Three-Dimensional Real-Space Simulation of Surface Roughness in Silicon Nanowire FETs
ABSTRACT We address the transport properties of narrow gate-all-around silicon nanowires in the presence of surface-roughness (SR) scattering at the Si/SiO2 interface, considering nanowire transistors with a cross section of 3 times 3 nm2 and gate length of 15 nm. We present transfer characteristics and effective-mobility calculations based on a full 3-D real-space self-consistent Poisson-Schrodinger solver within the nonequilibrium Green's function formalism. The effect of SR is included via a geometrical method consisting in a random realization of potential fluctuations described via an exponential autocorrelation law. The influence on transfer characteristics and on low-field mobility is evaluated by comparison with the clean case and for different values of the root mean square of potential fluctuations. The method allows us to exactly account for mode-mixing and subband fluctuations and to evaluate the effect of SR up to all orders of the interaction. We find that SR scattering is mainly responsible for positive threshold-voltage shift in the low-field regime, whereas SR-limited mobility slowly depends on the linear charge density, showing the inefficiency of mode-mixing scattering mechanism for very narrow wires.
- SourceAvailable from: Mahdi Pourfath
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- "Nonequilibrium Green's function (NEGF) formalism is used for this purpose. NEGF method is widely employed to model novel FETs, such as graphene –, carbon nanotube , , and silicon nanowire ,  FETs. We study the barrier shapes broadly used in literature and the WKB approximation for a wide range of barrier thicknesses, barrier heights, and applied voltages. "
ABSTRACT: A comprehensive study is conducted on the electron transport in conductive polymer matrix composites (CPMCs), employing the nonequilibrium Green’s function formalism. This paper provides a microscopic insight into the electron tunneling through the potential barriers existing between conducting sites. It is shown that Wentzel–Kramers–Brillouin approximation as well as other models with simple barrier shapes, which are widely used in literature, can lead to inaccurate results in comparison with the quantum mechanical approach using a hyperbolic barrier. In this paper, unlike most previous ones, percolation-related effects are disregarded for further focus on electron transport through the polymer potential barriers. It is assumed that a tunneling-conductive channel exists between the electrodes. This can be created either by applying electric field alignment or using a filler volume fraction higher than the percolation threshold. A two electrode resistive device is studied and the results indicate that a conductor–insulator transition occurs at a barrier thickness of $sim 1.7$ nm and the barrier thickness should be larger than several angstroms. Next, a novel tunneling field-effect structure based on CPMCs is introduced and its characteristics are comprehensively investigated. This device features a remarkably simple structure, an extremely high channel to gate coupling, a large transconductance, and a high current level. Besides, it has the advantage of being based on polymers. This ensures favorable physical properties, ease of fabrication, and low-cost processing techniques.IEEE Transactions on Electron Devices 05/2015; 62(5):1584-1589. DOI:10.1109/TED.2015.2411992 · 2.36 Impact Factor
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- "As the ON-state characteristics of a transistor are also related to their effective mobility , , the electron mobility reduction due to IRS at low drain bias should be understood in relation with the ON-current reduction at high drain bias. The low-field effective mobility in NWFETs is calculated from the electron density and the conductance using the expression ,  µ eff = G lin L ch qN ch (3) where G lin is the conductance in the linear region i.e. at low drain bias (V D = 5 mV in this work) and N ch is calculated by integrating the electron density in the subsection of the channel under the gate where the electron density is nearly uniform  as illustrated in Fig. 11(c). Assuming that the ballistic mobility is the effective mobility for a perfect NW, the IR-limited mobility µ IR can be calculated from the Matthiessen's rule, namely, µ −1 IR = µ −1 eff,IR −µ −1 ball where µ eff,IR is the effective mobility for rough NWs. "
ABSTRACT: The influence of interface roughness scattering (IRS) on the performances of silicon nanowire field-effect transistors (NWFETs) is numerically investigated using a full 3D quantum transport simulator based on the atomistic sp3d5s* tight-binding model. The interface between the silicon and the silicon dioxide layers is generated in a real-space atomistic representation using an experimentally derived autocovariance function (ACVF). The oxide layer is modeled in the virtual crystal approximation (VCA) using fictitious SiO2 atoms. <110>-oriented nanowires with different diameters and randomly generated surface configurations are studied. The experimentally observed ON-current and the threshold voltage is quantitatively captured by the simulation model. The mobility reduction due to IRS is studied through a qualitative comparison of the simulation results with the experimental results.
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ABSTRACT: In this paper, we detail the main numerical issues of the Monte Carlo method developed to solve the Wigner-Boltzmann transport equation and simulate the quantum transport in semiconductor nanodevices. In particular, we focus on the boundary conditions regarding the injection of particles and the limits of integration for the calculation of the Wigner potential which are of crucial importance for the physical correctness of simulation results. Through typical examples we show that this model is able to treat correctly purely quantum coherent and semi-classical transport situations as well. It is finally shown that to investigate devices operating in mixed quantum/semi-classical regimes and to analyze the transition between both regimes, this approach takes advantage of its full compatibility with Boltzmann algorithm. KeywordsWigner function-Quantum transport-Monte Carlo simulationJournal of Computational Electronics 12/2010; 9(3):224-231. DOI:10.1007/s10825-010-0319-6 · 1.37 Impact Factor