| Due to the low production cost and favourable electrical property, the applications of polysilicon thin film transistors (poly-Si TFTs) have been increasing in the active matrix liquid crystal display (AMLCD), three-dimensional (3D) integration, high-capacity memories, and so on. However, resulting from the existing of discrete grain boundaries and trap states in the polysilicon film, several physical effects are coupled together in poly-Si TFTs and the conduction mechanisms become complicated. As a result,it is difficult to correctly model poly-Si TFTs, and so far there is no industrial standard model. Thus it is important to propose a physics-based model for poly-Si TFTs, which can not only describe the electrical characteristics, but also be suitable for circuit simulation.According to the analysis of the conduction mechanisms of polysilicon and poly-Si TFTs, analytical and physics-based models for polysilicon and poly-Si TFTs have been developed in the present thesis,including the following issues:Firstly, based on the discontinuous energy structure and the grain-boundary dominating behavior, a conduction model for polysilicon has been proposed. Three primary conduction mechanisms, such as diffusion-drift, thermionic-emission and tunneling, have been invoked to depict the carrier characteristics of polysilicon. The effects of three mechanisms can be adjusted according to the temperature, doping level, density of the trap states, density of free carriers and the addition potential barriers. Thus, the proposed model can be interpret the conduction characteristics of polysilicon thin films over a wide range of doping levels with temperature. The expression of the conduction model is simple enough to be introduced into the model of poly-Si TFTs.Secondly, based on the proposed conduction model of polysilicon, Gaussian energy distribution model in the grain boundary and discrete grain boundaries in the channel, an effective mobility expression for poly-Si TFTs has been proposed by considering several important factors for mobility, such as the surface scattering effect, gate modulating effect, drain-induced-grain boundary barriers lowering effect and drain-caused grain-boundary-barrier asymmetrically distributing effect. Therefore, the present model can analytically and accurately describe the effective mobility of poly-Si TFTs. The calculated results are compared with the available experimental data and a good agreement is obtained.Thirdly, the threshold-voltage-based model must use unphysical smooth function, while the surface-potential-based model is not capable to consider the particular effects of grain boundaries. To solve the limitations of the two models, a drain direct current model for poly-Si TFTs has been presented by using the surface-potential-based approach and dividing the channel into several small TFTs according to the number of the grain boundaries. In the developed model, the discrete distribution of grain boundaries has been taken into account, the drain currents of the subthreshold, linear and saturated regimes can be analytically explained by a single expression, and the validity is verified by the experimental data.Fourthly, based on the model approach of double-gate (DG) MOSFET, a drain current model for DG poly-Si TFTs has been developed, which includes the discrete grain boundaries in the channel and the special conduction characteristics of DG-poly-Si TFTs. The drain current of DG poly-Si TFTs for different grain sizes, density of the trap states and carrier density can be described by the proposed model.In conclusion, the models for polysilicon, single-gate and DG poly-Si TFTs are developed by basing on the physics property of polysilicon, considering the discrete distribution of grain boundaries, using the surface-potential approaches. Therefore, they can interpret the electrical characteristics of polysilicon and poly-Si TFTs, and be suitable for circuit simulators, which have been fully verified by the experimental data. |
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