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Simulation of Interface States in MOSFETs
Student: Wee Chang Khor
Supervisor: Y Yeow
Category: Engineering Thesis Project - Electrical
Metal-Oxide-Semiconductor Field Effect transistor (MOSFET) has become the dominant device for modern VLSI and ULSI systems. The performance and reliability of these MOS devices are influenced by the quality and properties of the interface between the oxide and the silicon region directly beneath it. One of the most important properties is the amount of charges that exist in the Si-SiO2 interface.
There are generally four types of charges associated with the Si-SiO2 interface, they are fixed oxide charge, mobile oxide charge, oxide trapped charge and interface states. Interface states, whether inherent, process-related, or operationally generated was found to cause degradation in device parameters such as transconductance, carrier mobility and threshold voltage and generally reduce device reliability and lifetime.
Better fabrication techniques developed over the years, had managed to reduce the amount of interface traps. However, with the increasing integration density of Integrated circuits and reduction of dimensions of the device, even a small amount of traps can have adverse effects on the device performance.
The objective of this thesis is to investigate the effects of the interface states on the various electrical characteristics of MOSFETs using the 2D device simulation software, ATLAS from SILVACO.
Simulation results had shown that increase in the amount of acceptor type interface states in N-type MOSFETs will result in an increase in threshold voltage and a decrease in transconductance. While an increase in the amount of donor type interface states in N-type MOSFETs will have no effects on the threshold voltage and transconductance For P-type MOSFETs, increase in the amount of donor type interface states will cause a decrease in threshold voltage and transconductance. While an increase in the amount of acceptor type interface states in N-type MOSFETs will have no effects on the threshold voltage and transconductance.
The objective of this thesis has been met. The effects of the interface states on the various electrical characteristics of MOSFETs using ATLAS were presented. The simulated results were found to be consistent with interface traps theory proven by researchers.