Simulation study of deep sub-micron and nanoscale semiconductor transistors

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Xia, Tongsheng

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In accordance with Moore’s law, MOSFETs have been rapidly scaled down for the past thirty years to improve both the cost and performance of integrated circuits. Based on the 2003 International Roadmap for Semiconductors, MOSFETs will shrink to LG = 7nm by 2018. It will require the simulation tools to include quantum effects to study devices in this region. It has shown that the non-equilibrium green’s function (NEGF) formalism gives a sound base for this. The work is aimed at studying the properties of ultra-scaled transistors by the NEGF quantum transport formalism. We know that when it is far away from the band edge, there can be large differences between results obtained with the full band structure and those based on the effective mass model. However, not enough attention has been paid to the band characteristics where the imaginary part of the complex band structure appears to be a loop, which means the carriers can see a more transparent barrier and there can be vii more tunneling current in the off state for nano-scale transistors. We have incorporated the complex band structure into the NEGF formalism, and then studied its effect on the off state current for ultra-scaled semiconductor transistors. The silicon double gate MOSFET and carbon nanotube transistors, which have been proposed to improve the performance of transistors in the nanoscale region, are studied in detail. We also show other interesting results, such as pulling up the energy levels into the channel region from the valence band in the conventional deep sub-threshold region, transistor switching via the band alignment, and transmission of the band gap states in Schottky barrier carbon nanotube transistors. We then introduced the dephasing processes, using the phenomenological Büttiker probe method.