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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; Light-
emitting diode; Vacuum fluorescence;
Electroluminescence; Flat cathode-ray tube;
Non-emissive: Liquid crystal; Electromechanical;
Electrochromatic; Electroactive solids;
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
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 cellsReflective: 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.
Lee SH, Do HW, etal. Jpn. J Appl. Phys. Vol. 42 (2003) p1455
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.
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.