intro[1]

Thin, Fast Flexible semiconductors Seminar report 2011

1.INTRODUCTION

Imagine having a high-definition TV that is 80 inches wide and less than a

quarter-inch thick, consuming less power than most TVs on the market

today and can be rolled up when you're not using it. What if you could have

a "heads up" display in your car? These devices may be possible in the near

future with the help of the existing technologies.

Amorphous-oxide thin-film-transistor (TFT) arrays have been developed as

TFT backplanes for large-sized active-matrix displays. Displays with a-

IGZO TFT array are promising for large-sized TV because a-IGZO TFTs

can provide a large-sized backplane with excellent uniformity and device

reliability. Transistors made from amorphous silicon, such as those used in

liquid crystal displays, can also be grown at low temperature. They can even

be relatively transparent if produced from films that are sufficiently thin. But

when used to switch large currents, they become unstable.

A more promising material for making transparent transistors for driving

such displays is indium gallium zinc oxide—a wide gap semiconductor.

However, until now it has been difficult to grow high quality amorphous

oxide semiconductors and the aluminum oxide dielectrics used for

fabricating transistors, at low temperature. Poor quality materials are

electrically unstable and produce leak currents when exposed to light — a

particularly bad problem for making transparent displays.

Dept. of EEE Page 1 of 22 ASIET, Kalady

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2. LIQUID CRYSTAL DISPLAYS (LCDs):

LCDs are commonly used in televisions, mobile phones, computers etc.

LCD produces a black or colored image by selectively filtering a white light.

The light is provided by a series of cold cathode fluorescent lamps at the

back of the screen. Millions of individual LCD shutters arranged in a grid,

open and close to allow a metered amount of the white light through.

Liquid crystals encompass a wide range of rod-shaped polymers that

naturally form into thin layers, as opposed to the more random alignment of

a normal liquid. The nematic liquid crystals show an alignment effect

between the directors. In the case of an LCD, this effect is utilized by using

two directors arranged at right angles and placed close together with the

liquid crystal between them. This forces the layers to align themselves in

two directions, creating a twisted structure with each layer aligned at a

slightly different angle to the ones on either side. LCD shutters consist of a


Fig 1: Operation of LCD display

http://en.wikipedia.org/wiki/Liquid

http://en.wikipedia.org/wiki/Liquid_crystal

http://en.wikipedia.org/wiki/Cold_cathode


stack of three primary elements. Normally light cannot travel through a pair

of polarizers arranged in this fashion, and the display would be black. The

polarizers also carry the directors to create the twisted structure aligned with

the polarizers on either side. As the light flows out of the rear polarizer, it

will naturally follow the liquid crystal's twist, exiting the front of the liquid

crystal having been rotated through the correct angle that allows it to pass

through the front polarizer.

To turn a shutter off, a voltage is applied across it from front to back. The

liquid crystals align themselves with the electric field. The light no longer

changes polarization as it flows through the liquid crystal, and can no longer

pass through the front polarizer. By controlling the voltage applied across

the crystal, the amount of remaining twist can be selected.

2.1 LCD PIXELS

Each shutter is paired with a colored filter to remove all but the red, green or

blue portion of the light from the white source. Each shutter–filter pair forms

a single sub-pixel. The sub-pixels are so small that when the display is

viewed from even a short distance, the individual colors blend together to

produce a single spot of color, a pixel. The color shade is controlled by

changing the relative intensity of the light passing through the sub-pixels.


Fig 2: Pixels of a display

http://en.wikipedia.org/wiki/Pixel


3. ORGANIC LIGHT EMITTING DIODE

(OLED) DISPLAYS :

OLEDs are solid-state devices composed of thin films of organic molecules

that create light with the application of electricity. OLEDs can provide

brighter, crisper displays on electronic devices and use less power than

conventional light-emitting diodes (LEDs) or liquid crystal displays (LCDs)

used today.

OLEDs emit light

through a process called Electro

phosphorescence. When a voltage

is applied across the OLED, an

electrical current flows from the

cathode to the anode through the

organic layers. The cathode gives

electrons to the emissive layer of

organic molecules. The anode

removes electrons from the conductive layer of organic molecules. At the

boundary between the emissive and the conductive layers, electrons find an

electron hole and recombines with the hole. When this happens, the electron

gives up energy in the form of light. The color of the light depends on the

type of organic molecule in the emissive layer. The intensity or brightness of

the light depends on the amount of electrical current applied. OLED display

functions without a backlight and that makes it thinner and lighter than

LCDs.


Fig 3: OLED structure

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4.THIN FILM TRANSISTORS (TFTs):

The technical name for these devices is “thin film insulated-gate field-effect

transistors". The TFTs are the control elements that actively control the

individual pixels. For this reason, one speaks of so-called 'active matrix

TFTs'.TFTs in an active-matrix LCD act as simple ON/OFF switches, at

different speeds which depend on the refresh rate of the LCD, for example

60Hz. They are located in the left upper corner of each sub -pixel cell and

are the most important component of all displays.

Figure shows a simple structure of TFT, it consists of three terminals: the

gate for modulating the conductivity of the channel, the source for injecting

carriers and the drain for extracting carriers. The gate is insulated from the

semiconductor film by a gate insulation film; while the drain and source

directly contact the semiconductor film. They are fabricated by subsequent

deposition of conducting, insulating, and semiconducting thin films onto an

insulating substrate. Usually, amorphous silicon is the main constituent of

the semiconducting channel of a TFT.


Fig 4: Thin Film Transistor


4.1 TFT OPERATION

In a simple TFT, for example N-channel TFT, a positive voltage is applied

on the gate in order to switch it ON; the insulation layer can be considered as

the dielectric layer in a capacitor, hence negative charges are induced on the

semiconductor channel, which is the region between source and drain; these

negative charges create a electrons flow from source to drain to make the

channel conductive. When a negative voltage is applied on the gate,

electrons are depleted in the channel; hence almost no current is present. The

ON current depends on different parameters, for example channel width,

channel length, gate voltage and the threshold voltage of the TFT. When the

TFT is switched ON, a data voltage is applied on the source, the drain with

the LC load capacitance will charge up to the voltage with same amplitude,

i.e. transferring the data voltage from the data line to the pixel electrode.

When switched OFF, no current flows in the channel and the data voltage

cannot be transferred. While a P-channel TFT can be switched ON by

applying a negative voltage on the gate, and can be switched OFF by a

positive voltage on the gate.

TFTs can be made of crystalline silicon, poly-silicon or amorphous silicon.

Poly-silicon can be processed under low and high temperature and can be

built on common low-cost glasses or quartz plate. Crystalline Si owns higher

mobility. Amorphous silicon is widely used in LCD monitor and TV

because of its easy manufacturing on large glass substrates, but it has a

lower mobility.



5. PRESENT DAY DISPLAYS

Amorphous silicon has become the material of choice for the active

layer in thin-film transistors (TFTs), which are most widely used in large-

area electronics applications, mainly for liquid-crystal displays (LCDs).

Amorphous silicon (a-Si or α-Si) is the non-crystalline allotropic form

of silicon. In an amorphous solid, each atom and its nearest neighbors stand

in a not-quite-perfect order. The random arrangement of atoms tends to give

amorphous materials smooth surfaces and uniform structure when they are

produced as a film, which can measure anywhere from tens to hundreds of

nanometers thick. And these films are more robust than polycrystalline ones,

whose grain boundaries act like shunt paths for impurities.

Water vapor, for instance, would more readily penetrate polycrystalline

films. That’s why amorphous silicon has wrested the domain of TFTs from

crystalline silicon. But single crystals are too costly, brittle, and heavy to

cover 2-meter-wide glass panels. While thin-film polycrystalline silicon can

work well for transistors in large high-performance displays, it requires

costly, complex processing. Amorphous silicon, on the other hand, has just

what this job requires. Large glass panels can be cheaply coated with

uniform thin films of the stuff at high volumes, producing hundreds of

thousands of electronically consistent transistors.

The a-Si is formed by using SiH4, the hydrogen enters into the silicon film,

and can improve the loosen Si-lattice in a-Si, thus enhance its performance.

The a-Si can therefore be also referred as a-Si:H. The main advantage of a-

Si in large scale production is not efficiency, but cost. One further advantage


http://en.wikipedia.org/wiki/Silicon

http://en.wikipedia.org/wiki/Allotropic

http://en.wikipedia.org/wiki/Crystalline

http://en.wikipedia.org/wiki/Liquid-crystal_display

http://en.wikipedia.org/wiki/Large-area_electronics

http://en.wikipedia.org/wiki/Large-area_electronics

http://en.wikipedia.org/wiki/Thin-film_transistor

http://en.wikipedia.org/wiki/Active_layer

http://en.wikipedia.org/wiki/Active_layer


is that a-Si can be deposited at very low temperatures, e.g., as low as 75 °C.

This allows for deposition on not only glass, but plastic as well, making it a

candidate for a roll-to-roll processing technique. The normal electron

mobility of a-Si:H is ~0.3-1 cm2/Vsec, compared with c-Si’s >500 cm2/Vsec,

it is quite small. But for LCD’s TFT’s switch, it is enough.

On the other hand, its hole mobility is very low, therefore only N-channel

TFT can be practically used. Another drawback of a-Si is its high

photoconductivity, which cause the undesirable photo-leakage current in the

OFF state. To avoid it, a cover layer is used to shield it from ambient and

backlight.


Fig 5: Three forms of silicon

http://en.wikipedia.org/wiki/Roll-to-roll_processing

http://en.wikipedia.org/wiki/Plastic_electronics


6. REQUIREMENTS OF FUTURE

DISPLAYS:

Considering the future displays, amorphous silicon is just not good enough.

There are several drawbacks for a-Si that makes it unsuitable in this case.

For one thing, it isn’t fast enough Next-generation LCD TVs will be

refreshed at least 240 times a second, which is two to four times as quick as

today’s versions; that way, they’ll provide sharper fast-action sports and

movies. Three-dimensional displays will need refresh rates twice again as

high, to provide all that fast-motion goodness to each eye. Nor are today’s

thin-film transistors stable enough for displays that use the organic light-

emitting diode (OLED), a thin, efficient, high-contrast technology. Stability

matters because the “threshold voltage” that an amorphous silicon transistor

needs to turn on tends to drift as the transistor works. And both the problems

of slow switching and drift get worse when amorphous silicon devices are

made on flexible plastic, which is a critical design requirement for

tomorrow’s roll-up displays.

Amorphous silicon films are typically deposited at around 300 to 350 °C.

These temperatures work for glass, but they would distort plastic. At plastic-

friendly temperatures, below around 200° C, amorphous silicon transistors

degrade tremendously. And the most affecting factor is low carrier mobility

which causes slow switching of TFTs which wont be able to drive LCDs

operating at 120 Hz. Finally, they must be able to form a stable and uniform

amorphous phase they must be able to form uniform films at low

temperatures. In addition, the carrier concentration must be controlled at low

levels, <1015 cm3, with good stability and reproducibility to control device

characteristics such as threshold voltage and to suppress off current.



7. AMORPHOUS OXIDE

SEMICONDUCTORS

Transparent electronics devices formed on flexible substrates are expected to

meet emerging technological demands where silicon based electronics

cannot provide a solution. Researchers and display manufacturers need a

replacement in hand when the day comes for amorphous silicon to step

down from its throne. And they already have their eyes set on a promising

heir—in fact, a whole family of materials, known as amorphous oxide

semiconductors (AOSs). They’re amorphous because like today’s silicon

standby, they lack a regular crystalline structure, and they’re oxides because

they’re made of oxygen compounded with two or three metals, most

commonly selected from zinc, indium, gallium, and tin. Amorphous oxides

can form thin films that are transparent and electrically conductive, which is

why they already serve as the see-through electrode layer in displays and

solar cells. It was this quality that led to the surge in research that began in

1996, when Hideo Hosono and his colleagues at the Tokyo Institute of

Technology first noted the merits of amorphous transparent conducting

oxides.

AOSs could do more than simply serve as a passive electrode. They could

also replace amorphous silicon as the active semiconducting material that

does the heavy lifting as the channel in TFTs. The metal atoms for the

oxides are selected from the 15 elements that sit in rows 4 to 6 and columns

11 to 15 of the periodic table. This block includes rare elements like silver

and gold as well as environmentally unfriendly ones, like arsenic and lead.

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Eliminating toxic and costly materials leaves 8 elements that can give 28

dual-oxide combinations. The individual metal oxides, say, zinc oxide and

tin oxide, tend to become polycrystalline when deposited as thin films.

But an interesting thing happens when the two are mixed in similar

concentrations and deposited. Because they have different crystal structures,

the atoms cannot figure out which structure to adopt, and zinc tin oxide ends

up as an amorphous film.

The conduction band minimum (CBM) in typical wide-band-gap oxide

semiconductors is mainly made of unoccupied metal orbital. This suggests

that the incorporation of orbital is favorable for forming a greatly dispersed

CBM, which leads to a high electron mobility if the carrier relaxation time

does not differ largely between materials. This is the reason why many

transparent conductive oxides (e.g., SnO2 and In2O3:Sn) are composed of

heavy post transition metal cations such as In and Sn. Another way to form

greatly dispersed CBM is found in ZnO: a small Zn–Zn distance, due to

fourfold coordination, increases CBM dispersion and results in high electron

mobility. Another requirement to be considered is that many oxide

semiconductors have uncontrollable high carrier concentrations larger than

1017 cm3 because oxygen vacancies are easily formed and generate excess

free electrons. To suppress the generation of the free carriers via the

formation of oxygen vacancies, the incorporation of stronger metal–oxygen

bonds would be effective. Hence Ga and Al were chosen as third

components since Ga–O and Al–O are stronger chemical bonds than Zn–O

and In–O. In2O3-Ga2O3-ZnO (IGZO) is the material used for display studies

due to their good control in carrier concentration.


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8. FABRICATION OF IGZO TFT

The transparent amorphous oxide semiconductor, InGaZnO forms the active

channel in transparent thin-film transistors(TTFTs). The a-IGZO is

deposited on polyethylene terephthalate (PET) at room temperature and

exhibits Hall effect motilities exceeding 10 cm2V2s2, which is an order of

magnitude larger than for hydrogenated amorphous silicon.

The bottom-gate thin-film transistor (TFT) was fabricated using a-InGaZnO

film as an n-channel active layer on 125μm-thickness poly-ethylene-

naphthalate (PEN). A 30nm-thick a-InGaZnO layer was deposited by RF

magnetron sputtering technique using polycrystalline InGaZnO4 target in Ar

and O2 gas ambient. A 280nm-thick SiON layer was also deposited by RF

magnetron sputtering process for the gate insulator. As source, drain and

gate electrodes, ITO was formed by DC magnetron sputtering technique. All

the layers were deposited at room temperature. Source, drain, gate and

channel were defined by standard photolithography and lift-off techniques.

Organic semiconductors and hydrogenated amorphous silicon Films of a-

IGZO were prepared by pulsed laser deposition with a KrF excimer laser,

using a polycrystalline InGaZnO4 target at room temperature in an oxygen

atmosphere (oxygen pressure PO2). The chemical composition of the

obtained films measured by X-ray fluorescence spectroscopy was In:Ga:Zn

¼ 1.1:1.1:0.9 (in atomic ratio). The film is amorphous and optically

transparent in the entire visible and near-infrared region. The optical

transmittance is greater than 80%, including the reflection associated with

the film and glass substrate. The Seebeck coefficients obtained from thermo


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power measurements is negative, indicating that a-IGZO is an n-type

semiconductor. The amorphous phase is thermally stable up to, 500 °C in

air.

Incorporating Ga ions would be important in a-IGZO for suppressing carrier

generation via oxygen vacancy formation, because the Ga ion forms stronger

chemical bonds with oxygen than Zn and In ions. Top-gate flexible TTFTs

were fabricated using the a-IGZO film as an n-channel active layer on 200-

mm-thick PET films. Source, drain, gate contacts and a gate insulator were

defined by standard photolithography and lift-off techniques. A 140-nm-

thick Y2O3 layer was chosen for the gate insulator and ITO (Sn:10%) was

used for source, drain and gate transparent electrodes. These layers were

deposited by pulsed laser deposition at room temperature using Y2O3 and

ITO ceramic targets. The channel length and width were 50 mm and 200

mm, respectively. The TTFT sheet was bent into a curve with a surface

curvature radius of 30mm. The TTFT after bending maintained good

characteristics. Hence the TTFT performance is almost unaffected by

bending, although a slight decrease is observed in the saturation current.

After the initial bending, the TFT characteristics are stable and reproducible

during and after repetitive bending. The TTFT is stable at temperatures up to

120 °C, but becomes inoperative at higher temperatures, probably owing to

the softening of the PET substrate.

Pulsed laser deposition was used to form the active a-IGZO layer in this

study, but a sputtering or metal-organic chemical vapour deposition method

can be used for large-area uniform deposition and mass production, as

demonstrated for window electrodes of solar cells and flat-panel displays.


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9. FEATURES OF IGZO TFT

Thin film transistors made of IGZO has the following features:

TTFTs fabricated on PET sheets exhibit saturation motilities(μsat) of

11.8 cm2/V s. The mobility is larger than that for a-Si:H TFTs by more

than an order of magnitude.

Fabrication of low-temperature TFTs will allow flexible large-area

electronic devices to be developed. These devices are flexible,

lightweight, shock resistant and potentially affordable—properties that

are necessary for large, economic, high-resolution displays, wearable

computers and paper displays.

Small sub threshold voltage swing (S) of approximately 0.1 V/decade

(S = dV/dlogIDS) which is several times smaller.

Low operation voltage of < 5 V

Low leakage currents

an on-to-off ratio of 103—even during and after bending.

These achievements imply that transparent amorphous oxide semiconductors

have the potential to overtake a-Si:H, and are promising materials for

transparent flexible electronics. Furthermore, flexible TTFTs may be

integrated with other already developed devices that use a p-type amorphous

oxide semiconductor and p–n junction diodes fabricated at room

temperature; this would extend the possibilities of flexible transparent

electronic circuits. Further, when combined with ‘transparent circuit

technology’, TFTs can integrate display functions even on the windscreens

of cars.


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10. ADVANTAGES OF AOSs

AOS TFTs have several features that are attractive for flat-panel displays

and large-area integrated circuits that are summarized as follows:

Low processing temperature : AOS TFTs exhibit satisfactory operation

characteristics even if fabricated at room temperature.

Wide processing temperature window : By choosing an appropriate

chemical composition, AOSs form stable amorphous phases with high

crystallization temperatures of > 500 °C; therefore, an appropriate

temperature condition can be chosen to modify the TFT

characteristics.

Large electron mobility : AOSs exhibit large mobilities of

> 10 cm2/V s and even higher than 50 cm2/V s.

Simple electrode structure and low off current : Silicon-based field-

effect transistors require a p–n junction for the source and drain

electrodes to suppress inversion operation and the consequent increase

in off current.

Excellent uniformity and surface flatness : AOS TFTs exhibit excellent

short-range uniformity [18] and surface flatness (> 0.3 nm) owing to

the amorphous structure.

Ease of fabrication : It is rather easy to fabricate AOS TFTs. The

conventional direct-current (DC) sputtering methods can be used for

AOS TFTs.


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11. DISADVANTAGES OF AOS

Some issues must still be solved, beginning with device stability. Oxide-

based transistors are more stable than the amorphous silicon kind, but

they’re still not as stable as the industry would like. This can be a problem

even with LCDs, which are switched on and off by voltage, with a threshold

that is not perfectly constant. It’s an even bigger problem with OLEDs,

which are switched by current—an arrangement that makes the circuits even

more sensitive to transistor stability.

Another issue is regarding the expensive fabrication procedures and printing

inexpensive, flexible displays. The Toppan’s process could fabricate an e-

paper, but these black-and-white displays can get by with slow switching

speeds and low power. Roll-up color LCDs and OLEDs require oxide

transistors of higher quality.


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12. APPLICATIONSA flexible fully integrated transimpedance amplifier based on

amorphous oxide semiconductor for use in system-on-panel

has been built by researchers. Their device, made from an amorphous

indium-gallium-zinc-oxide (a-IGZO) thin-film transistor (TFT) and two a-

IGZO TFT-based resistors integrated on a flexible substrate, can be bent to a

radius of 5mm without significant degradation to the amplifier performance.

Mechanically flexible electronics have a wide range of possible applications.

These include flexible displays for future hand-held devices, flexible e-papers,

functionalised surfaces such as walls or furniture that can display information,

textiles that incorporate sensors, and medical applications such as implants or

prostheses.


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Fig 6: OLED display from SAMSUNGFig 7: Toppan’s E-paper


Toppan processed oxide TFTs on a plastic substrate under room temperature

and fabricated a flexible electronic paper display by combining it with an

electrophoretic E Ink front plane laminate. This is the first time an E Ink

electronic paper display has been driven by an oxide semiconductor TFT

array.This plastic substrate, amorphous oxide semiconductor TFT array is

thinner, lighter and more robust than glass substrate TFTs and is flexible.

Such mechanical properties bring next generation flexible displays closer to

reality. Toppan plans to develop flexible TFTs with goals to commercialize

thin, lightweight and flexible displays such as electronic paper, starting with a

practical prototype display in fiscal 2008. In parallel, we aim to introduce

printing methods into the fabrication process of flexible TFTs for

simplification and cost reduction. Lightweight and flexible displays are

considered a promising next generation product. Especially, with the recent

developments in the field of electronic paper, led by electrophoretic displays,

there is a heightened need to commercialize lightweight and robust flexible

TFTs on plastic substrates. Conventional flat panel displays, like LCDs are

driven by TFTs on glass, processed at high temperature (mainly amorphous

silicon TFT, fabricated at over 250 degrees centigrade). On the other hand,

such a glass-based TFT array has many disadvantages for electronic paper

because it is heavy, fragile and rigid (not flexible).

Making see-through circuits out of oxide semiconductors is becoming the

matter of interest among scientists. Besides holding out the possibility of

building displays into the windows of cars and trains, the materials' low cost

and low-temperature fabrication may suit them to future applications that

don't need transparency, notably roll-up electronic displays. Standard


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silicon-based techniques can't compete in this area, because even if they

could be made flexible, their processing temperatures, generally around 250

°C, are so high they would melt any plastic substrate holding the silicon in

place. The materials are transparent and hence they could be used in

electronic-ink screens that could be laminated on windshields, windows, and

eyeglasses.

Refresh rates are very important for 3-D television technology. A higher

refresh rate allows the television to display the different sets of images to

cerate the illusion of depth. Amorphous oxide semiconductors provide

higher refresh rates that can be used for 3-D displays. Recently, three-

dimensional (3D) displays have appeared on the market with panel sizes of


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Fig 8: Transparent display – A SAMSUNG prototype

Fig 9: A 3-D display by LG


55 inches and frame rates of 240Hz. However, higher frame rates of e.g. 480

Hz are required to improve the picture quality because a 3D display must

project two or more picture frames alternately for the left and right eyes.

13. CONCLUSION

Although no one has announced it formally, recent developments suggest

that display industry leaders are ramping up plans for large-scale production.

Companies are already making oxide transistor-based displays with sixth-

generation LCD panel manufacturing equipment, which uses glass substrates

185 centimeters wide and 150 cm high. What’s more, the scale of recently

demonstrated displays suggests that the industry will soon be able to use the

eighth-generation equipment that makes today’s large high-performance

LCDs. Look for the new materials to begin to appear in commercial displays

as early as 2012. In 2010, Samsung introduced a 14-inch OLED screen that

was 40 percent transparent. While the company hasn’t revealed its pixel-

controlling technology, there is no doubt that amorphous oxide transistors

are perfect for the application and could bring see-through displays into

consumer products. Given the astonishing developments of the past two

years, it’s easy to forget that amorphous oxide semiconductors are still in

their early adolescence; practical work started only about five years ago. In a

few more years, they’ll start appearing in your home, in our hand, and

perhaps on the lenses of our glasses.


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14. REFERENCE

Present status of amorphous InGaZnO thin-film transistors Toshio

Kamiya, Kenji Nomura and Hideo Hosono

Photosensitivity of Amorphous IGZO TFTs for Active-Matrix Flat-

Panel Displays Chiao-Shun Chuang, Hideo Hosono and Jerzy Kanicki

Program & Abstract of International Symposium on Transparent

Amorphous Oxide Semiconductor (TAOS 2006), Tokyo Institute of

Technology November 22, 2006

www.howstuffswork.com

H. Hosono, "Ionic amorphous oxide semiconductors: material design,

carrier transport, and device application“,Journal of Non-Crystalline

Solids, vol.352, pp.851-858, 2006.


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intro[1]