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The characteristics of amorphous silicon thin-film transistors fabricated by chemical vapor deposition method is discussed in detail in conjunction with posthydrogenation. As the hydrogenation around an active interface starts, the transistor performance is abruptly improved and then does not change with further hydrogenation. A critical condition for hydrogenation for such an abrupt improvement may be accounted for based on a simple formula using a transit time of hydrogen atoms. A desirable thickness of amorphous silicon films for high performance TFT is thicker than 100 nm. In addition, it is recommended that hydrogenation is to be made at a temperature as low as possible as long as it can be possible.

Arrays of large Si grains have been grown by phase-modulated excimer-laser annealing at room temperature. Home-plate- and square-shaped grains could be grown over the whole irradiated substrate area. These results indicate that the fabrication of x-Si TFTs on glass is a realistic development target.

— A chemical-vapor-deposition (CVD) method has been applied, for the first time, to a batch process for a-Si/SiN TFTs. The maximum process temperature was 500°C. High-performance TFTs with mobilities of 1.2 cm2/V-s, a threshold voltage of 4 V, and a subthreshold slope of 0.6 V/decade have been fabricated with good reproducibility and uniformity over a wide range of CVD conditions. This feature makes the a-Si/SiN TFT batch process practical.

This paper reports the study on characteristics control of amorphous silicon field-effect-transistors (a-Si FETs). For the application of a-Si FETs to switching elements in the display panel, it is necessary to have enhancement-type transistors. To do so, the impurity-doping method is investigated. First, a numerical analysis considering the localized states was carried out. It was shown that the characteristics control of a-Si FETs, such as single-crystal Si MOS FETs, can be achieved by impurity doping. Next, practical a-Si FETs were fabricated with phosphorus doping or boron-doping into the channels of transistors. Characteristics of these transistors were investigated and characteristics control was shown. Characteristics of the phosphorus-doped transistor as well as characteristics of boron-doped p-channel transistors showed the same properties as have been predicted by numerical analysis. By contrast, at the boron-doped transistors, characteristics of n-channel operation were deteriorated even at a small concentration of boron-doping. This is due probably to a large variation of the localized state density by boron-doping.

Since the technology to integrate amorphous-silicon thin-film transistors on a large area glass substrate is a key to realizing a flat panel display with high capacity, high resolution, and full color, it has been developed extensively in industry research organizations worldwide. Attempts have also been made to apply them for sensing devices and printing devices. The amorphous-silicon thin-film transistor is becoming central to man-machine interface devices, and the improvement of its performance has become important. This paper introduces these technological trends with emphasis on our own research results.