A. Abramo

Physically based modeling of low field electron mobility in ultrathin single- and double-gate SOI n-MOSFETs

In this paper, we have extensively investigated the silicon thickness dependence of the low field electron mobility in ultrathin silicon-on-insulator (UT-SOI) MOSFETs operated both in single- and in double-gate mode. A physically based model including all the scattering mechanisms that are known to be most relevant in bulk MOSFETs has been extended and applied to SOI structures. A systematic comparison with the measurements shows that the experimental mobility dependence on the silicon thickness (T/sub SI/) cannot be quantitatively explained within the transport picture that seems adequate for bulk transistors. In an attempt to improve the agreement with the experiments, we have critically rediscussed our model for the phonon scattering and developed a model for the scattering induced by the T/sub SI/ fluctuations. Our results suggest that the importance of the surface optical (SO) phonons could be significantly enhanced in UT-SOI MOSFETs with respect to bulk transistors. Furthermore, both the SO phonon and the T/sub SI/ fluctuation scattering are remarkably enhanced with reducing T/sub SI/, so that they could help explain the experimental mobility behavior.

Modeling of electron mobility degradation by remote Coulomb scattering in ultrathin oxide MOSFETs

This paper presents a comprehensive, numerical model for the remote Coulomb scattering (RCS) in ultrathin gate oxide MOSFETs due to ionized impurities in the polysilicon. Using a nonlocal screening approach, the model accounts for the static screening of the scattering centers produced both by electrons in the channel and in the polysilicon. Electron mobility is then calculated using a relaxation time approximation that consistently accounts for intersubband transitions and multisubband transport. Our results indicate that neglecting the screening in the polysilicon and making use of the Quantum Limit (QL) approximation can lead to a severe underestimate of the RCS limited electron mobility, thus hampering the accuracy of the predictions reported in some previous papers on this topic. Using our model, we discuss the oxide thickness dependence of the electron mobility in ultrathin gate oxide MOSFETs and the possible benefits in terms of RCS limited mobility leveraged by the use of high K dielectrics.

A comparison of numerical solutions of the Boltzmann transport equation for high-energy electron transport silicon

In this work we have undertaken a comparison of several previously reported computer codes which solve the semiclassical Boltzmann equation for electron transport in silicon. Most of the codes are based on the Monte Carlo particle technique, and have been used here to calculate a relatively simple set of transport characteristics, such as the average electron energy. The results have been contributed by researchers from Japan, Europe, and the United States, and the results were subsequently collected by an independent observer. Although the computed data vary widely, depending on the models and input parameters which are used, they provide for the first time a quantitative (though not comprehensive) comparison of Boltzmann Equation solutions. >

Two-dimensional quantum mechanical simulation of charge distribution in silicon MOSFETs

A solver for the two-dimensional (2-D) Schrodinger equation based on the k-space representation of the solution has been developed and applied to the simulation of 2-D electrostatic quantum effects in nano-scale MOS transistors. This paper presents the mathematical framework of the simulator, addresses the related accuracy and efficiency problems, and discusses the simulations performed to validate it. Furthermore, the 2-D quantum effects observed in the simulation of charge densities in tens-hundreds nanometer scale MOS structures are described.

Hot carrier effects in short MOSFETs at low applied voltages

In this paper a quantitative study of the electron energy distribution in silicon devices at low applied voltages is carried out by means of Monte Carlo simulations including the main mechanisms involved in the process of carrier heating. We present a clear-cut interpretation of the build up of the electron distribution at energies higher than that provided by the applied electric field (qV, V being the total voltage drop). Electron-electron interaction is analyzed and shown to be an effective process for the enhancement of the high-energy electron population.

Physical origin of the excess thermal noise in short channel MOSFETs

The physical origin of the excess thermal noise in short channel MOSFETs is explained based on numerical noise simulation. The impedance field representation and extraction method demonstrate that the drain current noise is dominated by source side contributions. Analysis identifies local ac channel resistance variations as the primary controlling factor. The nonlocal nature of velocity results in a smaller derivative of the velocity with respect to the field which in turn causes a higher local ac resistance near the source junction.

Study of low field electron transport in ultra-thin single and double-gate SOI MOSFETs

This paper studies the dependence on silicon film thickness (T/sub SI/) of the electron mobility in Single- (SG) and Double-Gate (DG) Ultra-Thin (UT) SOI MOSFETs. A comprehensive model was developed, including acoustic and optical phonon scattering and the scattering with possible interface states and microscopic roughness at both interfaces. The T/sub SI/ dependence of the effective mobility (/spl mu//sub eff/) predicted by simulations is, at moderate inversion densities (N/sub inv/), weaker than that observed in experiments. We analyze the physical origin of this discrepancy, with particular attention to the phonon limited mobility. Our results indicate that scattering with surface optical phonons is strongly enhanced in UT silicon layers and that it may help explain the experimental behavior of /spl mu//sub eff/.

A numerical method to compute isotropic band models from anisotropic semiconductor band structures

A numerical method for the determination of isotropic band models has been developed and applied to silicon. The resulting model accurately approximates both density of states and group velocity of the corresponding anisotropic band structure, thus providing an excellent agreement with both the collision and nonhomogeneous terms of the Boltzmann transport equation. The model, represented by a simple set of energy-wave vector tables, has been implemented in a Monte Carlo device simulator, but can also be extended to alternative methods for solving the Boltzmann equation. Simulation of homogeneous silicon shows a very good agreement with available experimental data. Comparison with results obtained using the complete anisotropic band structure, both in homogeneous and nonhomogeneous silicon devices, confirms the validity of the model. >

Electron energy distributions in silicon structures at low applied voltages and high electric fields

In this article a quantitative study of the electron energy distribution in silicon devices at low applied voltages is carried out by means of Monte Carlo simulations including the main mechanisms involved in the process of carrier heating. We present a clear‐cut interpretation of the build up of the electron distribution at energies higher than what is provided by the applied electric field. The influence of different boundary conditions on the simulation results is analyzed in detail. As a consequence, the hypothesis that the high energy tail simply represents the memory of the initial distribution at the injecting boundary due to ballistic transport is ruled out, even for highly inhomogeneous field profiles, such as in very short metal oxide semiconductor field effect transistors, for which a ballistic transport regime had been stated. In addition, the effect of short‐range electron‐electron interaction is examined and shown to be an effective process for the enhancement of the high‐energy electron population.

Analysis of quantum effects in nonuniformly doped MOS structures

This paper presents results from the self-consistent solution of Schrodinger and Poisson equations obtained in one-dimensional (1-D) nonuniformly doped MOS structures suitable for the fabrication of very short transistors. Different issues are considered and investigated, including quantum-induced threshold voltage shifts, low-field electron effective mobility and gate-to-channel capacitance. The reported results give indications for the optimization of n-MOS channel doping profiles suitable for the fabrication of ultrashort MOSFETs.