High-performance TFTs

Although high-performance TFTs are needed for several electronic applications, the potential for printed, inorganic electronics encompasses other devices and applications. A major opportunity is in optoelectronic applications, which impose different requirements, challenges, and opportunities (see Chapters 6, 7, 9, and 11 for discussion of solution-processed solar cells and other printed optical devices). 

Sera K, Okumura F, Uchida H, Itoh S, Kaneko S, Hotta K (1989) High-performance TFT s fabricated by XeCl excimer laser annealing of hydrogenated amorphous-silicon film. IEEE Trans Electron Device 36 2868-2872... 

It is important to note that the entire NW-TFT device fabrication process is performed essentially at room temperature. Therefore, the assembly of high-performance NW-TFTs can be readily applied to low-cost glass and plastic substrates. It has been demonstrated, for example, that NW-TFTs can be fabricated on plastic substrates (Fig. 11.14d) with carrier mobilities comparable with those made on silicon substrates. Moreover, studies demonstrate that... 

The reproducible assembly of high-performance NW-TFTs in high yield readily enables construction of more complicated high-speed circuits on... 

C.-Y. Chen and J. Kanicki, High-performance a-Si H TFT for large-area AMLCDs, Proc. 26th Eur. Solid State Dev. Reas. Conf., 1023-1031, 1996. 

Another advantage of OVPD over VTE is the ability to control surface morphology (Table 9.1, no. 10). Use of two different deposition modes in OVPD enables active design of layer morphology and interfaces with very valuable properties for device improvements this is of particular importance for high-performance organic TFTs. 

Fujisaki, Y. Inoue, Y. Sato, H. Kurita, T. Tokito, S. and Fujikake, H. (2003) Organic TFT-driven Liquid Crystal Cell with Anodic-oxidised Gate Insulator and Double Protection Layer. IDW Digest, 291-294. Gelinck, G.H. Geuns, T.C.T. and de Leeuw, D.M. (2000) High-performance all-polymer integrated circuits. Appl. Phys. Lett., 77, 1487-1489. 

The metastability phenomena influence the performance of the active matrix arrays. Defect creation in the channel causes a threshold voltage shift when a TFT is held on for an extended time and results in a slow drift of the on-current. Fortunately the rate of defect creation is low at room temperature and represents a minor problem. There is a larger effect on the characteristics of the high voltage TFTs. Resistors fabricated from n a-Si H change their resistance slowly because of defect equilibration and can affect the gain of amplifier circuits. 

In the form of amorphous silieon, thin flexible silieon films can be fabricated as well. Because of the absence of a erystal lattiee, eleetronic performance of amorphous silicon is multiple orders of magnitude worse than the performance of (poly)crystalline silicon, but still high enough for applications like TFT backplanes. However, processing temperatures required to obtain a-Si films of this high performance are not compatible with flexible substrates either. 

On the other hand, although TFTs cannot provide such high performance electronic devices, it can be manufactured on much cheaper substrates such as glass and even plastics, making the technology an ideal candidate for biosensor arrays in the view of cost. The main limitation of TFTs is the low mobility of the semiconductor material. This does not only affect the performance when it is used as a transducer, but, in case of amorphous TFT, it also prevents its use for the addressing logic and measurement circuit. 

The ideal solution to the drive-address circuitry is to incorporate it directly into the thin-film structure on the glass. TFTs can be fabricated for the active matrix and the drive-address circuitry at the same time. This is already being done commercially for projection LCDs, where the small size and 1000 X 1000-line matrix restricts the number of connections that can be made to the substrate. A 950 to 1000°C process utilizing fused silica substrates (Table 7.1) is used to make the high-performance poly silicon transistors. Larger- area displays utilize the lower-cost flat glass shown in Table 7.6 where the strain point is far below that of fused silica. In this case, a laser recrystalUzation process is used to create high-mobihty silicon films for developmental displays. 

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