MAIN Report a-Si:H

8/10/2019 MAIN Report a-Si:H

1/17

2014

SHANUJ GARG

2014H123162P

11/16/2014

Amorphous Silicon: Properties

and Applications


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Chapter 1

Introduction

Silicon is plenty available on earth surface in the form of dusts, sands, planetoids, silica etc.

Although most of the silicon is used in the steel refining, aluminium-casting, and fine

chemical industries, a relatively small portion (< 10%) of very high purity silicon is used in

semiconductor industries, which is called Electronic Grade Silicon (EGS).

EGS can be extracted directly from solid silica or other silicon compounds by purification

techniques like float zone refining and fluidized bed (FB) based on melting and resolidifying.

Silicon which we get from these techniques is polycrystalline in nature. From thispolycrystalline silicon Monocrystalline silicon is extracted using CZ growth which is

specifically used in fabrication of integrated circuits.[1]

Apart from these a non-crystalline allotrope of silicon also exists that is called Amorphous

Silicon.[2]It is often abbreviated to a-si.

1.1 Amorphous Silicon: Historical Background

In Dundee, Scotland, Walter Spear and Peter LeComber discovered around 1973 that

amorphous silicon prepared using a glow discharge in silane (SiH4) gas had unusually good

electronic properties; as they were building on earlier work by Chittick, Sterling, and

Alexander. Glow discharges are the basis for the familiar neon light; under certain

conditions, an electric voltage applied across a gas can induce a significant electrical current

through the gas, and the molecules of the gas often emit light when excited by the current.

Amorphous silicon was deposited as a thin film on substrates inserted into the silane gas

discharge. Spear and LeComber reported in 1975 that amorphous silicons conductivity could

be increased enormously either by mixing some phosphine (PH3) gas or some diborane (B2H6

) gas with the silane. Just as for crystal silicon, the phosphorus doping of the amorphous

silicon had induced a conductivity associated with mobile electrons (the material was n-

type), and the boron doping had induced a conductivity associated with mobile holes (the

material was p-type).[3]

In 1974 David Carlson discovered that he could make fairly efficient solar cells using a silane

glow discharge to deposit films. In 1976, he and Christopher Wronski reported a solar cell


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based on amorphous silicon with a solar conversion efficiency of about 2.4%. These two had

discovered, the optoelectronic properties of amorphous silicon made by glow discharge (or

plasma deposition) are very much superior to the amorphous silicon thin films prepared, for

example, by simply evaporating silicon. After several years of uncertainty, it emerged that

plasma-deposited amorphous silicon contained a significant percentage of hydrogen atoms

bonded into the amorphous silicon structure and that these hydrogen atoms were essential to

the improvement of the electronic properties of the plasma-deposited material. As a

consequence, the improved form of amorphous silicon has generally been known as

hydrogenated amorphous silicon (or, more briefly, a-Si:H) is found. In recent years, many

authors have used the term amorphous silicon to refer to the hydrogenated form, which

acknowledges that the unhydrogenated forms of amorphous silicon are only matter of study

today.[3]

Also amorphous silicon thin film transistors are the key semiconductor of an entire area in

the field of electronics, known as macro-electronics, as crystalline silicon FETs are the

key enablers for the integrated circuit field.[4]

1.2 Requirement of Macroelectronics

Micro-Electronics technology has revolutionized computing and communications associated

with all manner of systems. As device scaling continues, and we move fully into the age

of systems on a chip andsystems in a package, these advances will not only continue,

but become more pervasive. Yet, as revolutionary as microelectronics has been, there

are functions that are not well addressed by conventional microelectronics technology. Since

the driving force behind microelectronics has been smaller and smaller devices in smaller

and smaller areas, those applications that require the electronics to be spread over a large

area are difficult or costly to achieve with the conventional approach. For example flat panel

displays where requirement for electronic control at each pixel of the display is there.

Another example is solar cell arrays where photo diodes must be spread over areas as large as

several square meters to collect sunlight sufficient to run the needed equipment. Flexible and

large area electronics using amorphous silicon (a-Si), low temperature polysilicon (LTPS),

and organic & inorganic nanostructured semiconductor materials is a technology that is

beginning to show tremendous promise in these type of cases.Thin film transistors (TFTs)

using a-Si and LTPS on rigid silicon and glass substrates are employed for driving activematrix liquid crystal displays and image sensors and also in photovoltaics.[5]


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Chapter 2

Structure and Properties of a-Si

Till now we have seen the emerging need of large area electronics devices which will be

costlier if fabricated with microelectronic technologies; mainly concern about crystalline

form of silicon. In this chapter we will see the structural and behavioural properties of

amorphous silicon which makes it most important material of latest growing macroelectronic

technologies.

2.1Atomic structure

a-Si has no long range order like a crystal, in device-grade a-Si most silicon atoms still have

bonds to four neighbouring silicon atoms (Fig. 2.1). Surprisingly, these bonds have a bond

length and an angle between bonds close to those in crystalline silicon, despite the lack of

long range order. The words mostand close to are important. About 10% of the silicon

orbitals do not form bonds with other silicon atoms, but nearly all of these form bonds

to atomic hydrogen, which reduces the density of residual dangling bonds and states in

the band gap to ~1016 cm-3. Because the material contains on the order of ~10% hydrogen,

it is often referred to as hydrogenatedamorphous silicon, a-Si:H. Second, because the

bond lengths and angles are not all ideal, many of the bonds are strained.[4]

(a) (b)

Figure 2.1 (a) schematic representation of atomic structure of, single crystal silicon (left) and

hydrogenated amorphous silicon (right). (b) structure of hydrogenated amorphous silicon.

This whole structure can be understood by making a simple model as described above.If we

build a noncrystalline silicon structure with wooden sticks (to represent covalent bonds) andwooden balls drilled with four small holes for the sticks (to represent the silicon atoms), we


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will have some trouble in making a noncrystalline structure. To avoid a crystalline structure,

we will need to bend the sticks and also the fourth stick cant be connected to anywhere that

will represent the condition of dangling bond. The dangling bond problem is solved by

hydrogen atom which forms a covalent bond with existing vacant bond position, thats how

hydrogenated amorphous silicon forms which is often abbreviated to a-Si:H. Strained bond

is also called weak bond in amorphous silicon and it is generally a result of larger

deviations in bond angle and length. The energy in weak bond is always greater then non

strained bonds thats why the probability of their breaking and making defects in a-Si:H

atomic network is greater.

We should note that in the continuous random network like a-Si:H, the definition of a defect

is modified with respect to the crystal structure. In a crystal any atom that is out of place in a

lattice forms a defect. In the continuous random network an atom cannot be out of place.

Because the only specific structural feature of an atom in the continuous random network is

the coordination to its neighbours, a defect in a-Si:H is the coordination defect. This

happens when an atom has too many or too few bonds. In a-Si:H the defects are mainly

represented by Si atoms that are covalently bonded to only three Si atoms (threefold

coordinated) and have one unpaired electron, a so called dangling bond. Since this

configuration is the dominant defect in a-Si:H, the defects in a-Si:H are often related to thedangling bonds. Also because of hydrogen a significant percentage of dangling bond is been

passivated and this hydrogen passivation of the dangling bonds is responsible for

semiconductor properties of amorphous silicon.

2.2Density of States

In a-Si:H, there is a continuous distribution of density of states and no well-defined band gap

exists between the valence band and the conduction band. Due to the long range order

disorder in the atomic structure of a-Si:H the energy states of the valence band and the

conduction bands spread into the band gap and form regions that are called band tailstates.

The band tail states represent the energy states of electrons that form the strained bonds in the

a-Si:H network. The width of the band tails is a measure for the amount of disorder in a-Si:H

material. More disorder in a-Si:H means that the band tails are broader. In addition, the

dangling bonds introduce allowed energy states that are located in the central region between

the valence band and conduction band states.

[6]


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The electron and hole wavefunctions that extend over the whole structure are characteristic

for energy states, in which the charge carriers can be considered as free carriers. These states

are non-localised and are called extended states. The wavefunctions of the tail and defect

states are localized within the structure and therefore these states are called localised states.

Consequently, mobility that characterises transport of carriers through the localised states is

strongly reduced. This feature of a sharp drop of mobility of carriers in the localised states in

a-Si:H is used to define its band gap. This band gap is denoted by the term mobility gap, E

mob, because the presence of a considerable density of states in this gap. Conflicts the

classical concept of the band gap. The energy levels that separate the extended states from the

localised states in a-Si:H are called the valence band and the conduction band mobility edges.

The mobility gap of a-Si:H is larger than the band gap of single crystal silicon and has a

typical value of 1.8 eV. The localised tail and dangling bond states have a large effect on the

electronic properties of a-Si:H. The tail states act as trapping centres and build up a space

charge in a device, the dangling bond states act as very effective recombination centres and

affect in particular the lifetime of the charge carries. The Density of States of a-Si:H is been

shown in the figure below.[6]

Figure 2.2 The schematic representation of the distribution of density of allowed energy states for electrons for

a-Si:H.

Now after this much study about a-Si:H we can put these important differences between

Crystalline and amorphous silicon.

Crystalline Silicon Amorphous Silicon

1.1ev indirect band gap 1.7-1.9ev direct band gap

4 fold co-ordination 4 fold co-ordination; 3 fold commonly

Fixed bond lengths and angles Variable bond lengths and angles


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Long range order Very short range order

Higher mobility of carriers ~1000

. Lower mobility of carriers ~1-10

2

.

We are now able to understand the following properties of a-Si:H.

2.3Optical Properties

The optical properties of a-Si:H are usually characterised by its absorption coefficient and a

value of the optical band gap. The figure 2.3 shows that a-Si:H absorbs almost 100 times

more than c-Si (crystalline Si) in the visible part of the solar spectrum. The higher absorption

is due to the disorder in the atomic structure of a-Si:H that behaves like a direct gapsemiconductor. This means that 1 m thick a-Si:H layer is sufficient to absorb 90% of usable

solar light energy. In practice the thickness of a-Si:H solar cells is less than 0.5 m that is

about 100 times less than the thickness of a typical single crystal silicon cell. This results in

important savings in both material and energy in fabrication of a-Si:H solar cells.

Figure 2.3 Absorption coefficient as function of photon energy for a-Si:H[6]

Another advantage of a-Si:H is that the optical absorption can be slightly changed by

varying its hydrogen content, and it can be greatly changed by alloying with carbon


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or germanium. The absorption coefficient of hydrogenated amorphous silicon carbide (a-

SiC:H) and hydrogenated amorphous silicon germanium (a-SiGe:H) that are fabricated

at Delft University of Technology are shown along with absorption coefficient of a-Si:H in

Figure 2.3. This feature of easy alloying of a-Si:H allows to design solar cell structures in

which a-Si:H based materials with different absorption properties can be used as active

layers.[6]

2.4Electrical properties

The electrical properties of a-Si:H are usually characterised in terms of dark conductivity and

photoconductivity. The measurement of these two properties is a standard approach to obtain

information about the quality of a-Si:H material for application in solar cells. Also it givesinformation about the mobility-lifetime product and the influence of impurities in a-Si:H. The

mobilities of the charge carriers in the extended states of a-Si:H are about two orders of

magnitude lower than in single crystal silicon. Typically, the electron mobility is 10 cm2/Vs,

and the hole mobility is 1 cm2 /Vs in intrinsic a-Si:H. The low values of electron and hole

mobilities and the high mobility gap of a-Si:H result in a low dark conductivity, which in

device quality intrinsic a-Si:H is less than 110-10-1cm-1.[6]

2.5Some other properties

a-Si:H layers can be made thinner than c-Si, which may produce savings on silicon

material cost.

a-Si:H can be deposited at very low temperatures, e.g., as low as 75 degrees Celsius.

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

for a roll-to-roll processing technique.

Once deposited, a-Si can be doped in a fashion similar to c-Si, to form p-type or n-

type layers and ultimately to form electronic devices.

a-Si can be deposited over large areas by PECVD. The design of the PECVD system

has great impact on the production cost of such panel therefore most equipment

suppliers put their focus on the design of PECVD for higher throughput, that leads to

lower manufacturing cost. (PECVD is a a-Si:H deposition technique which is

discussed in the appendix part of this report)


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Chapter 3

Applications of a-Si:H

Based on the properties discussed above we can now understand the basic principle of

working of devices fabricated with amorphous silicon. In now a days amorphous silicn has

become a material of choice for TFTs and also solar cells. Basics of these applications are

discussed.

3.1 Amorphous Silicon based Thin Film Transistor Display

Amorphous silicon has become the material of choice for the active layer in thin-filmtransistors (TFTs), which are most widely used in large-area electronics applications, mainly

for liquid-crystal displays (LCDs). LCDs based on TFT are called TFT-LCD which are active

matrix displays contrasting to the previous passive matrix displays. Passive matrix displays

are flat-panel display consisting of a grid of horizontal and vertical wires. At the intersection

of each grid is an LCD element which constitutes a single pixel, either letting light through or

blocking it.[7]When a voltage is applied between two points one from each horizontal and

vertical wire, the corresponding crystal realigns and changes the light transmission properties

of the liquid crystal. By repeating this process, scanning through the pixels an image can be

formed on the display.[8]

Figure 3.1passive matrix addressing[8]

Problems arise as the number of rows and columns increase. With higher pixel

density, the electrode size must be reduced and the amount of voltage necessary to drive the


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display rapidly increases. The higher driving voltage creates a secondary problem; charging

effects. Even though only one row and column are selected, the liquid crystal material near

the row and column being charged are affected by the pulse. The net result is the pixel

selected is active (dark), but the areas surrounding the addressed point are also partially

active (greys). The partially active pixels reduce the display contrast and degrade image

quality. This effect is known as cross talk. Active matrix amorphous silicon TFT displays are

used to overcome this problem.[8]

Although AMLCD (Active Matrix Liquid Crystal Display) technology is more expensive but

solves the scanning limitations of passive displays, contrast ratio and grey scale, and the

flicker induced by holding signals in pixels in a previous LCDs.

Figure 3.2Active Matrix liquid crystal display[8]

The diagram above (Figure 3.2) shows how thin film transistors are incorporated into a

matrix isolating the pixels. The gates are connected together in rows called gate lines or

buses, and the data lines are connected to the transistor sources. The liquid crystal is

represented as a capacitor. pixel isolation eliminates crosstalk. Also isolation from thecolumn line permits the pixel capacitor to remain charged, so that faster responding liquid

crystals can be used. Because of these advantages larger displays can be realized.[8]

The dominant active matrix technology is thin-film transistors (TFTs) of a-Si:H. The lower

process temperature (


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3.2 Amorphous Silicon based TFT X Ray Sensor

Large format digital x-ray image sensors are a recent development in the fields of medical

imaging and non-destructive testing. Such image sensors have become practical through the

emergence of large-area, amorphous Silicon (a-Si) TFT and photodiode technologies.

[9]

X-ray imaging has been used in many ways, including high resolution imaging using

film, real-time video imaging and digital imaging for digital subtraction angiography

and computer-aided tomography. Additional uses of digital x-ray imaging include bone

mineral densitometry, portal imaging for radiotherapy, and many areas of materials

monitoring which use x-rays for non-destructive testing.[9]

In many of these applications, a large-area flat-panel imager based on a-Si thin-film

transistor (TFT) technology is an attractive component due to its light weight and small

form-factor, high photosensitivity, and lack of image distortion that is present in the

previously used X-Ray techniques.[9]

Many of these applications can be met using a large-area array of a-Si TFTs and photodiodes.

The TFTs are used as pixel switches which address each row of the array, and photodiodes at

each pixel location convert incident light to charge, which is read out by charge amplifiers

connected to each column of the array. The array is patterned on a large glass substrate,

using technology similar to that used in fabricating active-matrix, liquid crystal displays.

Gate driver and charge amplifier ICs are connected to each line on the glass.[9]The difference

between TFT based X-Ray sensor and the old X-Ray is shown by the figure below.

Figure 3.3 difference between the images of old X-Ray sensor (left) and a-Si:H TFT based sensor(right)

3.3 a-Si:H based Solar Cells

As we have seen in previous discussions that by desired doping we can make n type and p

type hydrogenated amorphous silicon, this kind of fabrication is used in a-Si:H based solar


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cells. With manufacturing advantages such as scalability and cross-compatibility with the flat

panel display industry, and with considerations for potential scarcity of silicon, the

amorphous silicon (a-Si) thin film photovoltaic devices (TFPD) are often the technology of

choice for high volume solar cell manufacturers.

For understanding the advantage of a-Si:H based solar cells we have to compare it with

conventional solar cells based on single crystal silicon structure.

There is a fundamental difference between c-Si and a-Si:H solar cells. A typical c-Si

solar cell, shown in Figure 3.4, consists of a p-type wafer, which is 300 to 500 m thick, with

a thin n-type layer on the top of the wafer, which forms a p-n junction. The width of the

depletion region of the p-n junction is less than 0.5 m. The depletion region, where

an internal electric field is created, represents only a tiny part of the wafer. Most electron-

hole pairs are generated in the bulk of the electrically neutral p-type region. Electrons, which

are the minority carriers in the p-type region, diffuse towards the p-n junction and in the

depletion region of the junction the electrons drift to the n-type layer under the influence of

the internal electric field. This solar cell is called a diffusion device because the

dominant transport mechanism of the photo-generated carriers is diffusion. Therefore,

the diffusion length of minority carriers (i.e. electrons in p-type material) is an

important material parameter that determines design and performance of c-Si solar cells.

Typical values of the diffusion length of electrons in the p-type float zone single crystal

silicon wafers are in the range of 250 m to 700 m. These values correspond to wafers with

a resistivity in the range of 0.25 to 1.0 cm,respectively.[6]

Figure 3.4 solar cell structure based on c-Si (left) and a-Si:H (right)[6]


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In a-Si:H the diffusion length of the charge carriers is much shorter than in single

crystal silicon. In device quality intrinsic a-Si:H the ambipolar diffusion length ranges from

0.1 to 0.3 m. In doped a-Si:H layers, in which the defect density due to the doping is two or

three orders of magnitude higher than in the intrinsic a-Si:H, the diffusion length of

the minority carriers is much lower. A solar cell structure based on the diffusion of the

minority carriers in the electrically neutral regions of the doped layers as in the case of c-Si

does not work in the case of a-Si:H. The photo-generated carriers would all recombine in the

doped layers before reaching the depletion region of the p-n junction due to the very short

diffusion length. Therefore, an a-Si:H solar cell is designed differently. A schematic lay-out

of an a-Si:H solar cell is shown in Figure 3.4. The active device consists of three layers: a p-

type a-SiC:H layer, an intrinsic a-Si:H layer, and an n-type a-Si:H layer. The doped

layers are usually very thin, a p-type a-SiC:H layer is ~10 nm thick and an n-type a-Si:H is

~20 nm thick. The active layer in the a-Si:H solar cell is the intrinsic layer which is typically

300 to 500 nm thick. This intrinsic layer is sandwiched in between the doped layers so that

an internal electric field is present across the intrinsic layer. The electron-hole pairs that

are generated in the intrinsic a-Si:H layer immediately experience the internal electric field

that separates electrons and holes from each other. The separated carriers drift under the

influence of the internal electric field towards the doped layers (electrons towards the n-type

layer and holes towards the p-type layer) and are collected by the electrodes. The dominant

transport mechanism of the photo-generated carriers is drift in the internal electric field and

therefore an a-Si:H solar cell is called a drift device. A stack, which consists of p-type

a-SiC:H, intrinsic a-Si:H and n-type a-Si:H layers, forms the p-i-n junction. This structure of

a-Si:H solar cell is presented in Figure 3.4 and it is generally called a single junction a-Si:H

solar cell. The band diagrams of c-Si and single junction a-Si:H solar cells are presented in

Figure 3.4, which also demonstrate the difference between the diffusion type and the drift

type solar cell.[6]

The thickness of the a-Si:H solar cell is comparable to the thickness of the depletion region

in the crystalline silicon solar cell, i.e. about 0.5 m. As mentioned earlier, this small

thickness is sufficient for absorption of the visible part of solar radiation. The small thickness

implies a large reduction in material and energy consumption during production when

compared to crystalline silicon solar cells. Furthermore, when deposited on a light substrate

such as a flexible foil the weight of a-Si:H solar modules is strongly decreased. This


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is desired specifically in space applications.The thicknesses of individual layers in c-Si and

a-Si:H solar cells are compared in Table below. [6]

Crystalline silicon solar cell a-Si:H solar cell

Al contact 0.5 to 1 m Top glass 2 to 4 mm

SiO2or Si3N4layer ~0.1 m Transparent electrode 0.5 to 0.9 m

n+ type emitter 0.3 to 0.5 m p type a-SiC:H 0.01 m

p type base 300 to 500 m Intrinsic a-Si:H 0.3 to 0.5 m

p++type region 1.0 m n type a-Si:H 0.02 m

SiO2 layer 0.05 to 0.1 m ZnO layer ~0.1 m

Al contact 0.5 to 1.0 m Al or Ag contact 0.5 to 1.0 m

Apart from these potential applications in recent time new applications for a-Si cells are

being sought such as building-integrated PV, space power, consumer electronics, grid

integration, and large scale power generation.


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Chapter 4Conclusion

Amorphous silicon has become the material of choice for large area electronics, with major

applications in displays, image sensing and solar power conversion. The technology

development has occurred because amorphous silicon is a thin film semiconductor that can be

deposited on large, low cost substrates. Furthermore, it retains the material properties of

silicon, and can therefore be patterned into complex electronic devices using the samephotolithographic techniques that have proved so powerful for crystalline silicon electronics.

The report describes the devices, arrays and applications enabled by amorphous silicon, and

how the unique properties of the material determines the design and performance of the

various applications. Active-matrix liquid crystal displays for high performance laptop

computers are a major business, and have established the array manufacturing technology.

Digital X-ray image sensors are poised to revolutionize medical diagnosis by replacing film

with flat-plate amorphous silicon arrays. Solar cells are the most mature application of

amorphous silicon, and many innovations like multi junction solar cell, to enhance the cell

efficiency have been introduced.


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Appendix

Plasma Enhanced Chemical Vapour Deposition (PECVD)

The most commonly used deposition method to produce "device quality" a-Si:H, both on the

laboratory and industrial scale, is the radio frequency (13.56 MHz) plasma

decomposition of SiH4 , known as the Plasma Enhanced CVD (rf PECVD) method or the

glow discharge (GD) deposition. The PECVD deposition system is schematically shown in

Figure below. The role of the plasma is to provide a source of energy to dissociate the SiH 4

molecules. This is done by collisions with electrons, which originate as secondary electrons

in the plasma and build up their energy by acceleration in an electric field. The growth of an

a-Si:H film is accomplished by attachment of reactive particles of dissociated SiH 4

molecules, called radicals, to the surface of the growing film. As the thickness of the a-Si:H

film for device applications is around half a micrometer, a-Si:H must be deposited on

an appropriate substrate carrier. Some of the energy transferred to the SiH 4 molecules in

the collisions with electrons is radiated as visible light, hence the deposition method is

also called the glow discharge.

Figure PECVD schematic view


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10, 20142.

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Xunming Deng and Eric A. Schiff Amorphous Siliconbased Solar CellsUniversity

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Silicon12th International Conference on Ultimate Integration on Silicon (ULIS), IEEE

(2011)

5. Chalamala B.R.,Moussessian A.,Kane, Michael G.Macroelectronics: Perspectives

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6. Arvind Shah Thin-Film Silicon Solar Cells Chapter 7 (Page 7.3-7.5)

7. Passive-Matrix Displays

(http://www.webopedia.com/TERM/P/passive_matrix_display.html)Dated October 29,

2014

8. Liquid Crystal Display (LCD) Passive Matrix and Active Matrix Addressing

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9. Richard L. Weisfield Amorphous Silicon TFT X-Ray Image SensorsInternational

Electronics Devices meeting (1998)
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