Flat Panel Displays --- Principles, Materials and

Processes

Jing Zhang04/06/2004

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; Light- emitting 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 AddressingRight: 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 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.

FED enabled by nanotubes

Wang QH, Yan M, Chang RPH, Applied Physics Letters, 78, p1294 (2001)

FED enabled by nanotubes

• Fabrication 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.