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Radio-Frequency Linearity of Carbon-Based Nanoscale Field-Effect Transistors Open Access


Other title
top-of-the-barrier model
carbon nanotube (CN)
small-signal circuit
field-effect transistor (FET)
contact resistance
third-order input-intercept point (IIP3)
tube pitch
high-frequency behavior
intermodulation distortion
density of states
band-to-band tunneling
nonlinear device modeling
Type of item
Degree grantor
University of Alberta
Author or creator
Alam, Ahsan U.
Supervisor and department
Vaidyanathan, Mani (Electrical and Computer Engineering)
Examining committee member and department
Pramanik, Sandipan (Electrical and Computer Engineering)
Wang, Xihua (Electrical and Computer Engineering)
DeCorby, Ray (Electrical and Computer Engineering)
Hossain, Masum (Electrical and Computer Engineering)
Department of Electrical and Computer Engineering
Solid State Electronics
Date accepted
Graduation date
Doctor of Philosophy
Degree level
This Ph.D. thesis focuses on the radio-frequency (RF) linearity of carbon-nanotube field-effect transistors (CNFETs) and graphene field-effect transistors (GFETs). The thesis can be categorized into three stages. In the first stage, the RF linearity potential of CNFETs has been investigated by considering an array-based device structure under the first approximation of ballistic transport. A nonlinear equivalent circuit for ballistic field-effect transistors is used to compare the linearity of CNFETs to conventional MOSFETs. It is shown that nanotube devices working at high frequencies are not inherently linear, as recently suggested in the literature, and that CNFETs exhibit overall linearity that is comparable to their MOSFET counterparts. The nonlinear quantum capacitance is identified to be a major source of high-frequency nonlinearity in CNFETs. The impact of device parameters such as oxide capacitance, channel width, and tube pitch are also investigated. In the second stage, a modified top-of-the-barrier model (MTBM) capable of simulating ballistic transport in GFETs is developed. The model captures band-to-band (Klein-Zener) tunneling, which is important in zero-bandgap materials, and it accounts for variations in the densities of states between the channel and the source and drain regions. The model is benchmarked against a sophisticated solver (based on the non-equilibrium Green’s function approach) and is shown to have very good quantitative agreement. The utility of the modified TBM is demonstrated by investigating and comparing the RF linearity of GFETs to that of CNFETs and conventional MOSFETs. In the third stage, the RF linearity potential of short-channel GFETs is assessed by using the modified top-of-the-barrier approach developed in stage 2, again under the first approximation of ballistic transport. An intrinsic GFET is examined to reveal the key features of GFET linearity, and extrinsic parasitics are then included to assess the overall RF linearity. It is shown that short-channel GFETs can be expected to have a signature behavior versus gate bias that includes a constant-linearity region at low gate bias, sweet spots of high linearity before and after the gate bias for peak unity-current-gain frequency, and poor linearity at the gate bias corresponding to the peak unity-current-gain frequency. It is otherwise found that a GFET offers overall linearity that is comparable to a conventional MOSFET and a CNFET, with the exception that the amount of intermodulation distortion in a GFET is dominated by the drain-injected carriers, a unique outcome of graphene’s lack of a bandgap.
Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.
Citation for previous publication
A. U. Alam, C. M. S. Rogers, N. Paydavosi, K. D. Holland, S. Ahmed, and M. Vaidyanathan, "RF linearity potential of carbon-nanotube transistors versus MOSFETs," IEEE Trans. Nanotechnol., vol. 12, no. 3, pp. 340-351, May 2013.A. U. Alam, K. D. Holland, S. Ahmed, D. Kienle, and M. Vaidyanathan, "A modified top-of-the-barrier model for graphene and its application to predict RF linearity," in International Conference on Simulation of Semiconductor Processes and Devices, 2013 (SISPAD), Sep. 2013, pp. 155-158.A. U. Alam, K. D. Holland, M. Wong, S. Ahmed, D. Kienle, P. Gudem, and M. Vaidyanathan, “RF linearity performance potential of short-channel graphene field-effect transistors,” Submitted to IEEE Trans. Microwave Theory Tech., Manuscript ID TMTT-2014-08-0894, 12 pages, Aug. 2014.

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