Theory - 24 :- LCD vs TFT works?

 History of TFT LCD

Liquid crystal was discovered by the Austrian botanist Fredreich Rheinizer in 1888. "Liquid crystal" is neither solid nor liquid (an example is soapy water).

In the mid-1960s, scientists showed that liquid crystals when stimulated by an external electrical charge could change the properties of light passing through the crystals.

The early prototypes (late 1960s) were too unstable for mass production. But all of that changed when a British researcher proposed a stable, liquid crystal material (biphenyl).

Today's color LCD TVs and LCD Monitors have a sandwich-like structure (see figure below).

LCDs are a type of television screen that uses liquid crystals sandwiched between two sheets of polarizing material.

TFT (Thin‐film transistor) is a field‐effect transistor that is used to build the LCD screen and is embedded in every

pixel, making it faster and giving a better image quality.

What is TFT LCD?

TFT LCD (Thin Film Transistor Liquid Crystal Display) has a sandwich-like structure with liquid crystal filled between two glass plates.

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.

 

TFT LCD - Electronic Aspects of LCD TVs and LCD Monitors

Electronic Aspects of AMLCDs

The most common liquid-crystal displays (LCDs) in use today rely on picture elements, or pixels, formed by liquid-crystal (LC) cells that change the polarization direction of light passing through them in response to an electrical voltage.

As the polarization direction changes, more or less of the light is able to pass through a polarizing layer on the face of the display. Change the voltage, and the amount of light is changed.

There are two ways to produce a liquid-crystal image with such cells: the segment driving method and the matrix driving method.

The segment driving method displays characters and pictures with cells defined by patterned electrodes.

The matrix driving method displays characters and pictures in sets of dots.

Structure of Color TFT LCD TVs and LCD Monitors

A TFT LCD module consists of a TFT panel, driving-circuit unit, backlight system, and assembly unit.

Structure of a color TFT LCD Panel:

1. LCD Panel
   - TFT-Array Substrate
   - Color Filter Substrate
2. Driving Circuit Unit
   - LCD Driver IC (LDI) Chips
   - Multi-layer PCBs
   - Driving Circuits
3. Backlight & Chassis Unit
   - Backlight Unit
   - Chassis Assembly

It is commonly used to display characters and graphic images when connected a host system.

The TFT LCD panel consists of a TFT-array substrate and a color-filter substrate.

The vertical structure of a color TFT LCD panel.

The TFT-array substrate contains the TFTs, storage capacitors, pixel electrodes, and interconnect wiring. The color filter contains the black matrix and resin film containing three primary-color - red, green, and blue - dyes or pigments. The two glass substrates are assembled with a sealant, the gap between them is maintained by spacers, and LC material is injected into the gap between the substrates. Two sheets of polarizer film are attached to the outer faces of the sandwich formed by the glass substrates. A set of bonding pads are fabricated on each end of the gate and data-signal bus-lines to attach LCD Driver IC (LDI) chips

Driving Circuit Unit

Driving an a-Si TFT LCD requires a driving circuit unit consisting of a set of LCD driving IC (LDI) chips and printed-circuit-boards (PCBs).

The assembly of LCD driving circuits.

A block diagram showing the driving of an LCD panel.

To reduce the footprint of the LCD module, the drive circuit unit can be placed on the backside of the LCD module by using bent Tape Carrier Packages (TCPs) and a tapered light-guide panel (LGP).

How TFT LCD Pixels Work  

A TFT LCD panel contains a specific number of unit pixels often called subpixels.

Each unit pixel has a TFT, a pixel electrode (IT0), and a storage capacitor (Cs).

For example, an SVGA color TFT LCD panel has total of 800x3x600, or 1,440,000, unit pixels.

Each unit pixel is connected to one of the gate bus-lines and one of the data bus-lines in a 3mxn matrix format. The matrix is 2400x600 for SVGA.

Structure of a color TFT LCD panel.

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.

In a TFT LCD's unit pixel, the liquid crystal layer on the ITO pixel electrode forms a capacitor whose counter electrode is the common electrode on the color-filter substrate.

Vertical structure of a unit 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.

Polarity-inversion driving methods. 

In an active-matrix panel, the gate and source electrodes are used on a shared basis, but each unit pixel is individually addressable by selecting the appropriate two contact pads at the ends of the rows and columns.

Active addressing of a 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.

Generating Colors

The color filter of a TFT LCD TV consists of three primary colors - red (R), green (G), and blue (B) - which are included on the color-filter substrate.

How an LCD Panel produces colors.

The elements of this color filter line up one-to-one with the unit pixels on the TFT-array substrate.

Each pixel in a color LCD is subdivided into three subpixels, where one set of RGB subpixels is equal to one pixel.

(Each subpixel consists of what we've been calling a unit pixel up to this point.)

Because the subpixels are too small to distinguish independently, the RGB elements appear to the human eye as a mixture of the three colors.

Any color, with some qualifications, can be produced by mixing these three primary colors.

The total number of display colors using an n-bit LDI is given by 23n, because each subpixel can generate 2n different transmittance levels.