LCD

TFT

LED-OLED

CCD

CMOS

DISPLAY SYSTEMS AND PHOTOSENSORS (PART 2)

Thin Film Transistors(TFT)

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Field effect transistors (FET) consist of a piece of semiconductor made into four regions that are referred to as the source, drain, gate, and body (Figure 1). The source and drain components are doped with charge carriers that will provide excess holes (p-doped) or electrons (n-doped) in the semiconductor. Between the source and drain is the body, which issemiconductor that is either non-doped or oppositely doped from the source and drain.

Figure 1: Diagram of a planar Field Effect Transitor (FET) with labeled parts.

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Field Effect Transistors(FET)

The gate is placed near the body region, but separated by an electrical insulator, (Often oxides) and may be either a conducting metal or semiconductor oppositely doped from the source and drain. In an n-channel, or npn, FET, applying positive voltage to (a negative voltage in the case of a hole-doped p-channel, or pnp, transistor) to the gate, the gate provides an electric field that causes electrons to be mobile in the body of the transistor, which becomes conductive and even current amplifying.

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FET configurations

Operation gate volt controls the thickness of the channel consider an n-channel device

making the gate more positive attracts electrons to the gate and makes the gate region thicker – reducing the resistance of the channel. The channel is said to be enhanced

making the gate more negative repels electrons from the gate and makes the gate region thinner – increasing the resistance of the channel. The channel is said to be depleted

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Effect of varying the gate voltage

The Thin Film Transistor (TFT) is a special kind of field-effect transistors that are made by the deposition of thin films of the different materials that make up the transistor. In an a-Si TFT, the conducting carriers are electrons. Electrons drift from the source electrode to the drain, which results in current flow from drain to source. The switching structure is based on the metal–insulator–semiconductor (MIS) structure, which is also used widely in the IC industry with crystalline silicon. In such a structure, the metal can attract and repel carriers at the semiconductor side, which changes the carrier concentration of the semiconductor. It effectively engineers the conductance and hence can be used to switch on and off with high and low conductance, respectively. Although operation modes for crystalline silicon, a-Si and poly-Si are different, the basic physics is similar.

7Schematic representation of and notation for a TFT.

Thin Film Transistors(TFT)

Typical TFTs can be categorized as normal or inverted structures, depending on whether their gate is on the top or bottom, respectively.

Another classification depends on whether the drain/source and gate areon the same or opposite sides; these are called coplanar or staggered structures, respectively. Hence, there are basically four different kinds of TFT structures. Different structures may be the result of different fabrication procedures and may result in different device performance.

Typically, a-Si TFTs use the inverted staggered structure configuration because it makes them easy to fabricate and gives them better electrical characteristics. The a-Si channel is sensitive to photons of the external ambient that mainly illuminates from the bottom side in a display system: an inverted structure can shield the device from this light by virtue of the gate metal.

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Thin-film transistor characteristics

Non-silicon-based thin-film transistors

this section, will introduce two kinds of TFTs where the active layer consists of organic molecules and ZnO-based materials. Organic TFT (OTFT) can be fabricated with a low temperature deposition and a solution process, which is less complex compared with conventional Si-based technologies. In addition, due to the mechanical flexibility of organic material, OTFT can be used as the backbone of the flexible display, and this has the benefit of being rugged, thin and lightweight. Also, it is potentially of low cost due to the possibility of using the roll-to-roll process. When fabricating a flexible display on a plastic rather than a glass substrate, one important issue arises concerning the durability of the substrate.

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chemical structure of some common organic semiconductors are showed below. Intrinsically, organic materials are ambipolar which means that they can conduct both holes and electrons. Extrinsically, however, electron conduction is often several orders of magnitude smaller: first of all, injection of electrons from a contact into organic materials is more problematic than hole injection. Second, electron-rich molecules and organic anions are very sensitive to oxidation by water or oxygen and the resulting oxidized molecules have a strong tendency to trap electrons which effectively lowers electron mobility. In practice, it is very difficult to exclude water and oxygen and therefore organic molecules are sometimes substituted with electron-withdrawing groups like fluor to make them more oxidation-resistant which often yields higher extrinsic electron mobilities. Pentacene is one of the most popular and promising ‘small molecule’-semiconductors with reported p-type room-temperature mobilities typically around 100 cm2/Vs.

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materials

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P-type organic semiconductor materials

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n-type organic semiconductor materials

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For practical applications, organic semiconductors are typically used as thin films ranging from a few nanometers to a few micrometers thick. To deposit such thin films, several methods can be used depending on the organic material’s properties, cost, tolerances, substrate type etc.For polymers, deposition typically occurs from solution phase which of course requires the polymer to be soluble. The polymer solution in a volatile solvent is dispensed on a substrate surface. When the solvent evaporates, it leaves behind a coating of the polymer. The resulting polymer film is amorphous or semicrystalline depending on the processing conditions. The simplest methods, are:· solution casting (where the solution is simply poured on a substrate, A),· inkjet printing (where small drops of solution are ejected in a patterned way on a substrate, B),· aerosol spray (where a ‘mist’ of droplets is homogenuously sprayed on a substrate, C),· ‘doctor blading’ (where a blade scrapes the solution over a substrate, D),· screen printing (where the solution is pressed through a screen onto a substrate, E),· dip coating (where a substrate is dipped in the polymer solution, F),· spin coating (where the solution is ‘spun’ on a rotating substrate, G).

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Processing

Schematical depiction of the various methods for obtaining thin films by solution processing.

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Optical microscopic image of the 100 ppi organic thin-film transistor array fabricated entirely by printing process (pixel size: 254 μm).

Organic thin-film transistor array fabricated entirely by printing process on a flexible plastic film

The availability of organic semiconductors allows the construction of organic electronic devices analogous to classic devices such as transistors, solar cells and (light-emitting) diodes and gave rise to the field of organic or ‘plastic’ electronics. Due to their relatively low performance, they are no competition for classic, inorganic semiconductors in high performance applications but they can maybe bring electronics to areas where silicon can not. Thanks to their relative ease of fabrication and deposition and their compatibility with printing techniques and low-cost, flexible substrates, they could be used for flexible, large area and potentially low-cost applications includes:

Sensors

Electronic papers

memory devices including radio frequencyidentification cards (RFIDs)

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Applications

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Sony Corporation (‘Sony’) announced that it developed a super-flexible 80 μm-thick 4.1-in 121 ppi OTFT*1-driven full color OLED display which can be wrapped around a thin cylinder.To create the display, Sony developed OTFTs with an original organic semiconductor material (a PXX derivative) with eight times*2 the current modulation of conventional OTFTs.

OLED

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Sony will continue to improve the performance and reliability of its flexible organic displays because the application of these developments are expected to yield thin, light-weight, durable and mobile devices with enhanced form-factor.

Technology Features1. High performance OTFT with originally developed high-mobility and highly-

stable organic semiconductor materials, PXX derivatives.Sony has developed organic semiconductor material which is stable under

exposure to oxygen, moisture, light and heat and improved current modulation of eight-times*2 that of conventional OTFT with organic semiconductor of pentacene. Improvement of this OTFT achieved the world’s highest-resolution OTFT-driven OLED display with resolution of 121 ppi and 432 x 240 x RGB (FWQVGA) pixels*4.

2. Integration of a flexible gate-driver circuit with OTFTsThis is the world’s first demonstration*3 of an OLED display with an integrated gate-driver circuit with OTFTs.

3. Enhancement of flexibility with all organic insulators in the OTFT and OLED integration circuit In order to enhance flexibility of the display, Sony has developed organic insulators for all the insulators in the OTFT and OLED integration circuit.

OLED

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4. Achieved display capable of reproducing moving images while rolled-up around cylinder with 4mm radius.Even after 1000 cycles of repeatedly rolling-up and stretching the display, there was no clear degradation in the display’s ability to reproduce moving images.

Specification of the OTFTorganic semiconductor: peri-Xanthenoxanthene(PXX) derivativehole mobility: 0.4 cm2/Vscurrent on/off ratio: 106channel length: 5μmthreshold voltage: -5VSpecification of the rollable OTFT-driven OLED displaysize of a panel: 4.1 inch widenumber of pixels: 432 x 240 x RGB pixelssize of a pixel: 210μm x 210μmresolution: 121 ppi (pixels per inch)number of colors: 16,777,216peak luminance: >100 cd/m2contrast ratio: >1000:1minimum bending radius: 4 mmdriving scheme: 2T-1C voltage programming with OTFTsthickness of a panel: 80μm

OLED

There is two common device configurations. Both are of the inverted structure type, since the insulator must be formed before the organic thin film. For a bottom contact device, the organic layer is deposited at the top of the device. Drain, source, gate and insulators can be defined by conventional photolithography which provides a high resolution (less than 1 m). For a top contact device, the organic material is formed first, followed by drain and source electrode evaporation through a shadow mask. The resolution is limited to several tens of micrometers in this configuration. As regards device performance, OTFTs with a top contact typically exhibit a superior performance compared to bottom contact types due to the larger contact area and lower contact resistance. Typically, a molecular thin film exhibits a much lower mobility value than semiconductor materials with covalent bonds, which means the electric current provided by OTFTs is typically smaller than that of a-Si and poly-Si based TFTs.

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Device configurations

BENEFITS AND CHALENGES

• Benefits of an OTFT:

• Does not require glass substrate as amorphous Si does. It could be made

on a piece of plastic.

• Manufactured at lower temperatures

• Deposition techniques could reduce costs dramatically.

• Challenges involved:

• Work arounds for complications with photo resists.

• To find organic semiconductors with high enough mobilities and switching times.

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• OTFT technology’s application is diverse.

Organic thin-film transistor (OTFT) technology

involves the use of organic semiconducting

compounds in electronic components,

notably computer displays. Such displays are

bright, the colors are vivid, they provide fast

response times (which need to be developed

in OTFT), and they are easy to read in most

ambient lighting environments.

• Organic substrates allow for displays to be fabricated on flexible surfaces, rather than on

rigid materials as is necessary in traditional TFT displays. A piece of flexible plastic might be

coated with OTFT material and made into a display that can be handled like a paper

document. Sets of such displays might be bundled, producing magazines or newspapers

whose page contents can be varied periodically, or even animated. This has far-reaching

ramifications. For example, comic book characters might move around the pages and

speak audible words. More likely, such displays will find use in portable computers and

communications systems.

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Future

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1. Jiun-Haw Lee , David N. Liu, Shin-TsonWu, “ Introduction to Flat Panel Displays”, chapter3: thin film transistors, Wiley-SID Series in Display Technology.

2. Antonio Facchetti, Tobin J. marks, “Transparent Electronics”.

3. Alberto Salleo, “Fabrication and characterization of highperformancepolymer thin-film transistors”, Palo Alto research center.

References: