Download Flat Panel Displays --- Principles, Materials and Processes

Flat Panel Displays --Principles, Materials and
Processes
Jing Zhang
04/06/2004
Outline
• CRT (Cathode Ray Tube) Displays
• Flat Panel Displays
- Classification;
- Liquid Crystal Displays;
- Basic Principles;
- Addressing Issues;
- Modified LCD
- Field Emission Displays;
• Conclusions
CRT
• CRT lives on: 50s – VS EL; 60s – VS
Plasma panels; now – VS LCD.
• Advantage: Available in a variety of sizes
with a wide selection of phosphors; Fast
response time; Low cost, etc.
• Disadvantage: Depth needed to focus or
scan the beam; High power consumption.
CRT Displays – Basic Operations
Left: Basic structure of Cathode-ray tube.
Right: Addressing method of CRT – Raster.
Flat Panel Displays
• Flatness Ratio: Depth to picture diagonal
• Definition: Flat; Light; Require less
power…
• Classification:
Emissive: Gas discharge; Plasma panel; Lightemitting diode; Vacuum fluorescence;
Electroluminescence; Flat cathode-ray tube;
Non-emissive: Liquid crystal; Electromechanical;
Electrochromatic; Electroactive solids;
Liquid Crystal
• Shape & Phases:
Liquid Crystal Displays
• Principle of a
twisted nematic
(TN) LCD
• Supertwisted
nematic LC cell:
Larger twisted angle;
Smaller voltage required
Addressing Schemes for LCD
Left: Direct Addressing
Right: Passive Matrix LCD with row and column
electrodes
TFT matrix Addressed LCD
• Motivation: To improve the response speed
• Circuit & Schemes:
Fabrication of TFTs
• TFTs are Metal-Insulator-Semiconductor
Field Effect Transistors, which are used
more often as bottom-gate.
• For gate-dielectric: CdSe – aSi – PolySi
• A-Si: Low mask count; Small number of
mask alignments and processing steps
• P-Si: Higher mobility; but needs to develop
low temperature processing.
Guest – Host (GH) Displays
• TN & STN:
Low transmittance; High power
consumption.
• GH Displays: Fewer polarizer and color filters;
Bright; Wide viewing angle.
• Host – Liquid Crystal; Guest – anisotropic dyes
Guest - Host Displays
Left: Basic GH cell. Off state: Polarized white light is absorbed by the dye.
Remaining spectrum exits as the colored light. On state: Dyes are
tuned by electric field and allow the light to pass as white light.
Right: Double GH cell. The polarizer is replaced by the second cell, in
which the LC molecule orientation is rotated by 90 degree with
respect to the first cell.
Guest – Host Displays
Left: Reflective GHD. Off state: Dye absorption yields a dark state. On
state: Smooth metal electrode reflects almost 100 percent of the
incoming light, which is forward scattered by the front scattering film.
Right: Structure of a stacked three – layer GHD. It generates color
without using a color filter, by relying on the combined absorption
of the dyes. This sequence generates largest contrast.
Transflective with VA cells
Reflective:
If no E field: 2* (Pi/2);
If E applied: 2* (Pi/2+Pi/2).
Transmissive:
If no E field applied:
Two WRFs make no
retardation, which obtains
a dark state;
If E applied:
Polarization twisted by LC.
Field Emission Displays
• Electrons are generated by field emission rather than thermal
emission.
• Less power consumption and instant turn-on; wide viewing
angle; high color saturation.
• Sub-micron tips: Low work function material, sharp tips,
suitable emitter materials.
FED enabled by nanotubes
Wang QH, Yan M, Chang RPH, Applied Physics Letters, 78, p1294 (2001)
FED enabled by nanotubes
• Fabricatio
n
Process:
Conclusion
• Flat panel displays are playing more
important roles with increasing quality and
decreasing cost; CRT displays still share
the display market at the same time.
• Mature technology is developed for liquid
crystal displays, as well as other flat panel
displays.
• Performance on power, image quality can
be improved.