Amorphous Silicon TFT LCD Displays Jonathan Brownstein EECS 277A.

Amorphous Silicon TFT LCD Displays

Jonathan BrownsteinEECS 277A

Components of The LCD

1. Backlight

2. Polarizer

3. TFT Active Matrix on Glass

4. Liquid Crystal Material

5. Glass with Color Filter

6. Polarizer

7. Surface of Display

Backlight

• CCFL (Cold Cathode Fluorescent Lamp) - Apply enough voltage for little heat• Shines light through LCD• LCD based on allowing light through

Polarizers

• Polarizers allow only properly oriented light through

TFT

• Thin Film Transistors• Put onto glass • Common substrate is glass• Common semiconductor layer is amorphous Si

- Bonds to glass - Roll to roll processing

Fabrication of TFT- Amorphous Si on Glass

- Cleaning

- Photoresist Coating

- Exposure to light

- Remove all photoresist except the mask

- Expose to plasma

- Wet etch gets rid of excess Si

- Take away remaining photoresist

- Left with layer you want on glass

Placement in LCD

• Protective coating

• Pixel Electrode

• Placed on corner

• Put into array

• Gate electrode

• Data Bus

Active Matrix

• Addressing scheme• TFT set in an active matrix• Maintains pixel state• Black matrix surrounding pixel elements• Prevents cross-talk - Creates higher contrast - Only desired pixel receives a charge• Pixel acts as a capacitor to hold the charge until

the next refresh cycle

Equivalent Circuit• Data bus line - Determines intensity• Capacitors - Storage capacitance part of gate bus line - Stores voltage to activate liquid crystal cells - Larger aperture ratio - Reduces switching time

Liquid Crystals

• Relaxed phase is twisted - Orients light so it passes through• Matrix element is selected - LC untwists and does not reorient light• LC loses transparency as voltage

increase• Loses transparency with increasing

voltage

Color Filter• RGB filter for each pixel• Design varies - Stripe is more common• Filter blocks wavelengths except ones in range of that pixel• Polarizer sharpens color

Overview

Summary

• TFT LCDs represent the cutting edge in flat-panel-display technology

• Best technology yet developed for Active-matrix displays

• Improvements being made:

- Lighter

- Smaller

- Faster

- Consuming less power

Thank You