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

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.

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.

Fig: A Pixel

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 substrate consists of a matrix of pixels and a region called ITO (a transparent electrically conductive film) each having a TFT device. Thousands or millions of these pixels together create an image on the screen. The diagram shows the structure 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.

PolarizerColor filter

ArraySubstrate

Bonding Pad

Seal

Black Matrix

LCD Crystals

Polarizer

Pixel ElectrodeLayer (ITO)

Space

Pixel Electrode

Alignment Layer

Alignment Layer

TF

Backlight

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

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.

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

Chart of Number of Pixels:

Display Format Columns Rows Number of pixels

VGA 640 480 307.200

SVGA 800 600 480.000

XGA 1024 768 786.432

SXGA 1280 1024 1310720

UXGA 1600 1200 1920000

QXGA 2048 1536 3145728

QSXGA 2560 2048 5242800

QUXGA 3200 2400 7680000

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).

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-matrix vs. active-matrix driving of LCD Monitors.

In passive- matrix LCDs (PMLCDs) there are no switching devices, and each pixel is addressed for more than one frame time. The effective voltage applied to the LC must average the signal voltage pulses over several frame times, which results in a slow response time of greater than 150 msec and a reduction of the maximum contrast ratio. The addressing of a PMLCD also

produces a kind of crosstalk that produces blurred images because non-selected pixels are driven through a secondary signal-voltage path. In active- matrix LCDs (AMLCDs), on the other hand, a switching device and a storage capacitor are integrated at the each cross point of the electrodes.

The active addressing removes the multiplexing limitations by incorporating an active switching element. In contrast to passive-matrix LCDs, AMLCDs have no inherent limitation in the number of scan lines, and they present fewer cross-talk issues. There are manykinds of AMLCD. For their integrated switching devices most use transistors made ofdeposited thin films, which are therefore called thin-film transistors (TFTs).

The most common semiconducting layer is made of amorphous silicon (a-Si).a-Si TFTs are amenable to large-area fabrication using glass substrates in a low- temperature (300°C to 400°C) process.

An alternative TFT technology, polycrystalline silicon - or polysilicon or p-Si-is costly to produce and especially difficult to fabricate when manufacturing large-area displays.

Nearly all TFT LCDs are made from a-Si because of the technology's economy and maturity, but the electron mobility of a p-Si TFT is one or two orders of magnitude greater than that of an a-Si TFT.

This makes the p-Si TFT a good candidate for an TFT array containing integrated drivers, which is likely to be an attractive choice for small, high definition displays such as view finders and projection displays.

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 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.