Transistor, thin film

Despite of its much reduced manufacturing cost and versatile form factor, the main drawback of TFTs compared with single crystalline silicon devices is the low electrical performance. This is a direct result of the low electron mobility of the semiconductor material employed for TFT fabrication. 

While amorphous silicon TFT sulfers from low electronic performance, it is very flexible in application and manufacturing. One important advantage is that amorphous Si can be deposited at temperatures as low as 75°C. This makes it possible for the device to be made not only on glass, but also on plastics. In addition, amorphous silicon can be deposited over very large areas by plasma-enhanced chemical vapor deposition (PECVD) with standard industrial equipments. Both features make mass-scale production of amorphous silicon TFT-based devices relatively easy and economic. The main application for amorphous silicon TFT is on liquid crystal displa (LCDs), in which each pixel is individually driven by a TFT transistor. 

The fabrication of a polycrystalline silicon film can be achieved through various CVD methods or crystallization of amorphous silicon. But these processes require high temperatures of at least 300°C, making the deposition only possible on glass but not plastic. A relatively new technique called laser recrystallization has been devised to crystallize a precursor amorphous silicon film by localized heating without damaging the plastic substrate. A transfer process has also been developed to fabricate poly-Si TFT circuits on plastic substrates [14]. 

In recent years, organic or polymer semiconductor materials have been intensively researched to make TFTs. These organic TFTs can be manufactured with very low cost using much simpler 

The variation of the source-to-drain current, 7, as a function of the source-to-drain voltage, V, and of the grid voltage, V, is a well-established equation (Guillaud et al. 1990)  

Korotcenkov, Handbook of Gas Sensor Materials Properties, Advantages and Shortcomings for Applications Volume 1 Conventional Approaches, Integrated Analytical Systems, 

The curve = f(Va) enables deduction of the mobility p, which is called the field-effect mobility. 

The channel length L (distance between the source and drain electrodes) and the thickness of dielectric layer determine the range of the voltage sweeps. The magnitude of L is also important in minimizing the effect of the contact resistance, which must be kept to a small fraction of the total channel resistance (Torsi 

It is clear that the sensing effect in TFT can be improved by reducing the free charge carrier density and the thickness of the active part of the TFT and by increasing the mobility of charge carries and capacitance of the insulating material, i.e., decreasing the thickness of the insulator (Bouvet 2006). 

Amorphous semiconductors are employed where crystalline substrates are unusable or impractical such as when a large number of discrete devices need to be integrated onto the surface of a material such as glass. Even when a single-crystal substrate could be used the maximum area is limited to the size of a single-crystal substrate, while amorphous semiconductors are easily adapted to large substrates. Devices on flexible surfaces and on materials that cannot be heated significantly (polymers for example) are also commonly constructed from amorphous semiconductors. 

More recent applications are beginning to require higher speeds, which a-Si H devices with carrier mobilities of Icm V s are not always capable of meeting. This is driving movement away from amorphous semiconductors to nanocrystalline 

Therefore, there is still an important market for a-Si H switching devices. 

The lesson of the thin film transistor is that amorphous semiconductors work well when the demands on the performance of the device are not great and where light emission is not important. When such requirements are coupled with restrictions on the surface on which the material is to be applied, amorphous semiconductors can become essential. The major question at this point is becoming - will organic materials replace inorganic materials in amorphous thin film devices  

A significant advantage of a-Si H and alloys with Ge and C is that these are direet-gap materials, unlike their parent erystalhne semiconductors. Consequently, they have at least 100 times higher optieal absorption coefficients than the equivalent erystalline materials. The benefit is that only 1% of the thickness of the crystalline deviees is required to absorb an equivalent amount of light. At the same time, one needs earriers to move only 1% as far as in erystalline Si or Ge before they reach a contact. Therefore, the electrieal quality of the material may be much lower and earriers ean still be eoUeeted. For a-Si H the very high absorption coefficient allows aU tight to be eoUeeted in lpm of material. This means it needs only a relatively thin film of a-Si H to produce a useful deviee. Hence the name thin film solar cells. 

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Phthalocyanines have been used to incorporate semiconductor properties in polymers (182) or to develop a thin-film transistor (183). Phthalocyanines and their derivatives can act as dyes in color photography (qv) (184) or electrophotography (185). Light-sensitive compositions for use on Hthographic plates are comprised in part of copper phthalocyanine blue (186). Dichlorosilicon phthalocyanine [19333-10-9] has been used in the... 

Fig. 4. Some electronic device applications using amorphous silicon (a) solar cell, (b) thin-fiLm transistor, (c) image sensor, and (d) nuclear particle detector.

ORGANIC SEMICONDUCTOR THIN-FILM TRANSISTOR SENSORS... 

Polymers such as PTV have potential applications as the active semiconductor layer in thin-film transistors (TFTs). 

The synthesis of two a,a -disubstituted bis(dithienothiophene) (BDT) derivatives 163 and 166 for organic thin film transistors (TFT) has been performed. Substitution of dithienodithiophene 15 and subsequent oxidation with -BuLi and Fc(acac) gave the two derivatives (Scheme 18) <1999SM(102)987>. 

The a,a-disubstituted bis(dithienothiophene)derivatives 116 (Table 2) have been deposited as active layers in organic thin-film transistors <1999SM(102)987>. 

Au-gate poly-Si thin film transistor Au-gate MIS capacitance 18-mer ssDNA, 1012 - 1013 ssDNA/cm2 5 mM phosphate buffer pH 7.2 18-mer cDNA A Eh 355 mV AVfi, 140mV 1 h 30 min Ag/AgCl [36]... 

Klauk H, Halik M, Zschieschang U, Schmid G, Radlik W, Weber W (2002) High-mobility polymer gate dielectric pentacene thin film transistors. J Appl Phys 92(9) 5259-5263... 

As described earlier, the covalently bonded hydrogen, by passivating dangling bond defects and removing strained weak Si—Si bonds from the network, dramatically improves the semiconducting quality of amorphous silicon. Hence without the presence of hydrogen, effective amorphous semiconductor devices such as solar cells or thin film transistors would not be possible. Unfortunately, low defect density, high electronic quality... 

Lee, S. Koo, B. Park, J.-G. Moon, H. Hahn, J. Kim, J. M. 2006. Development of high-performance organic thin-film transistors for large-area displays. MRS Bull. 31 455 459. 

Sazonov, A. Striakhilev, D. Lee, C.-H. Nathan, A. 2005. Low-temperature materials and thin-film transistors for flexible electronics. Proc. IEEE 93 1420-1428. 

Kagan, C. R. Andry, P. (Editors). 2006. Thin-Film Transistors. CRC Press, Boca Raton, FL. 

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