TFT LCD Basic Knowledge
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.
Fig. 1 TFT LCD Structure
- TFT Glass has as many TFTs as the number of pixels displayed. 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.
- Liquid crystal will rotate into different angles according to the charges applied to each pixel.
Why we need to control the LC’s standing angle lies within millions of pixels? Because we need to use LC’s optic rotation nature to control the amount of light passing through the LCD panel. Light from the backlight module travels from the TFT panel through ITO electrode, turned by Liquid Crystal and reaches the LCD panel on the top.
Fig. 2 LCD Lighting Theory
- With no voltage applied across the pixel, the LC molecules twist to align to the rubbing of the glass plates. Light entering the first polarizer is twisted and can exit the second polarizer –> pixel is ON. With a voltage applied across the pixel, the LC molecules untwist to align with the electric field. Light entering the first polarizer cannot exit the second polarizer –> pixel is OFF.
Fig. 3 TFT LCD ON and OFF
- A TFT substrate is composed of a matrix of pixels and ITO region (a transparent electric conducting film) each with a TFT device and is so called array. Thousands or millions of these pixels together create an image on the display. The diagram below shows the simple structure of a pixel.
Fig. 4 TFT LCD Single Pixel Structure
Driving Methods of LCD
- Passive Matrix (PMLCD)
Simple matrix type was used in the first stage of LCDs. In this method, the transparent electrodes are set on X and Y axis. There is not switching device.
- Active Matrix (AMLCD)
A switching device and a storage capacitor are integrated at each cross point of the electrodes.
Fig. 5 Passive and Active Matrix TFT LCD
Fig. 6 Schematic representation of simple matrix LCDs
Fig. 7 Basic configuration of an AMLCD; this structure is the typical transmissive color TFT-LCD
Passive Matrix LCD Problems:
- Display Size is limited because the more rows, the shorter time the on-voltage can be applied, resulting in poor contrast ratio, narrow viewing angle, and fewer gray levels.
- Crosstalk occurs when neighboring pixel voltages affect each other, reducing the gray scale, contrast, and viewing angle.
- Submarining occurs when slow-to-respond LC materials cannot respond quickly enough and the picture can disappear temporarily.
- One Solution: placing a switch at each pixel, such as a transistor or diode –> pixel matrix becomes “active.”
Fig. 8 TFT-based Active Matrix LCDs (AMLCD)
Fig. 9 Schematic Diagram of TFT-LCD Array with Controllers, Power supply, and Driver circuits
- Switching element at each pixel. Individual pixels isolated from each other. Thin Film Transistors most commonly used.
- Horizontal scan lines address gates of the TFTs.
- Data applied through vertical lines (drain), changing the polarization and optical transparency of the liquid crystal cell.
- Many passive display problems eliminated:
- Pixel isolation eliminates crosstalk
- Isolation from the column line permits the pixel capacitor to remain charged, so that faster responding liquid crystals can be used.
- Larger displays can be realized.
Advantages of the active matrix (AM) approach:
- Higher sizes
- Higher contrast
- Higher gray scale
- Higher resolution
- Higher viewing angle
- Faster response. Eliminates “ghosting”
- Better control of the color
Color Filters (RGB)
- Conventional color displays use a pixel arrangement called RGB. In this arrangement, red, green and blue pixels are arranged in equal proportion.
- At high pixel densities, RGB arrangement is adequate.
- When the number of pixels is limited, the image may appear fuzzy. To compensate for this, a GRGB arrangement can be used.
Fig. 10 TFT LCD RGB Patterns
- Transmissive type TFT LCD: the light travels from the backlight through color filter and LC then appears on the panel. (high brightness but more power consumption).
- Reflective type TFT LCD contains a reflective mirror, utilizing the external light for image display. Power saving, and light-weight (without backlight). Ideal for viewing with external light sources.
- Transflective type TFT LCD is a promising displaying device for both outdoor and indoor applications. Benefits of adopting Transflective technology includes:
- Power saving
- Sun light readability
- Indoor readability
Fig. 11 TFT LCD Generations-1
Fig. 12 TFT LCD Generations-2
Fig. 13 TFT LCD Generations-3
Fig. 14 Generation Scaling and Cost Reduction
TFT Production Process
Please view the video below for more details.