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; 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 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. 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 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.