Polycrystalline Silicon TFTs for Large Area Electronic Application
This work focuses on low and high voltage Thin Film Transistors (TFTs) for large area flat panel displays and the related physical phenomena. All TFT samples in this work were fabricated using a low temperature, glass compatible poly-Si technology. Extensive numerical simulation and experimental work has been carried out on different high voltage TFT structures to investigate performance related problems, for example the ’current crowding’ effect under the on-state condition and optimisation of blocking capability, transient switching characteristics. Significant improvements in the device performances have been demonstrated by using novel device structures. In addition, the search for a higher blocking capability is a constant aim throughout the work. To ensure that the numerical analysis has a sound base, extensive studies have been carried out on low voltage TFT’s. The device phenomena of low voltage poly-Si TFTs have been analysed using experimental measurement and numerical simulations. In particular, a representative poly-Si density of states (DOS) has been successfully extracted using a 2-D simulation instead of using the conventional experimental methods. Based on our numerical simulation model, a variety of novel high voltage structures have been proposed and carefully examined in this work. The mechanisms that govern the device behaviour have been investigated. Transient characteristics, impact of hydrogenation, the floating body effect and the structural sensitive hydrogenation effect have been investigated. High voltage TFTs And their primary applications in large area electronics and flat panel displays in particular. A steady demand for a wider viewing angle and a faster response from the Active Matrix-addressed Liquid Crystal Display (AMLCD) demands higher voltage operation of the pixel switches. As a novel aspect in this work, the transient characteristic of the SI (semi-insulating held plate) high voltage poly-Si TFT has been experimentally examined. The resistivity of the held plate affects the transient response. In addition, the performances of three novel high voltage poly-Si TFT’s have been presented. They are (1) conductivity modulated SI HVTFT, (2) conductivity modulated SI HVTFT with buffer and (3) an HVTFT with variable doping slots in the offset region. Using these structures, significant improvement in the device performances has been realised.
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