
  A model of transistor operation based on measured data, as opposed to a theory-based model.

Construction:-

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:-

There are many types of TFT available in the markets.

· TN+Film :- The TN display suffers from limited viewing angles, especially in the vertical direction. For colour representation many panels use 6 bits per colour, instead of 8, and are consequently unable to display the full 24-bit truecolor (16.7 million colour shades) available from modern graphics cards.

· IPS :- IPS (in-plane switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Most panels also support true 8-bit per channel color.Their are few types of IPS in market.

1. AS-IPS.

2. AS-TW-IPS

· MVA :- 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.

· PVA :- PVA (patterned vertical alignment) and S-PVA (super patterned vertical alignment) are alternative versions of MVA technology offered by Samsung.

· CVA :- CPA (Continuous Pinwheel Alignment) was developed by Sharp.

Electrical interface:-

External consumer display devices like a TFT LCD mostly use an analogue VGA connection, while newer, more expensive models mostly feature a digital interface like DVI, HDMI, or DisplayPort as well.

Inside an external display device there is a controller board that will convert VGA, DVI, HDMI, CVBS etc. to digital RGB at native resolution that the display panel can make use of. In a laptop the graphics chip will directly produce a signal suitable for connection to the builtin TFT. A control mechanism for the backlight is usually included on the same controller board.

The lowlevel interface of STN, DSTN, or TFT display panels use either single ended TTL 5V or TTL 3.3V 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") TFT displays often use LVDS or TMDS signaling that is the same as the parallel interface 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.

Backlight intensity is usually controlled by varying a few volts DC to the backlight highvoltage (1.3kV) DC-AC inverter. It can also be controlled by a potentiometer or be fixed. Some models use PWM signal for intensity control.

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

Applications

Organics have long been attractive for use in electronics because of their light weight, flexibility, and low cost compared with their Si counterparts. Recent increases in performance, however, have rapidly expanded organic FETs from niche markets, making them targets for a wider range of applications.

Organics offer potential advantages in displays, where TFTs are implemented as switches to activate individual pixels. Hand-held devices (cell phones, PDAs, etc.) with ultrathin displays can achieve higher resolution and information content, while new technologies, such as flexible displays and electronic paper, are potentially revolutionary advancements. Integrated smart pixels, with an OTFT switching an organic light-emitting diode (OLED) pixel, have been demonstrated, even though actual OTFT active-matrix OLED displays are yet to be demonstrated.

An alternative to active-matrix flexible displays is an innovative example by E-ink utilizing an OTFT backplane with a laminated electronic ink frontplane, consisting of a layer of electrophoretic microcapsules on a transparent electrode. The OTFT backplane controls the contrast of the display by moving charged black and white pigments to the transparent layer. In late 2000, E-Ink presented the world's first flexible (16 cm × 16 cm) electronic ink display using an OTFT backplane circuit created by Lucent, consisting of an array of 256 transistors fabricated using a low-cost, rubber stamp printing process. The printed transistors from Lucent and a typical flexible. Plastic Logic, a company actively developing ink-jet-printed plastic TFTs, subsequently has demonstrated a bistable reflective display driven by an ink-jet-printed active-matrix backplane together with Gyricon Media, the provider of SmartPaper™ reusable display material. This first experimental prototype is a display featuring 3024 pixels (63 × 48) at 50 dpi on a glass substrate. More recently, Philips and E-Ink jointly demonstrated a similar electronic ink display driven by OTFTs with 320 × 240 pixels, a diagonal length of 127 mm, a resolution of 85 dpi, and a bending radius of 2 cm. Philips has also announced that it had formed a technology incubator company, Polymer Vision, to partner with other companies interested in ultrathin, rollable displays that could double as electronic paper.

Schematic of an E-ink display. An OTFT backplane addresses each element, with encapsulated, charged pigments shifting to the transparent electrode surface.

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A 256-transistor array produced by Lucent using a rubber stamp printing process. (Reprinted with permission from.

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The world's first flexible electronic ink display driven by organic transistors. (Reprinted with permission fromal Academy of Sciences USA.)

Safety Measures:-

The liquid crystals inside the display are poisonous and must not be ingested or brought into contact with skin. Spills from a cracked display should be washed off immediately with soap and water.

The leading manufacturer of liquid crystal materials for display applications states as follows:

Merck KGaA has committed itself to not introduce into the market liquid crystal materials which are either acutely toxic or mutagenic.

The complete report "Toxicological and Ecotoxicological Investigations of Liquid Crystals; Disposal of LCDs" is available from Merck KGaA