Silicon on insulator & thin film transistor

Introduction:  Herein this article, the SOI and thin-film transistor (TFT) will be described in the following way.

SOI:  In contrast to thin-film transistor, top silicon layer of an SOI (silicon-on-insulator) wafer is a premium-quality single-crystalline material i.e. apt for high-density and high-performance integrated
Several forms of SOI structures have been demonstrated using different holding substrates and insulator materials.

These consist of silicon-on-oxide, silicon-on-sapphire (SOS), silicon-on-zirconia (SOZ), and siliconon-nothing(air gap). In SOZ and SOS technologies, upon a crystalline insulating substrate, single-crystalline silicon film is epitaxially grown. In these, insulators themselves serve as the substrates, ZrO₂ in SOZ and Al₂O₃ in SOS.

The difficultywith these techniques is that the material quality gets disturbed when the film gets thinner. The most popular is the one that was the first option and which used oxide as an insulator and another that used Si wafer as the holding substrate. There are several ways to construct this structure. One is where SIMOX (separation by oxygen implantation) is used and where high-dose oxygen is implanted onto a silicon wafer followed by annealing at high temeparture to create buried SiO₂ layer.

In another technique, bonding of two silicon wafers is involved. One silicon wafer has an oxidized layer followed by thinning or completely removed majority of the top wafer until a thin silicon layer is left. In one technique, lateral epitaxial growth of silicon over an oxide layer is usedthat starts from a seed opening to the substrate.

Another technique employs laser recrystallizationthat transforms amorphous silicon deposited on the oxide layer to single-crystalline material, or poly-crystalline form with large grain size. A schematic diagram of n-channel MOSFET made on an SOI substrate is shown in Figure (a) with its typical I-Vcharacteristics illustrated in Fig. (b). The kinks which are associated with floating body lacking a substrate tie are easily noticeable.

Thin-Film Transistor (TFT): The thin-film transistor is usually referred to as MOSFET unlike other types of transistors. Their structure is same as MOSFET that is built on SOI except for that their active film is a deposited thin film while it may have any form of substrate.

Since a semiconductor layer is formed by deposition, there are more defects and imperfections in amorphous material than single-crystalline semiconductors and that results in more complex transport processes in TFT. To improve the performance, reliability and reproducibility of a device, the bulk and interfacetrap densities must be diminished to reasonable levels.
Due to lower mobility, current in a TFT is always quiterestricted and leakage current is always higher because of defects.

Its main applications are in those areas where a flexible substrate or large-area is required and it is not feasible to carry out conventional semiconductor processing. Its fine example is a large-area display where an array of transistors in turn controlsan array of lighting elements.

In such applications, device performance parameters such as speed or current are not critical.