Amorphous Silicon TFTs

Thin-film Transistor Device Structures 11.3.2.1 Amorphous Silicon TFTs 

As a first demonstration of the fabrication of TFT devices by digital lithography, tri-layer insulated-gate thin-film transistors were fabricated on four-inch glass substrates using a three-layer wax-mask process. Mask layers were used to define the 

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Water is known to cause electrical instability in a variety of electronic devices. For example, if water is absorbed on the surface of an unencapsulated amorphous silicon TFTs it induces a charge in the semiconducting film that acts as an additional channel layer for conduction, the back-channel effect [56]. The situation for semiconducting polymers is more complex, because water can be absorbed by the bulk of the film and interact directly with the accumulated carrier states. 

The departure of the experimental data from the y/Ici,sat versus Vg straight line at low gate biases is generally attributed to a subthreshold regime (like in conventional MISFETs). We note, however, that such an interpretation is at variance with Eq. (14.47), which predicts a positive value for V,. As noted above, another origin of this departure would be a decrease in the mobility at low gate voltages, as in amorphous silicon TFTs [13, 18]. 

Figure 6.2. Schematic structures of [a] a single-crystal Si MOSFET, [b] amorphous silicon TFT, and [c] polycrystalline silicon TFT.

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. 

Liquid-cr57Stal displays normally employ a matrix of amorphous silicon TFTs to control the voltage applied to the individual pbcels. In order to drive an active-matrix addressed flat-panel LCD, it is necessary to make contact to each of the row and... 

The use of TFTs in electronic biosensor array is still limited. Various TFT-based DNA and protein transducers have been developed either with poly-Si or amorphous silicon TFTs [4,58]. Although proved to be successful as sensor transducers, working TFT-based sensor arrays have not reported in the literature. This is largely attributed to the fact that TFT foundries are mostly specialized for the manufacturing of LCD backplanes and not commercially available to researchers. 

For the measurement circuit, the amorphous silicon TFT has not been considered suitable for analog circuit or high frequency digital circuit, due to its low mobility and transconductance, while the poly-Si TFT has been developed into a large variety of analog circuits with moderate performance. The suitability of the three FET technologies for biosensor array applications is summarized in Table 6.1. 

Amorphous silicon TFTs are the more developed of the two, research on them having started earlier. Impressive active matrix LCDs have been fabricated using this technology, although longer term polysilicon offers a number... 

In the field of flexible plastic screens the largest model Samsung has produced is believed to be the 17.5 cm (640 pixels by 480 pixels) prototype which comprises an amorphous silicon TFT which is attached to a colour filter. 

It is of crucial importance that the extracted field-effect mobility and threshold voltage obtained from transistor characteristics are therefore not exclusively channel properties but influenced by the contact formation at the source and drain electrodes. Commonly, the transfer line method (TLM) is used to extract the contact resistance from the OFET current/voltage dependence [69]. The method stems from a conventional technique to estimate contact resistances, and was developed for amorphous silicon TFTs. The prevailing contact resistance is determined by varying the channel length L of the transistor. Since the total resistance is the sum of the channel resistance Vd//d and the total contact resistance R =Ris, +I id), the total resistance 7 tot in the linear region can be written as... 

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