Bandstructure Effects in Silicon Nanowire Electron Transport
Date of this Version
6-15-2008Acknowledgements
The computational resources for this work were provided thrugh nanoHUB.org by the Network for Computational Nanotechnology (NCN). The authors would like to acknowledge Prof. Mark Schilfgaarde of Arizona State University for ab initio GW calculations. Dr. Tony Low of Purdue University for pseudopotential calculations for benchmarking of bandstructure results, and Prof. Timothy Boykin of the University of Alabama at Huntsville for tight-binding discussions.
Abstract
Bandstructure effects in the electronic transport of strongly quantized silicon nanowire field-effect-transistors (FET) in various transport orientations are examined. A 10-band sp3d5s∗ semiempirical atomistic tight-binding model coupled to a self-consistent Poisson solver is used for the dispersion calculation. A semi-classical, ballistic FET model is used to evaluate the current-voltage characteristics. It is found that the total gate capacitance is degraded from the oxide capacitance value by 30% for wires in all the considered transport orientations ([100], [110], [111]). Different wire directions primarily influence the carrier velocities, which mainly determine the relative performance differences, while the total charge difference is weakly affected. The velocities depend on the effective mass and degeneracy of the dispersions. The [110] and secondly the [100] oriented 3 nm thick nanowires examined, indicate the best ON-current performance compared to [111] wires. The dispersion features are strong functions of quantization. Effects such as valley splitting can lift the degeneracies particularly for wires with cross section sides below 3 nm. The effective masses also change significantly with quantization, and change differently for different transport orientations. For the cases of [100] and [111] wires the masses increase with quantization, however, in the [110] case, the mass decreases. The mass variations can be explained from the non-parabolicities and anisotropies that reside in the first Brillouin zone of silicon.
Keywords
Anisotropy, bandstructure, effective mass, injection velocity, MOSFETs, nanowire, nonparabolicity, quantum capacitance, tight binding, transistors
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