Numerical simulation of thin film transistors

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Numerical simulation of thin film transistors


Author: Fung, Lai-wah
Title: Numerical simulation of thin film transistors
Degree: M.Sc.
Year: 1998
Subject: Thin film transistors -- Simulation methods
Thin film transistors -- Mathematical models
Hong Kong Polytechnic University -- Dissertations
Department: Multi-disciplinary Studies
Dept. of Electronic Engineering
Pages: vi, 90 leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: Thin-film transistors (TFTs) have emerged as key elements for large scale matrix displays for both Liquid Crystal Displays and Electroluminience Displays. Amorphous TFTs have been developed first, and the technology is now mature enough to be implemented in most commercial applications. But polysilicon TFTs are also receiving great attention because of their high current-drive-capability. It is necessary to have detailed understanding of the TFT current characteristics in order to evaluate its application in display systems, and develop a device model for design of TFT circuits. The static transistor characteristics are determined by the localized electronic states that occur in the bandgap. The localized states are divided into deep and tail states. The deep states, mostly consisting of Si dangling bonds, determine the threshold voltage and the tail states determine the field effect mobility. This dissertation is on the development of a two-dimensional numerical simulation program for TFTs. Both amorphous silicon and polysilicon thin-film transistors can be modelled whereby the defects and grain boundaries in the material are treated as a spatially uniform density of localized states in the bandgap. The fundamental semiconductor equations such as Poisson's and Current Continuity Equation are solved by numerical methods based on Stone's strongly implicit method. The program is used to derive the electric charge, potential and the drain current at various drain voltages and gate voltages. In the simulation of TFTs, it is usually desirable to obtain accurate results at low calculation costs. However, it is difficult to be satisfied since it needs a large number of grid points for the numerical solution of governing equations. Since the adaptive grid generation method is known to be effective in dealing with a trade-off problem of this kind, in this dissertation, an adaptive grid generation algorithm had been introduced. Finally, the validity of the simulation program is shown by the comparison of the two-dimensional numerical simulation results of TFTs with experimental data reported in other papers.

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