Marco Saraniti

Hybrid fullband cellular automaton/Monte Carlo approach for fast simulation of charge transport in semiconductors

We present a fullband cellular automaton (CA) code for simulation of electron and hole transport in Si and GaAs. In this implementation, the entire Brillouin zone is discretized using a nonuniform mesh in k-space, and a transition table is generated between all initial and final states on the mesh, greatly simplifying the final state selection of the conventional Monte Carlo algorithm. This method allows for fully anisotropic scattering rates within the fullband scheme, at the cost of increased memory requirements for the transition table itself. Good agreement is obtained between the CA model and previously reported results for the velocity-field characteristics and high field distribution function, which illustrate the potential accuracy of the technique. A hybrid CA/Monte Carlo algorithm is introduced which helps alleviate the memory problems of the CA method while preserving the speed up and accuracy.

The Upper Limit of the Cutoff Frequency in Ultrashort Gate-Length InGaAs/InAlAs HEMTs: A New Definition of Effective Gate Length

Ultrashort gate-length pseudomorphic high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. The RF response and the cutoff frequency fT have been obtained for physical gate lengths ranging from 10 to 50 nm. These results, in turn, have been used in a transit-time analysis to determine the effective gate length in each case. By interpolation, one can make an estimate of the absolute upper limit for fT, which we find to be 2.9 THz in the device studied. Importantly, the effective gate lengths are considerably shorter than the depletion lengths. Thus, in general, any estimate of fT based on the latter quantity is likely too small by a quite significant amount.

An efficient multigrid Poisson solver for device simulations

The aim of this paper is to show that the multigrid approach can provide an efficient two-dimensional Poisson solver used in the analysis of realistic semiconductor devices based on particle simulators. Our robust implementation of the multigrid method is faster by one or two orders of magnitude than standard successive over-relaxation solvers and is capable, at the same time, of efficiently handling highly inhomogeneous grids and irregular boundary conditions relevant for realistic devices. All essential parts of the algorithm, such as coarsening, prolongation, restriction, and relaxation, have been adapted and optimized to deal with these complex geometries and large variations in the charge density. In particular, a new variant of the Gauss-Seidel-type relaxation scheme is introduced that is particularly suited for grids that lack globally dominant directions. As an example, the multigrid Poisson solver has been applied to two different electronic devices, a GaAs High Electron Mobility Transistor and a Si Metal Oxide Semiconductor Field Effect Transistor.

Field effect modulation of ionic conductance of cylindrical silicon-on-insulator nanopore array

Results demonstrating the field effect modulation of ionic transport through an array of cylindrical nanopores fabricated in silicon-on-insulator substrates are presented. Pronounced modulation of the conductance is observed at low electrolyte concentrations when the electric double layers within the nanopores are overlapping. A numerical model based on Brownian dynamics reproduces the measured data.

Influence of the electron–phonon interaction on electron transport in wurtzite GaN

A full-band electron transport calculation in wurtzite phase GaN based on an accurate model of electron–phonon interactions using a rigid pseudo-ion model is reported. The calculated deformation potentials are used to estimate the scattering rate in a cellular Monte Carlo (CMC) simulator. Longitudinal optical (LO)-like and transverse optical (TO)-like polar optical phonon scatterings are also employed in this work. The calculated velocity is lower than the previous works which used fixed deformation potentials.

Teflon™-coated silicon apertures for supported lipid bilayer membranes

We present a method for microfabricating apertures in a silicon substrate using well-known cleanroom technologies resulting in highly reproducible giga-seal resistance bilayer formations. Using a plasma etcher, 150μm apertures have been etched through a silicon wafer. Teflon™ has been chemically vapor deposited so that the surface resembles bulk Teflon and is hydrophobic. After fabrication, reproducible high resistance bilayers were formed and characteristic measurements of a self-inserted single OmpF porin ion channel protein were made.

Aspect Ratio Impact on RF and DC Performance of State-of-the-Art Short-Channel GaN and InGaAs HEMTs

We report a comparison between state-of-the-art GaN and InGaAs HEMTs in terms of the minimum aspect ratio required to limit short-channel effects. DC and RF simulations were carried out through our full-band cellular Monte Carlo simulator, which includes the full details of the band structure and the phonon spectra. Our results indicate that the minimum aspect ratio for GaN devices is 15 for negligible short-channel effects and 10 for reduced short-channel effects. On the other hand, InGaAs devices perform well for lower aspect ratio values such as 7.5 and 4-5 for negligible and reduced effects, respectively. The origin of this difference between GaN and InGaAs HEMTs is believed to be related to the different dielectric constants of the two materials and the corresponding difference in the electric field distributions related to short-channel effects.

Simulation of Ultrasubmicrometer-Gate

Pseudomorphic delta-doped ultrasubmicrometer-gate high-electron mobility transistors have been modeled using a full-band cellular Monte Carlo simulator. Reasonable agreement between experimental and numerical results is obtained for a 70-nm gate length. We discuss the scaling of this device to shorter gate lengths and the role played by various dimensions in the structure. Devices with 20-nm gate lengths should produce fTs above 1.5 THz without difficulty. This paper demonstrates the power of particle-based simulation tools in capturing the relevant physics responsible for device operation and key to performance optimization.

\hbox{In}_{0.52} \hbox{Al}_{0.48}\hbox{As/In}_{0.75}\hbox{Ga}_{0.25}\hbox{As/In}_{0.52}\hbox{Al}_{0.48}\hbox{As/InP}

A 50 nm nMOSFET has been studied by Ensemble Monte Carlo (EMC) simulation including a novel physical model for the treatment of surface roughness and impurity scattering in the Si inversion layer. In this model, we use a bulk-like phonon and impurity scattering model and surface-roughness scattering in the silicon inversion layer, coupled with the effective/smoothed potential approach to account for space quantization effects. This approach does not require a self-consistent solution of the Schrodinger equation. A thorough account of how these scattering mechanisms affect the transport transient response and steady-state regime in a 50 nm gate-length nMOSFET is given in this paper. A set of I/sub ds/-V/sub ds/ curves for the transistor is shown. We find that the smoothing of the potential to account for quantum effects has a strong impact on the electron transport properties, both in transient and steady-state regimes. We also show results for the impact that impurity and surface-roughness scattering mechanisms have on the average velocity of the carriers in the channel and the current flowing through the device. It was found that time-scales as short as 0.1-0.2 ps are enough to reach a steady-state channel electron average velocity.

Pseudomorphic HEMTs Using a Full-Band Monte Carlo Simulator

The authors present a novel hydrodynamic approach to the study of DC and AC hot-carrier transport in semiconductors. To this end use is made of a total-energy scheme which incorporates simultaneously the kinetic and potential energy associated with different conduction band minima. Furthermore, convective and diffusive contributions are considered by including the variance of velocity-velocity and velocity-energy fluctuations. Together with static characteristics, a small-signal analysis under spatially homogeneous conditions of the most important response functions of the electron system (e.g. differential mobility, diffusivity, velocity-noise spectrum, etc) is developed consistently. The validation of the present approach is supported by an excellent comparison with Monte Carlo results carried out for the case of n-Si at 300 K.