Plasma Technology for Advanced Devices
Plasma Display Panels (PDP)
The color plasma display panel (PDP) is one of the leading types of high definition television (HDTV) displays. It's small depth, the ability to provide very high-resolution, and it's scalability to very large sizes make this technology very attracktive for the TV set market.
Depending on the type of voltage that is used to generate the plasma, AC and DC PDP's can be distiguished. The AC PDP was invented in 1964 at the Coordinated Science Laboratory at the University of Illinois. The TV industry gave a 2002 Emmy award for technological achievement to the original U of I inventors of the plasma display: Larry Weber, Donald Bitzer and the late Gene Slottow, and their first graduate student, Robert Willson, whose name appears alongside those of Bitzer and Slottow on the original plasma display patent. Fujitsu, the leading manufacturer of plasma displays, also shared the award. Read about the early beginning of plasma displays at the UIUC ECE alumni news website and in an article by Gene Slottow (H.G. Slottow, IEEE Trans. Electron Devices ED-23 (7), 760 (1976)). The DC PDP was invented by Philips in 1968.
Almost all the PDP companies have now adopted the AC PDP. The plasma in each cell of an an AC PDP is generated by dielectric barrier discharges (DBDs) operating in a glow regime in a rare gas mixture (typically 500 Torr). Slide 1 shows a collection of simplified views of PDP's from various sources. Depending on the design details, the front side can have two parallel transparent ITO (indium tin oxide) electrodes and two non-transparent Cr/Cu/Cr bus electrodes. The top side is coated with protective materials such as MgO which has a high coefficient of secondary electron emission. The rear side of the display panel t has an address electrode perpendicular to the sustaining and scanning electrodes. These two sides are covered with dielectric materials such as PbO and separated by a barrier rib of 100-150 mirometer thickness. The barrier rib has one of red, green, and blue color phosphors. The phosphor covers only the dielectric of the address electrode. It does not cover the sustaining and scanning electrode sides on the front side where the surface discharge occurs. This minimizes the flux of charged particles to the phosphor covered surfaces. The PDP cell is filled with a mixture of He, Ne, and Xe gases.
Principle and Operating Conditions
Slides 2 and 3 show cross sections of plasma display panels which illustrate the functional principle. A detailed description of the physics and technological challenges of plasma display panels can be found in a review by Jean-Pierre Boeuf (J. Phys. D, 36 (2003) R53). Microdischarges in plasma displays have been extensively modeled.
The standard electrode geometry in commercially available AC PDP's is called a coplanar electrode geometry, or short ACC. The alternative, simpler but less efficient, electrode geometry is called matrix configuration, or ACM. The ACM cofiguration is comprised of orthogonal electrodes on the front and the back of the PDP. The discharge is ignited in the gas filled gap between two intersecting electrodes onto which the appropriated potentials are applied. In the ACC configuration, the plasma is struck between two parallel electrodes on the front plane and a perpendicular electrode on the back plane. The plasma of a PDP is essentially a pulsed DC plasma which is created by voltage pulses of several ten to hundred volts (200 to 250 V for ACM displays) and durations in the order of several nanoseconds. An emitting cell has first to be activated (by a writing pulse) before it goes through several plasma puls cycles (sustaining pulses). The writing pulse creates trapps charges in the dielectric layers of the front and back plates. These charges add to the potentail difference during the sustaining pulse in such away that the total potential exceeds the breakdown voltage of the gas mixture during the sustaining pulses. In a cell which has not been "written", the sustaining pulse will be below the breakdown voltage and hence a plasma won't be ignited. At the end of the emission period of the cell, a so called erasing pulse is applied which brings both electrodes of the cell back to zero potential. This scheme with writing, sustaining and erasing pulses pertains to the ACM cell geometry. The pulsing scheme for the ACC geometry is somewhat more complicated, however more addressing schemes are possible.
Issues and Challenges
The question of addressing becomes much more complicated when one has to deal with addressing and sustaining one or several million cells at a frequency of 60 Hz (60 frames per second) and with the possibility of displaying more than 16 million colours. Each pixel is comprised of three individual cells in the three primary colors. In order to display 16.7 millions colors (256 x 256 x 256) each cell has to be able to display 256 intensity or "grey scale " levels.The grey scale is obtained by modulating the number of current pulses in a given discharge cell during a TV field (16.7 ms, corresponding to 60 Hz). A binary coding scheme is used to achieve 256 grey levels. This binary coding leads to temporal non-uniformity of the light emissionf and in some cases to motion artefacts such as dynamic false contours. Various methods have been introduced to reduce these motion artefacts.
For high resolution displays the conditions on addressing speed become more severe since the addressing time is inversely proportional to the number of lines of the screen. One of the solutions is dual scan, where the panel is divided in two parts which are scanned simultaneously. The downside of this method is that two drivers are needed which increases the cost of the display.
Besides false contours, the luminous efficiency and the lifttime are the major concenrs for plasma display panels.
A term denoting the ratio of total luminous flux to total radiant flux, luminous efficiency is measured in lumens per watt. Higher luminous efficiency can be obtained through better design of the cell and electrode structure, gas mixtures, phosphor, and driving circuit. The efficiency of PDPs on the market is 1.8 lumens/W by Pioneer and 2 lumens/W by Samsung SDI. Fujitsu is working on a new approach, called plasma tube technology and claims that 5 lumens/W can be achieved. For comparison, the luminous efficiencyof fluorescent light sources is 80 lumens/W which gives rise to optimism for further improvements for PDP's.
The useful lifetime of a PDP or Plasma TV is measured by the period of time it takes for the display to appear half as bright compared to the original state. Today, many plasma manufacturers are listing 60,000 hours as the life span to half life - matching the life span listed by top LCD manufaturers. This corresponds to over 7 years of continuous operation, 24 h per day, 7 days a week !
- Organic Light Emitting Diodes (OLED's): Today mostly used for small displays in MP3 players and mobile phones
- Liquid Crystal Displays (LCD's): Competing with PDP's for sub 42 inch TV's and being the display of choice for computer screens.
- Field Emission Displays with Carbon Nanotubes: In a very early development phase.
- Surface-Conduction Electron-Emitter Displays: Developed and commercialized by Canon and Toshiba