1,720,992 research outputs found
Impact of the gate and external insulator thickness on the static characteristics of ultra-scaled silicon nanowire FETs
No full textPhysically based diagonal treatment of the self-energy of polar optical phonons. Performance assessment of III-V double-gate transistors
No full textWe propose a diagonal approximation for the self-energy that describes the interaction between electrons and polar optical phonons in the framework of nonequilibrium Green's function transport simulations. Our model is based on the definition of a scaling factor, which renormalizes the local electron-phonon coupling, to take into account the nonlocality of the interaction and provide the correct scattering rates. While previous studies relied on empirical values of this factor, we derive, from basic physical relationships, analytical expressions in the presence of the one- and two-dimensional confinement of phonons. We apply our model to the self-consistent simulation of double-gate p-type transistors made of technologically relevant III-V materials (InAs, InSb, and GaSb). Their performance is benchmarked, for different crystallographic orientations and strain constraints, against the corresponding Si and Ge devices. We find that the electron-polar optical phonon scattering plays a major role in degrading the performance of the III-V devices and typically results in a widening of the performance gap existing between III-V and Si or Ge devices in ballistic transport condition
Anharmonic Phonon-Phonon Scattering Modeling of Three-Dimensional Atomistic Transport: An Efficient Quantum Treatment
No full textQuantum treatment of inelastic interactions for the modeling of nanowire field-effect transistors
Full text linkDuring the last decades, the Nonequilibrium Green's function (NEGF) formalism has been proposed to develop nano-scaled device-simulation tools since it is especially convenient to deal with open device systems on a quantum-mechanical base and allows the treatment of inelastic scattering. In particular, it is able to account for inelastic effects on the electronic and thermal current, originating from the interactions of electron-phonon and phonon-phonon, respectively. However, the treatment of inelastic mechanisms within the NEGF framework usually relies on a numerically expensive scheme, implementing the self-consistent Born approximation (SCBA). In this article, we review an alternative approach, the so-called Lowest Order Approximation (LOA), which is realized by a rescaling technique and coupled with Padé approximants, to efficiently model inelastic scattering in nanostructures. Its main advantage is to provide a numerically efficient and physically meaningful quantum treatment of scattering processes. This approach is successfully applied to the three-dimensional (3D) atomistic quantum transport OMEN code to study the impact of electron-phonon and anharmonic phonon-phonon scattering in nanowire field-effect transistors. A reduction of the computational time by about×6 for the electronic current and×2 for the thermal current calculation is obtained. We also review the possibility to apply the first-order Richardson extrapolation to the Padé N/N-1 sequence in order to accelerate the convergence of divergent LOA series. More in general, the reviewed approach shows the potentiality to significantly and systematically lighten the computational burden associated to the atomistic quantum simulations of dissipative transport in realistic 3D systems
Anharmonic phonon-phonon scattering modeling of three-dimensional atomistic transport. An efficient quantum treatment
No full textWe propose an efficient method to quantum mechanically treat anharmonic interactions in the atomistic nonequilibrium Green's function simulation of phonon transport. We demonstrate that the so-called lowest-order approximation, implemented through a rescaling technique and analytically continued by means of the Padé approximants, can be used to accurately model third-order anharmonic effects. Although the paper focuses on a specific self-energy, the method is applicable to a very wide class of physical interactions. We apply this approach to the simulation of anharmonic phonon transport in realistic Si and Ge nanowires with uniform or discontinuous cross sections. The effect of increasing the temperature above 300 K is also investigated. In all the considered cases, we are able to obtain a good agreement with the routinely adopted self-consistent Born approximation, at a remarkably lower computational cost. In the more complicated case of high temperatures (300 K), we find that the first-order Richardson extrapolation applied to the sequence of the Padé approximants N-1/N results in a significant acceleration of the convergenc
Quantum treatment of phonon scattering for modeling of three-dimensional atomistic transport
No full textBased on the nonequilibrium Green's function formalism, we show a numerically efficient method to treat inelastic scattering in multidimensional atomistic codes. Using a simple rescaling approach, we detail the calculations of the lowest-order approximation (LOA) [Y. Lee et al., Phys. Rev. B 93, 205411 (2016)2469-995010.1103/PhysRevB.93.205411] series to the usual, computationally intensive, self-consistent Born approximation (SCBA). This, combined with the analytic continuation technique of Padé approximants, is applied to an atomistic code based on a tight-binding sp3d5s∗ model for electrons and holes, and a modified valence-force-field method for phonons. Currents in Si and Ge gate-all-around nanowire transistors are then computed considering the main crystallographic transport directions ((100), (110), (111)) for both n-type and p-type devices. Our results show that in most configurations, third-order LOA currents are enough to achieve a high agreement with SCBA results, while reducing the calculation time by about one order. In addition, we propose a criterion to determine the validity of such expansion technique
Dual-gated WTe2/MoSe2 van der Waals tandem solar cells
No full textWe propose and numerically investigate, through a multiscale approach, a tandem solar cell based on a van der Waals heterostructure comprising of two monolayers of transition-metal dichalcogenides. The electronic connection between the two subcells is obtained via tunneling through the van der Waals heterojunction, which is electrostatically controlled by means of a dual gate. Furthermore, by adjusting the dual-gate voltages, the photocurrents in the two subcells can be matched and the tandem cell performances can be optimized. Assuming an optimal absorptance, as expected in light-trapping systems, we predict that a power conversion efficiency of 30.7%, largely exceeding that of the single subcells, can be achieved. The proposed design being suitable for other van der Waals heterojunctions shows that it represents a viable option for future high-efficiency photovoltaic system
Physically based diagonal treatment of polar optical phonons in III-V p-type double-gate transistors: comparison of InAs vs Ge and Si
No full textImpact of the gate and insulator geometrical model on the static performance and variability of ultrascaled silicon nanowire FETs
No full textWe investigate the effect of the geometrical model adopted for the gate electrode and for the insulator enveloping the access regions on the full-quantum simulation of ultrascaled nanowire FETs (NW-FETs). We compare the results obtained in the 'minimal' geometry commonly used in simulations with those obtained in a more realistic one, able to fully account for the gate fringing effects. We evaluate the impact of the model geometry on the static performance of NW-FETs and discuss the interplay with the surface roughness and the random distribution of dopants. We find that the I-scriptscriptstyle ON I-scriptscriptstyle OFF ratio evaluated in the minimal geometry can be remarkably underestimated in short devices, notably in the case of small length-To-width ratio. The roughness-induced current degradation and the sensitivity to the surface roughness variability can also suffer from nonnegligible underestimations when evaluated in this geometry. Finally, we point out that an inaccurate description of the device electrostatics is expected to result in an overestimation of the sensitivity to the doping-induced variabilit
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