3D Monte Carlo simulation of current trends and performance in scaled trigate MOSFET

Silicon nanowires with multiple-gates provide better source drain isolation and are thought to be the most promising candidate to replace bulk Si MOSFETs as we downscale deep into the nanometer regime. In this study we utilize a 3D full-band particle Monte Carlo (MOCA3D) simulator to investigate the performance and current trends of fully-depleted Trigate MOSFETs with different cross-sections. Results indicate that as we reduce the cross-section, the increased coupling between the top and lateral gates reduces the channel potential in different axes. In addition, device current normalized with respect to the Trigate perimeter alleviates as we scale the cross-section.     

Full band Monte Carlo simulation of dislocation effects on 250 nm AlGaN-GaN HEMT

In this work, we present a full band Monte Carlo simulation of the effects of dislocation scattering on the performance of a 0.25 μm AlGaN/GaN HEMT (high electron mobility transistor). We performed a full characterization of the device and validated the simulation results with experimental data (Lee et al. in IEEE Electron. Dev. Lett. 24:613–615, [2003]). Here we show a study of the DC device performance as a function of the density of thread dislocations.     

Use of density gradient quantum corrections in the simulation of statistical variability in MOSFETs

With the scaling of field-effect transistors to the nanometre scale, it is well recognised that TCAD simulations of such devices need to account for quantum mechanical confinement effects. The most widely used method to incorporate quantum effects within classical and semi-classical simulators is via density gradient quantum corrections. Here we present our methodologies for including the density gradient method within our Drift-Diffusion and Monte Carlo simulators and highlight some of the additional benefits that this provides when dealing with the charge associated with random discrete dopants.     

Electron–phonon dissipation in quantum nanodevices

Microscopic modelling of electronic-phase coherence versus energy dissipation plays a crucial role in the design and optimization of new-generation electronic quantum nanodevices, like quantum-cascade light sources and quantum logic gates; in this context, a variety of simulation strategies have been proposed and employed. The aim of this article is to discuss virtues versus intrinsic limitations of non-Markovian density-matrix approaches. More specifically, we shall show that the usual mean-field treatment employed to derive quantum-kinetic equations may lead to highly unphysical results, like negative distribution functions and non-dissipative carrier–optical phonon couplings. By means of a simple two-level model, we shall show that such limitations are expected to be particularly severe in zero-dimensional electronic systems—like quantum-dot nanostructures, potential constituents of quantum-computation devices—coupled to dispersionless phonon modes. Such a behaviour is in striking contrast with the case of Markovian treatments, where a proper combination of adiabatic limit and mean-field approximation guarantees a physically acceptable solution.     

Boundary conditions for Density Gradient corrections in 3D Monte Carlo simulations

Monte Carlo remains an effective simulations methodology for the study of MOSFET devices well into the decananometre regime as it captures non-equilibrium and quasi-ballistic transport. The inclusion of quantum corrections further extends the usefulness of this technique without adding significant computational cost. In this paper we examine the impact of boundary conditions at the Ohmic contacts when Density Gradient based quantum corrections are implemented in a 3D Monte Carlo simulator. We show that Neumann boundary conditions lead to more stable and physically correct simulation results compared to the traditional use of Dirichlet boundary conditions.     

Unified simulation of transport and luminescence in optoelectronic nanostructures

Computer simulation of microscopic transport and light emission in semiconductor nanostructures is often restricted to an isolated system of a single quantum well, wire or dot. In this work we report on the development of a simulator for devices with various kinds of nanostructures which exhibit quantization in different dimensionalities. Our approach is based upon the partition of the carrier densities within each quantization region into bound and unbound populations. A bound carrier is treated fully coherent in the directions of confinement, whereas it is assumed to be totally incoherent with a motion driven by classical drift and diffusion in the remaining directions. Coupling of the populations takes place through electrostatics and carrier capture. We illustrate the applicability of our approach with a well-wire structure.     

Fully 3D self-consistent quantum transport simulation of Double-gate and Tri-gate 10 nm FinFETs

We utilize a fully self-consistent 3D quantum mechanical simulator based on the Contact Block Reduction (CBR) method to investigate the effects of fin height and unintentional dopant on the device characteristics of a 10-nm FinFET device. The per-fin height off-current is found to be relatively insensitive to fin height while the corresponding per fin height on-current may significantly depend on fin height due to the stronger confinement with decreasing fin height. Also gate leakage is found to show similar behavior as device on-current with decreasing fin height. Tri-gate (TG) FinFET is found to show better performance compared to Double-gate (DG) FinFET, with the exception of gate leakage current. Simulation results show that an unintentional dopant within the channel can significantly alter device characteristics depending on its position and applied biases. In addition, the effects of unintentional dopant are found to be stronger at high drain bias than at low drain bias.     

A new approach to modelling quantum distributions and quantum trajectories for density matrix and Green function simulation of nano-devices

In this paper we present a new formalism for quantum transport distribution functions based on density matrices and non-equilibrium Green’s functions that have practical computational advantages and better interpretive power than Wigner function and other phase-space distributions that rely on the centre-relative construction thus leading to non-compact support on phase-space. The new approach uses a mixed position-momentum basis and has manifest compact support in phase space, proving detailed accounting for nodal regions in both position and momentum. The relevant equations of motion and possible computational schemes are discussed within exactly soluble models that illustrate the formalism and its interpretation. The near-classical limit is easily obtained and lends itself to path-variable iterative methods including Monte Carlo schemes. The new distribution functions are complex valued and lead to coupled drift-diffusion style equations of motion. The methodology is compared with Wigner functions, density gradient and Bohm trajectory methods.     

Surface roughness induced device variability: 3D <Emphasis Type="Italic">ab initio</Emphasis> Monte Carlo simulation study

It is expected that published results from drift diffusion simulation of oxide thickness fluctuations in nano-scale devices underestimates the true intrinsic device parameter variation by neglecting local variations in surface roughness scattering. We present initial results from 3D ‘bulk’ Monte Carlo simulation including an ab initio treatment of surface roughness scattering capable of capturing such transport variation. The scattering is included directly through the real space propagation of carriers in the fluctuating potential associated with a randomly generated interface. We apply this approach to simulate inversion layer mobility in order to validate the model before its possible application in device variability simulations. Qualitative agreement is found with universal mobility data and avenues for possible calibration of surface and simulation parameters are highlighted.     

Role of inelastic electron–phonon scattering in electron transport through ultra-scaled amorphous phase change material nanostructures

The electron transport through ultra-scaled amorphous phase change material (PCM) GeTe is investigated by using ab initio molecular dynamics, density functional theory, and non-equilibrium Green’s function, and the inelastic electron–phonon scattering is accounted for by using the Born approximation. It is shown that, in ultra-scaled PCM device with 6 nm channel length, \(<\) 4 % of the energy carried by the incident electrons from the source is transferred to the atomic lattice before reaching the drain, indicating that the electron transport is largely elastic. Our simulation results show that the inelastic electron–phonon scattering, which plays an important role to excite trapped electrons in bulk PCM devices, exerts very limited influence on the current density value and the shape of current–voltage curve of ultra-scaled PCM devices. The analysis reveals that the Poole–Frenkel law and the Ohm’s law, which are the governing physical mechanisms of the bulk PCM devices, cease to be valid in the ultra-scaled PCM devices.     

A method for performing fully quantum mechanical simulations of gated silicon quantum wire structures

As transistors get smaller, fully quantum mechanical treatments are required to properly simulate them. Most quantum approaches treat the transport as ballistic, ignoring the scattering that is known to occur in such devices. Here, we review the method we have developed for performing fully quantum mechanical simulations of nanowire transistor devices which incorporates scattering through a real-space self-energy, starting with the assumption that the interactions are weak. The method we have developed is applied to investigate the ballistic to diffusive crossover in a silicon nanowire transistor device.     

Atomistic non-equilibrium Green’s function simulations of Graphene nano-ribbons in the quantum hall regime

The quantum Hall effect in Graphene nano-ribbons (GNR) is investigated with the non-equilibrium Green’s function (NEGF) based quantum transport model in the ballistic regime. The nearest neighbor tight-binding model based on p _z orbital constructs the device Hamiltonian. GNRs of different edge geometries (Zigzag and Armchair) are considered. The magnetic field is included in both the channels and contact through Peierls substitution. Efficient algorithms for calculating the surface Green function are used to reduce computation time to enable simulating realistically large dimensions comparable to those used in experiments. Hall resistance calculations exactly reproduce the quantum Hall plateaus observed in the experiments. Use of large dimensions in the simulation is crucial in order to capture the quantum Hall effect within experimentally magnetic fields relevant 10–20 T. R. Golizadeh-Mojarad and A.N.M. Zainuddin contributed equally to this work.     

Three-dimensional optimal multi-level Monte–Carlo approximation of the stochastic drift–diffusion–Poisson system in nanoscale devices

The three-dimensional stochastic drift–diffusion–Poisson system is used to model charge transport through nanoscale devices in a random environment. Applications include nanoscale transistors and sensors such as nanowire field-effect bio- and gas sensors. Variations between the devices and uncertainty in the response of the devices arise from the random distributions of dopant atoms, from the diffusion of target molecules near the sensor surface, and from the stochastic association and dissociation processes at the sensor surface. Furthermore, we couple the system of stochastic partial differential equations to a random-walk-based model for the association and dissociation of target molecules. In order to make the computational effort tractable, an optimal multi-level Monte–Carlo method is applied to three-dimensional solutions of the deterministic system. The whole algorithm is optimal in the sense that the total computational cost is minimized for prescribed total errors. This comprehensive and efficient model makes it possible to study the effect of design parameters such as applied voltages and the geometry of the devices on the expected value of the current.     

Optimal recloser and autosectionalizer allocation in distribution networks using IPSO–Monte Carlo approach

Determination of optimal location of automatic devices such as reclosers and autosectionalizers (R&AS’s) for reliability improvement of distribution systems is a combinational and complex problem. In this paper, a methodology based on cost/benefit analysis for optimal R&AS’s allocation in distribution systems is proposed. The objective function is considered minimizing the costs of reliability of the distribution systems. Besides, the effect of load types is considered in the optimal R&AS’s allocation. A hybrid method based on Improved Particle Swarm Optimization (IPSO) algorithm and Monte Carlo simulation is proposed for solving the problem. The proposed algorithm is applied to the actual 50-bus distribution system in Qazvin-Iran. Numerical studies are representing of the effectiveness of the proposed method.     

Analytical and self-consistent quantum mechanical model for a surrounding gate MOS nanowire operated in JFET mode

We derive an analytical model for the electrostatics and the drive current in a silicon nanowire operating in JFET mode. We show that there exists a range of nanowire radii and doping densities for which the nanowire JFET satisfies reasonable device characteristics. For thin nanowires we have developed a self-consistent quantum mechanical model to obtain the electronic structure.     

Electromagnetic transient scattering analysis in time-domain—comparison of TLM and TDIE methods

Here we consider two popular techniques useful for time domain signature analysis of direct scattering problem i.e., given the incident waveform and the target geometry, find the scattered field. We present in this paper a comparison of two popular time domain numerical techniques, viz., the transmission line matrix (TLM) method and the finite-difference time-domain (FDTD) method for electromagnetic scattering problems. Even though there are many similarities between the two methods, the modeling philosophy is different. Whereas in FDTD Maxwell’s equations are solved using a differencing scheme, in TLM a scattering approach akin to Huygens principle is implemented by replacing the space domain with a system of interconnected transmission-lines. The comparison is made via standard canonical shapes, a dielectric cube, and a dielectric sphere, to address the factors affecting accuracy, efficiency, and the required computer resources.