A model capturing the effect of general strain on the electron effective masses and band-edge energies of the lowest conduction band of silicon is developed. Analytical expressions for the effective ...mass change induced by shear strain and valley shifts/splittings are derived using a degenerate kldrp theory at the zone-boundary X point. Good agreement to numerical band- structure calculations using the nonlocal empirical pseudopotential method with spin-orbit interactions is observed. The model is validated by calculating the bulk electron mobility under general strain with a Monte Carlo technique using the full-band structure and the proposed analytical model for the band structure. Finally, the impact of strain on the inversion-layer mobility of electrons is discussed.
Due to the rapid decrease in device dimensions the well-established TCAD tools are pushed to the limits of their applicability. Since conventional MOSFETs are already operating in the sub-100
nm ...range, new physical effects and principles begin to determine the transport characteristics, and the validity of conventional current transport models is in question. The drift-diffusion model, which has enjoyed a remarkable success due to its relative simplicity, numerical robustness, and the ability to perform two- and three-dimensional simulations on large unstructured meshes, must be generalized to include hot-carrier and classical non-local effects. This motivated the development of higher order moments transport models such as the hydrodynamic, the energy-transport, and the six-moments models. After the introduction of stress for device performance enhancement the demand for accurate carrier mobility calculations based on full-band Monte Carlo algorithms has significantly increased, since they allow calibration of phenomenological mobility models and thus justify closure relations for higher order moments equations.
The transport models based on the semi-classical Boltzmann transport equation already contain information which can only be obtained from quantum-mechanical consideration. These are the band structure, expressions for the scattering rates, and the Pauli exclusion principle reflecting the Fermi statistics of carriers. With scaling continuing, other quantum-mechanical effects begin to affect transport properties. Quantum confinement in the direction orthogonal to transport in inversion layers makes the energy spectrum discrete. For sufficiently long channels, however, the carrier motion in transport direction can still be treated semi-classically, and development of transport models based on a set of subband Boltzmann equations is possible.
A useful approximation to mimic the quantum-mechanical carrier concentration profile is to introduce an effective potential into otherwise classical transport models. Transport calculations can then be carried out using conventional TCAD tools providing accurate and timely results. However, when modeling transport in ultra-scaled structures with only a few subbands occupied the full subband method must be applied.
Parallel to the search for new technological solutions for MOSFET scaling, the development of conceptually new devices and architectures is becoming increasingly important. New nanoelectronic structures, such as carbon nanotubes, nanowires, and even molecules, are considered to be prominent candidates for the post-CMOS era. At this small device size the geometrical spread of the carrier wave packet in transport direction can no longer be ignored. When the device size becomes shorter than the phase coherence length, the complete information about carrier dynamics inside the device including the phase of the wave function is needed and one has to resort to a full quantum-mechanical description including scattering. Transport in advanced nanodevices is determined by the interplay between coherent propagation and scattering. Numerical methods for dissipative quantum transport based on the non-equilibrium Green’s function formalism, the Liouville/von-Neumann equation for the density matrix, and the kinetic equation for the Wigner function are attaining relevance.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Low field mobility in double- and single-gate structures is analyzed for (1
0
0) and (1
1
0) SOI substrate orientation. A Monte Carlo algorithm for vanishing driving fields allows the calculation of ...the mobility for arbitrary scattering rates and band structure without further approximations. Due to volume inversion, mobility in double-gate ultra-thin body (1
1
0) SOI FETs is enhanced in comparison with the mobility of single-gate structures in the whole effective field range. In double-gate (1
0
0) structures the mobility decreases below the single-gate value for high effective fields. It is argued that the twice as high carrier concentration in double-gate FETs causes significant occupation of higher subbands, where mobility is low, and that additional inter-subband scattering channels for the lowest subband are opened. These effects partly compensate the mobility enhancement due to volume inversion and lead to a mobility decrease in double-gate (1
0
0) structures.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Strained-Si material has emerged as a strong contender for developing transistors for next-generation electronics, because this material system offers superior transport properties. We suggest a ...model describing the low-field bulk mobility tensor for electrons in strained-Si layers as a function of strain. Our analytical model includes the effect of strain-induced splitting of the conduction band valleys in Si, intervalley scattering, and doping dependence. Intervalley scattering has been modeled on the equilibrium electron distribution and the valley splitting for a given strain tensor. The effect of different substrate orientations is considered by performing coordinate transformations for the strain tensor and effective masses. Monte Carlo simulations accounting for various scattering mechanisms and the splitting of the anisotropic conduction band valleys due to strain in combination with an accurate ionized impurity scattering model were carried out to verify the results for the complete range of Ge contents and for a general orientation of the SiGe buffer layer. Our mobility model is suitable for implementation into a conventional technology CAD simulation tool.
The effect of uniaxial-strain, band-structure, mobility, effective masses, density of states, channel orientation and high-field transport on the drive current, off-state leakage and switching delay ...in nano-scale, Silicon (Si) and Germanium (Ge), p-MOS DGFETs is thoroughly and systematically investigated. To accurately model and capture all these complex effects, different simulation techniques, such as the Non-local Empirical Pseudopotential method (bandstructure), Full-Band Monte-Carlo Simulations (transport), 1-D Poisson-Schrodinger (electrostatics) and detailed Band-To-Band-Tunneling (BTBT) (including bandstructure and quantum effects) simulations, were used in this study.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The band structure of silicon (Si) under arbitrary stress/strain conditions is calculated using the empirical nonlocal pseudopotential method. The method is discussed with a special focus on the ...strain induced breaking of crystal symmetry. It is demonstrated that under general stress the relative movement of the sublattices has a prominent effect on the conduction band masses. This displacement, which cannot be determined from macroscopic strain, is extracted from ab initio calculations. The transport properties of strained Si are investigated by solving the semi-classical Boltzmann equation using the Monte Carlo (MC) method. It is shown that the change of the electron effective mass induced by uniaxial stress has to be included in accurate models of the electron mobility.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The effect of degeneracy both on the phononlimited mobility and the effective mobility including surface-roughness scattering in unstrained and biaxially tensile strained Si inversion layers is ...analyzed. We introduce a new method for the inclusion of the Pauli principle in a Monte Carlo algorithm. We show that incidentally degeneracy has a minor effect on the bulk effective mobility, despite non-degenerate statistics yields unphysical subband populations and an underestimation of the mean electron energy. The effective mobility of strained inversion layers slightly increases at high inversion layer concentrations when taking into account degenerate statistics.
The effect of degeneracy both on the phonon-limited mobility and the effective mobility including surface-roughness scattering in unstrained and biaxially tensile strained Si inversion layers is ...analyzed. We introduce a new method for the inclusion of the Pauli principle in a Monte Carlo algorithm. We show that incidentally degeneracy has a minor effect on the bulk effective mobility, despite non-degenerate statistics yields unphysical subband populations and an underestimation of the mean electron energy. The effective mobility of strained inversion layers slightly increases at high inversion layer concentrations when taking into account degenerate statistics.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Carbon nanotube field-effect transistors (CNTFETs) have been studied in recent years as a potential alternative to CMOS devices, because of the capability of ballistic transport. CNTFETs can be ...fabricated with Ohmic or Schottky type contacts. We focus here on Schottky barrier CNTFETs which operate by modulating the transmission coefficient of carriers through the Schottky barriers at the interface between the metal and the carbon nanotube (CNT). The behavior of these devices has been studied by solving the coupled Schrödinger–Poisson equation system. In agreement with experimental results, simulations indicate the ambipolar behavior of these devices. However, the ambipolar behavior limits the performance of these devices in both on and off regimes. To suppress this effect a double gate structure is proposed. Simulations demonstrate that this structure exhibits improved device characteristics.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
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