17 Field Emission Display Screen

Field Emission Display Screen Seminar Report ‘10

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

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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The light emitting principle of the field emission display screen is as

shown in the figure below.

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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:

1. Brightness

2. Speed

3. Compact and lightweight

4. Display size

5. Low driving voltage

6. Wider viewing angle

7. High illumination

8. Wide temperature extremes

9. 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.

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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.

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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

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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.

Voltage Modulation

This is the classic analog method of producing grey levels and gives a

luminance response similar to that of a CRT. However, it requires accurate

low-power drivers and very uniform tip response.

Current or Charge Control

This method corrects for tip nonuniformity but requires complex

drivers to control the emitted charge.

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Mixed-Mode Modulation

This is the method most display integrators use. Some gray scale is

obtained from partial use of two or more of the above modes, thus avoiding

the extreme conditions of any one method.

FED technology offers an array of display characteristics, ranging from

efficient high-voltage focused versions to cost-effective low-voltage proximity

focused iterations. Extracting electrons from microtips and modulating them

with a G-2 gate provides flexibility and allows display designers to specify

visual performance. Because of the simpler assembly, custom performance

and special sizes are less costly to produce

View

angles

Brightness Contrast Speed Colour

1600 To 3500 fL <100:1 20 ns TV

colours

Table I. Field emission display characteristics

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DRAWBACKS

Even though the field emission display screens has many advantages as

mentioned above it also have some disadvantages which may be listed below:

1. Vacuum tubes do require maintenance.

2. Current FEDs often suffer from variation in screen brightness across the

display, and also within each pixel.

3. Durability due to electrical discharge in the small gaps everywhere in

FED prototypes.

4. The killing problem was durability: the tips couldn’t survive under

severe conditions of arcing (i.e. electrical discharge) due to the small

gaps everywhere in FED prototypes.

5. Another big problem for the FED concept is the cathode driver. For big

screen applications, such as HDTV, it is difficult (if not impossible) to

build a feasible high voltage (several hundred of switching voltage)

driver for operating multiple (thousands) cathodes – power consumption

will exceed several kilowatts for such a driver (note that modern TV set

consumes only ~20-150 Watts of energy).

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Since the FED uses the vacuum tubes like the cathode ray tubes it

requires frequent maintenance. This drawback cannot be eliminated under any

conditions.

The second, third and fourth drawbacks can be eliminated by using

ballast resistors. The ballast resistors are those resistors that form a thin layer

below the electron guns or micro tips. They are highly resistive in nature and it

restricts the amount of current flowing through the micro tips.

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FAQS

1. What is field of view?

Field of View (FOV) describes how large the virtual image can appear

to be to the viewer and is measured in degrees. >50 degree FOV per eye is

possible using OLED micro displays.

2. Why didn't the FED industry already take over?

The short answer is that the fundamental FED idea was not supported

by some advanced decision making technologies, such as the Ideality

Approach As a result of that, the FED industry has been depressed for many

years.

3. What is display speed?

Display speed is the rate at which the image can be changed while

maintaining image detail.

4. Why can it be used in Laptop computers?

The FED promises full colour at low power consumption in a form

factor that is compatible with laptop computers. Also it will be attractive.

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5. Is there any radiation for the field emission display screen?

Since the field emission display screen uses the vacuum tubes and the

phosphor screens there will be some radiation for it. The radiation effect of the

FED screen will be similar to that of the cathode ray tube (CRT) display.

6. Is there any interference among the electrons while it propagates?

At a time the cathode emits electrons that will hit the phosphor screen on

only one colour. So even when the electrons interfere among themselves there

will not be any loss of information.

7. What is display size?

Display size is the total size of the display in which the information can be

obtained. For the field emission display screen the display size is about 40-inches

diagonally.

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FED APPLICATIONS

1. Sonographs

2. X-ray imaging

3. Heart-rate monitors

4. Laptop computers

5. Hang-on-the-wall televisions

6. Big screen and PC monitors

7. High-definition TV

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CONCLUSION

CRT technology has already reached its technological and marketing

limits and will likely be replaced in 10 years. The modern world needs

substances that are small in size. This shows that the cathode ray tube do not

have much to do anything in the market in future. And it would die already, if

Field Emission Display (FED) technology or any other displays would bring

anything to the market.

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BIBLIOGRAPHY

BOOKS

ELECTRONICS FOR YOU, JUNE 2002

ELECTRONICS FOR YOU, JULY 2002

WEB

WWW.SHARPWORLD.COM

WWW.WTEC.ORG

WWW.VIRTUALVISION.COM

WWW.EOFOUNDRY.COM

WWW.ISIS-INNOVATION.COM

WWW.ATIP.ORG

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ACKNOWLEDGEMENT

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ABSTRACT

With a 100-year head start over more modern screen technologies, the

CRT is still a formidable technology. It’s based on universally understood

principles and employs commonly available materials. The result is cheap-to-

make monitors capable of excellent performance, producing stable images in

true colour at high display resolutions. But in the world of miniaturization,

Cathode ray tubes (CRT) are giant dinosaurs waiting for extinction. A CRT

uses a single-point hot electron source that is scanned across the screen to

produce an image.

The CRT’s most obvious shortcomings are well known:

It uses too much electricity.

Its single electron beam design is prone to misfocus, misconvergence and

colour variations across the screen.

Its clunky high-voltage electric circuits and strong magnetic fields create

harmful electromagnetic radiation.

It’s physically too large.

Attempts to replace bulky Cathode ray tubes resulted in the

introduction of the field emission display screens (FED) screens. It will be the

biggest threat to CRT’s dominance in the panel display arena. Instead of using

a single bulky tube, FEDs use tiny ‘mini tubes’ for each pixel, and the display

can be built in the same size as a CRT screen.

The FED screens are lightweight, low power consuming and compact.

The FEDs can be used instead of some other technologies are gaining market

share in big screen and PC monitors, such as Projection TV, Plasma Displays,

Liquid Crystal, and Organic Transistor Displays.

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CONTENTS

INTRODUCTION 01

FED TECHNOLOGY 04

WORKING 11

CHARACTERISTICS 14

DRAWBACKS 19

FAQS 21

APPLICATIONS 23

CONCLUSION 24

BIBLIOGRAPHY 25

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Documents

17 Field Emission Display Screen