Massimo (Max) V. FischettiProfessor, Texas Instruments Distinguished Chair in Nanoelectronics
1978 PhD Physics, University of California, Santa Barbara
Electronic transport in ‘large’ semiconductor devices can be studied using simplifying assumptions afforded by the large size of the devices, mainly, the assumption of incoherent transport --, amenable to the semiclassical Boltzmann picture -- and of a ‘bulk-like’ electronic structure of the conductive channel, assumption which allows the use of either the effective-mass approximation or of a `full-band’ (but still bulk) description. Neither of these assumptions is likely to remain valid in the `10 nm era’: Novel materials, the small size of the devices, the strong confinement effects induced by such small length scales and the likelihood of quasi-coherent transport require two major changes in how we study electronic transport:
- The excitation spectrum of the systems (i.e., their band structure) must be recalculated for each system under investigation, since their finite size and quantum confinement effects will alter it dramatically from the bulk counterpart.
- Transport must be treated in ways which allow for incomplete decoherence, and thus transcend the validity of the semiclassical Boltzmann picture.
Our research proceeds on three fronts in order to tackle the problems described above: Band-structure calculations, semiclassical electronic transport, and quantum transport
- Band-structure calculations: We use of empirical pseudopotentials, together with supercell techniques, to study the electronic structure of several systems of technological interest: Thin semiconductor bodies (the active layers of many field-effect transistors (FETs), such as Ultra-Thin Body Silicon on Insulator (UTBSOIs), FinFETs, Double-Gate FETs), hetero-channels (as those employed in III-V MOSFETs and HEMTs), graphene, graphene nanoribbons, semiconductor quantum wires, and carbon nanotubes. While not as predictive and ‘transferable’ as first-principle ab-initio pseudopotentials, nevertheless it possible to use quasi-transferable local empirical pseudopotentials suitably calibrated to capture the main physical features of these interesting structures, while retaining a degree of simplicity and flexibility not afforded by more expensive ab-initio methods. These simplifications allow us to study effects caused by strain, disorder, and additional features of carrier scattering at an atomistic level.
- Semiclassical Transport: The calculations described above are tools which allow us to study low-field and high-field electronic transport in these nanometer-scale structures. The low-field mobility, calculated usually using a Kubo-Greenwood linearization of the Boltzmann equation, is a useful quantity to compare with experiments and calibrate the physics controlling scattering processes. In addition, it allows us to study interesting phenomena arising from the coupling of interfacial excitations (optical phonons and plasmons) with the low-dimensionality electron gas. High-field transport is studied using Monte Carlo techniques and allows to study a regime closer to the real operation of devices.
- Quantum Transport: Ballistic transport is studied using an open-boundary-conditions, self-consistent Schrödinger/Poisson system and a Pauli Master equation scheme is used to handle dissipative transport, accounting for electron-phonon , Coulomb, interface/edge roughness scattering.
By necessity, we are also interested on the numerical issues associated with the large size of the eigenvalue and linear-systems we must face for band-structure calculations of large systems, problems faced in this study will also be briefly discussed.
- M. V. Fischetti and S. E. Laux, “Monte Carlo Analysis of Electron Transport in Small Semiconductor Devices Including Band-Structure and Space-Charge Effects”, Phys. Rev. B 38, 9721-9745 (1988) .
- M. V. Fischetti, “Monte Carlo Simulation of Electron Transport in Technologically Significant Semiconductors of the Diamond and Zinc-blende Structures. Part I: Homogeneous Transport”, IEEE Trans. Electron Devices, ED-38, 634-649 (1991) .
- M. V. Fischetti and S. E. Laux, “Monte Carlo study of electron transport in Si inversion layers”, Phys. Rev. B 48, 2244-2274 (1993) .
- M. V. Fischetti and S. E. Laux, “Band structure, deformation potentials, and carrier mobility in strained Si, Ge, and SiGe alloys”, J. Appl. Phys. 80, 2234-2252 (1996) .
- M. V. Fischetti, “Master equation approach to the study of electronic transport in small semiconductor devices”, Phys. Rev. B 59, 4901-4917 (1999) .
- Massimo V. Fischetti, Deborah A. Neumayer, and Eduard A. Cartier, “Effective electron mobility in Si inversion layers in MOS systems with a high-κ insulator: The role of remote phonon scattering”, J. Appl. Phys. 90, 4587-4608 (2001) .
- S. Jin, M. V. Fischetti, and T.-w. Tang, “Modeling of electron mobility in gated silicon nanowires at room temperature: Surface roughness scattering, dielectric screening, and band nonparabolicity”, J. Appl. Phys. 102, 083715 (2007) .