F. Venturi

A many-band silicon model for hot-electron transport at high energies

Abstract A new silicon model for electron transport at high electric fields is presented. The model features an original conduction-band structure consisting of three isotropic bands together with the lowest non-parabolic band in a finite spherical Brillouin zone. The bands are given by analytic expressions whose parameters are fixed by best fitting the density of states taken from band-structure calculations. Such a model is consistently used in electron dynamics and in the evaluation of the scattering probabilities. The coupling constants to the scattering agents are determined by best fitting the available experimental data on transport properties. The effect of the new model on the results is discussed for a bulk system with particular attention to the features (e.g. the detailed shape of the electron distribution function) which are important for device applications.

A general purpose device simulator coupling Poisson and Monte Carlo transport with applications to deep submicron MOSFETs

An efficient self-consistent device simulator coupling Poisson equation and Monte Carlo transport suitable for general silicon devices, including those with regions of high doping/carrier densities, is discussed. Key features include an original iteration scheme and an almost complete vectorization of the program. The simulator has been used to characterize nonequilibrium effects in deep submicron nMOSFETs. Substantial overshoot effects are noticeable at gate lengths of 0.25 mu m at room temperatures. >

Simple and efficient modeling of EPROM writing

A simple and efficient model for first-order simulation of the writing of n-channel erasable programmable ROM (EPROM) cells is presented. It allows the current injected into the gate insulator of the cell transistor to be calculated, accounting (at first order) both for the nonMaxwellian form of the electron energy distribution and for the nonlocal nature of carrier heating. The model is implemented as a postprocessor of a two-dimensional device simulator, and it is validated by means of a comparison with experimental data obtained with devices with effective channel lengths ranging from 1.4 to 0.5 mu m. >

MOS/sup 2/: an efficient MOnte Carlo Simulator for MOS devices

An efficient Monte Carlo device simulator has been developed as a postprocessor of a two-dimensional numerical analyzer based on the drift-diffusion model. The Monte Carlo package analyzes real VLSI MOSFETs in a minicomputer environment, overcoming some existing theoretical and practical problems. In particular, the particle free-flight time distribution is obtained by a new algorithm, leading to a CPU time saving of at least one order of magnitude compared with the traditional approach. To describe rare electron configurations, such as the high-energy tails of the distributions and the particle dynamics in the presence of large retarding fields, a multiple repetition scheme was implemented. Selected applications are presented to illustrate the simulators capabilities. >

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.

The impact of voltage scaling on electron heating and device performance of submicrometer MOSFETs

A study is presented on the effects of voltage scaling on hot-electron phenomena and intrinsic device performance in submicrometer MOSFETs. A Monte Carlo device simulator featuring a suitable band model for high-energy electrons is used. An interesting finding is that at very short channel lengths the high energy tail of the electron distribution function, the most important quantity in determining hot-carrier reliability, is controlled by the applied bias and not by local electric fields. As confirmed by recently reported experimental work, the results of this study indicate that the conventional, linear voltage scaling can be weakened using a more relaxed voltage reduction law that leads to improved performance without threatening device reliability. >

Monte Carlo simulations of high energy electrons and holes in Si-n-MOSFET's

Monte Carlo simulations of high-energy electrons and holes in Si n-MOSFETs are presented. Key features of this work include the use of a suitable silicon model for carrier transport at high electric fields, an original impact ionization model, and sophisticated numerical techniques to speed up the calculation. The case of submicrometer Si n-MOSFETs is considered as a relevant application. >

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. >

A multiband Monte Carlo approach to Coulomb interaction for device analysis

This paper presents an extension of the theoretical approach for both the short‐ and long‐range components of the Coulomb interaction among carriers in semiconductors to the case of an arbitrary isotropic multiband model, devised for Monte Carlo simulation of silicon devices. The analytical and numerical aspects of the model are discussed in detail. Results for the effect of the Coulomb interaction on the carrier distribution function and on the energy‐loss properties of the carrier gas are presented for the case of electrons in homogeneous and inhomogeneous silicon structures.

Assessment of quantum yield experiments via full band Monte Carlo simulations

In this paper we present an in-depth analysis of Quantum Yield (QY) data by means of Full Band Monte Carlo (FBMC) simulation including data from stressed oxides. The effect of device structure and initial energy distribution on QY efficiency is explored and the consequences of oxide stress on QY data are analyzed. In particular, we show that: (a) there is universal shape for QY curves in fresh oxides independent of oxide thickness and substrate doping; (b) QY data can be used to gain important insight into possible Stress Induced Leakage Current (SILC) mechanisms and discriminate between different SILC models.