Field Emission Display Screen Seminar Report ‘03
Introduction
Various types of displays have become common in the every day life. The displays are used in televisions, computers etc. They also have wide use in laboratories and in medical applications. The displays are those devices by which we can view moving objects. The displays are manufactured depending upon their application.
One of the hottest markets driving physics research is the demand for a perfect visual display. People want, for example, large, thin, lightweight screens for high-definition TV and outside displays and very high resolution flat computer monitors that are robust and use little power. Several types of flat display are competing for these applications. Not surprisingly, the research departments of universities and the big electronics companies around the world are bustling with exciting ideas and developments. New university spinout companies are developing many new devices. The different types displays available are:
· Liquid crystal displays
· Plasma displays
· Electro luminescent displays
· Field emission displays
· Projection displays
Liquid crystal displays
Even the liquid crystal display (LCD), which has 85 per cent of the flat-screen market, is still a young technology and the subject of very active research. LCDs depend on arrays of cells (pixels) containing a thin layer of molecules which naturally line up (liquid crystals); their orientation can be altered by applying a voltage so as to control the amount of light passing through. Their main drawbacks have been poor viewing characteristics when seen from the side and in bright light, and a switching speed too slow for video. Electrically sensitive materials called ferroelectric and antiferroelectric liquid crystals show potential. These work slightly differently and are bistable so should use less power. They can respond 100 to 1000 times faster than current displays, and should give brighter images from all angles. One solution to the drawbacks of LCDs is to combine them with another technology. Indeed, the latest, high quality LCDs on the market incorporates a tiny electronic switch (a thin film transistor, TFT) in each pixel to drive the display.
Plasma displays
Although LCDs up to a 42-inch diagonal have been demonstrated, for larger flat TV screens, companies have instead turned to plasma display panels. These employ gas discharges (as in a fluorescent tube) controlled by an electrical signal. The ionised gas, or plasma, emits ultraviolet light which stimulates red, green and blue phosphors inside each pixel making up the display panel to produce coloured light. The images on the latest displays are very clear and bright. Unfortunately they are still expensive.
Electro luminescent displays
One of the most promising emerging display technologies exploits ultra thin films of organic compounds, either small molecules or polymers, which emit light (luminescence) when subjected to a voltage. These organic light-emitting diodes (OLEDs) produce bright, lightweight displays.
Field emission displays
The other major technology competing for the flat screen, market is the field emission display. This works a bit like a cathode-ray tube, except that electrons are emitted from thousands of metal ‘micro-tips’, or even a diamond film, when an electric field is applied between the tips and a nearby phosphor coated screen. Printable Field Emitters, based at the Rutherford Appleton Laboratory near Oxford, has come up with a novel idea employing low-cost composite materials deposited and patterned using screen printing and simple photolithography. This technology could produce affordable large displays in the 20 to 40-inch diagonal range suitable for TVs.
Projection displays
Finally, a completely different approach showing potential is to direct light from an image source using wave-guides through a glass or plastic sheet onto a screen. A clever variation of this is ‘the Wedge’ developed by Cambridge 3D Display. Light rays pass up a thin wedge-shaped glass plate and emerge at right angles at various points depending on the angle of entry. The beauty of this device is that it could be used to project any kind of micro-display – LCD or OLED, for example – onto a large screen.
All of the technologies described here still have drawbacks and no one yet knows which will win the big prize of flat screen TVs. It is likely that all of them will find niche markets. The next five years will certainly see a revolution in flat screen development.
FED Technology
The FED screen mainly contains three parts:
1. Low-voltage phosphors.
2. A field emission cathode using a thin carbon sheet as an edge emitter.
3. FED packaging, including sealing and vacuum processing.
Low voltage phosphors
The low voltage phosphors are the screens in which the images are displayed. In the display technology the phosphor screens act as anode, which receives the electrons emitted from the cathode. The phosphor glows when the electrons bombards with it to show the images. The phosphors are made up of layers of three primary colours -green, red and blue. These colour phosphors are displayed by the “field sequential colour” in which the green information is read first then redrawn with red information and finally with blue colour. The FED may have pixel pitches of about 0.2mm.
Field emission cathode
In the field emission display screen the cathode are electron guns that emit electrons. Here there are about 200-million electron guns called “micro tips”. The emission of electrons is called “cold cathode emission”. Each of these micro tips is smaller than one micrometer and they are deposited into a dense grid. They are made up of materials such as molybdenum.
The micro tips can be of different types:
1. Wedge type emitter using silicon.
2. Silicon tips with continuous coating of diamond particles.
3. Single-crystal diamond particle on silicon tips.
4. Planar diode emitter.
5. Metal-insulator-semiconductor type planar emitter
Wedge type emitter using silicon
The out standing features of wedge type emitter using silicon are its brightness and low vacuum requirements. It has a packaging density of 106 emitters per mm2 at the rate of 103 emitters per pixel. It has an accelerating electrode potential of 40V and low power consumption. However this display has to go miles in the case of price and mass production status.
Silicon tips with continuous coating of diamond particles
These cone-shaped blunt emitters have a radii of curvature ranging from 0.3 to 3 pm. The low work function can offer considerable current at low voltage field emission.
Single-crystal diamond particle on silicon tips.
Instead of plating the polycrystalline diamond particles on silicon tips, diamond particles can be placed on the tips of silicon needle to form a field emitter. The only drawback is the expenditure involved in placing diamond particles on the tips of silicon needle.
Planar diode emitter.
The planar diode emitter configuration uses a diamond like carbon emitter. They are easy to fabricate and much suited for mass production. One disadvantage for this type of displays is that once failed, the display will have to work with out that pixel.
Metal-insulator-semiconductor type planar emitter
A new type of field emission display (FED) based on an edge-enhance electron emission from metal-insulator-semiconductor (MIS) thin film structure is proposed. The electrons produced by an avalanche breakdown in the semiconductor near the edge of a top metal electrode are initially injected to the thin film of an insulator with a negative electron affinity (NEA), and then are injected into vacuum in proximity to the top electrode edge. The condition for the deep-depletition breakdown near the edge of the top metal electrode is analytically found in terms of ratio of the insulator thickness to the maximum (breakdown) width of the semiconductor depletition region: this ratio should be less than 2/(3 \pi - 2) = 0.27. The influence of a neighboring metal electrode and an electrode thickness on this condition are analyzed. Different practical schemes of the proposed display with a special reference to M/CaF_2/Si structure are considered.
FED packaging
The field emission display screens are comprised of a thin sandwich. In this the back is a sheet of glass or silicon that contains millions of tiny field emitters which is the cathode. The front is a sheet of glass coated with phosphor dots, which is the anode. The anode and cathode are a fraction of millimeter apart.
The final packaging of the field emission display screen is as shown in the figure above. The front portion here is the Phosphor and the back represents the emitter or micro tips.
WORKING
The field emission display works a bit like the cathode ray tube except that electrons are emitted from thousands of metal micro tips or even from a diamond film. This emission of electron occurs from the cold cathode when a voltage is applied between the cathode and anode. These electrons propagate from cathode to anode. They bombard with the phosphor, which is the anode and causes it to glow. This reproduces the image on the screen by the mixing of colours present in the screen.
There are two basic ways in which working of an FED can be explained:
1. Low voltage anode
2. High voltage anode
Low voltage anode
The low voltage approach uses the “field sequential colour” method as I mentioned earlier. In this method the entire screen is individually painted in each of the three primary colours, one at a time. As each of the colours are painted separately only that colour phosphor is grounded, so that all the electrons can strike that particular colour. This prevents any of the electrons to strike accidentally the other colours present in the screen. This may be a problem in the case of the low voltage approach.
High voltage anode
In the high voltage approach the emission from micro tip radiate in a roughly 600 cone. When these tips are very close to anode, the spread to emitted stream of electron is small enough to result in a spot size of nearly 0.33mm diameter. When the anode voltage is increased further greater phosphor efficiency is required and also the distance between anode and cathode should be increased to prevent arcing. Also focusing will be required in this case.
The light emitting principle of the field emission display screen is as shown in the figure below.
FED Characteristics
In the world of miniaturization, Cathode ray tube (CRT) is giant dinosaurs waiting for extinction. A CRT uses a single-point hot electron source that is scanned across the screen to produce an image. Comparing with the CRT displays the field emission displays has many advantages. They are:
- Brightness
- Speed
- Compact and lightweight
- Display size
- Low driving voltage
- Wider viewing angle
- High illumination
- Wide temperature extremes
- Colour Quality
Brightness
Most displays are adequate in normal (50–100 fc) room lighting. However, in dimly lit situations, such as a patient bedside at night, dim (reflective) displays are difficult to read. Most alarming, a dim display may be deceptively easy to misread.
Because an FED is an emissive display that produces its own light, it can be dimmed continuously from full brightness to less than 0.05 fL. In direct sunlight applications there will be a problem of low contrast This often requires the use of special contrast enhancement filters, such as 3M micro louver filters to generate contrast.
Speed
Display speed is the rate at which the image can be changed while maintaining image detail. Displays with inadequate response times will create
image "smear" that can be confused with defective blood flow, or will hide jitter that can indicate instability or electrical interference. With a response time of 20 nanoseconds, FED technology produces smear-free video images.
Compact and lightweight flat panel displays
Far less bulky than the CRT or plasma emission based displays, and are also significantly brighter than back lit LCDs.
Display size
This technology could produce affordable large displays in the 20 to 40-inch diagonal range suitable for TVs.
Low driving voltage
As discussed earlier the field emission displays can be made to work in extremely low voltage conditions with some limitations.
Wider viewing angle
A main advantage of the field emission display screens when compared with the ordinary cathode ray tube display is its wider viewing angles. The FED s can attain a viewing angle of 1600.
High illumination
The FED glows by itself by the bombarding of the electrons on the phosphor screen. So the FEDs can attain high illumination.
Wide temperature extremes
Unlike CRTs, FEDs have no cathode heater, no deflection system, and no shadow mask. Because of the cold cathode emission, instant-on is available at wide temperature extremes (–40 to 85°C).
Colour Quality
FEDs use conventional TV phosphors. This is of particular importance in such areas as telemedicine. The ability of a display to show true flesh tones depends in large part on the colorimetry of the display. TV phosphors have been fine-tuned for decades to provide the most natural skin tones possible, and, although not yet widely used, are unchanged in some FEDs.
FED technology provides a wide color gamut with continuous dimming and 8-bit gray scale. Its image is equally bright from any viewing angle, and power efficiency is high (from 3 to 40 lm/W, depending on voltage and phosphor).
FEDs produce gray scale by a number of different methods.
a. Frame Rate Control
b. Pulse Width Modulation (PWM)
c. Voltage Modulation
d. Current or Charge Control
e. Mixed-Mode Modulation
Frame Rate Control
Running at, for example 400 Hz, a 50% gray level can be obtained by alternating a white and a black field every other frame. A 25% gray level can be achieved by alternating one of four frames to white, or one out of 400 frames. This method is simple, allowing the use of digital on/off drivers, but the FED runs into flicker at low, and capacitive switching problems at high, frequency.
Pulse Width Modulation (PWM)
PWM requires the column to switch off earlier than the row time to decrease the pixel brightness level. The advantage to this method is that when on, the tips are always operated at maximum voltage, but rate control delays can add up at short switching rates.