Amorphous Silicon TFT LCD Displays Jonathan Brownstein EECS

Amorphous Silicon TFT LCD Displays Jonathan Brownstein EECS 277 A

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 flatpanel-display technology • Best technology yet developed for Active-matrix displays • Improvements being made: - Lighter - Smaller - Faster - Consuming less power

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