Monte Carlo simulations of double-gate MOSFETs

A fullband Monte Carlo simulator has been used to analyze the performance of scaled n-channel double-gate (DG) MOSFETs. Size quantization in the channel is accounted for by using a quantum correction based on Schrodinger equation. Scattering induced by the oxide interface is included with a model calibrated with measurements for bulk devices. The detailed self-consistent treatment of quantum effects leads to several interesting observations. We observe that the sheet charge in DG devices does not decrease as much as expected in bulk devices when quantum-mechanical effects are included. The average carrier velocity in the channel is also somewhat reduced by quantum effects, as a second-order effect. In the test cases studied here, application of quantum effects causes a reduction in simulated current not exceeding 15%. In a DG structure, quantum effects tend to concentrate the charge density in the center of the channel, where transverse fields are lower. Because of this, interface scattering appears to be less pronounced when quantum effects are included.     

MC simulation of strained-Si MOSFET with full-band structure and quantum correction

A new two-dimensional full-band Monte Carlo simulator, "Monte Carlo University of Texas" (MCUT) is introduced and described in this paper. MCUT combines some of the best features of semiclassical MC device simulation including full-band structure and flexibility of scattering processes, with generality of material composition and the ability to address degeneracy breaking among energy valleys and the associated effects on scattering and transport due to quantum confinement and strain effects. The latter capability derives from extension of a prior crystal-momentum-independent self-consistent Poisson-Schro/spl uml/dinger-based quantum corrected potential, to a valley dependent quantum correction via, in part, a new modeling concept of "effective strain" within the full-band structure code. Low field mobility simulation results for large tensile strained-Si channel nMOSFETs and unstrained-Si channel nMOSFETs device are compared with other simulation methods and experimental data to demonstrate the effectiveness of the approach, and the abilities to simulate high-field transport and transport in devices of a few 10s of nanometer channel lengths are briefly demonstrated.     

A novel hydrodynamic model for nanoscale devices simulation

This paper presents a novel scheme to incorporate quantum effect in classical hydrodynamic model. The scheme can be applied to multi-dimensional and transient conditions and no additional equations are required to solve quantum potential, so complexity of equations is drastically reduced. Simulation results show consistent with that of Monte Carlo simulation. This technology provides an efficient method for investigating quantum effect in small size semiconductor devices. A new guess method for hydrodynamics model has also been proposed in this paper and a 2D hydrodynamic simulator based on quantum correction and new initial guess method has been developed. The solution obtained from DD model gives a good initial guess of HD model. Its advantage is it can achieve convergence after a few iterations because initial guess is closed to final solution. Two-dimensional simulations have been carried out on a few nanoscale devices. The results have been compared with that of other initial guess methods and the significant differences have been found, especially in numerical stability.     

Study of a 50 nm nMOSFET by ensemble Monte Carlo simulationincluding a new approach to surface roughness and impurity scattering inthe Si inversion layer

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_ds-V_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     

Numerical analysis of nonequilibrium electron transport inAlGaAs/InGaAs/GaAs pseudomorphic MODFETs

Nonequilibrium electron transport in InGaAs pseudomorphic MODFETs has been analyzed with the moment equations approach. In the model, the momentum and energy balance equations for the two-dimensional electrons in the InGaAs channel are solved with relaxation times generated from a Monte Carlo simulation. The two-dimensional electron wave functions and the quantized state energies in the InGaAs quantum well are calculated exactly from the Schrodinger equation along the direction perpendicular to the quantum well. Also included is a two-dimensional Poisson equation solver. In the calculation, all of the equations are solved iteratively until a self-consistent solution is achieved. The simulation results for a realistic device structure with a 0.5-μm recessed gate show a significant overshoot velocity of 4.5×10⁷ cm/s at a drain bias of 1.0 V. Electron temperature reaches a peak value of around 2500 K under the gate. In energy transport, the diffusive component of the energy flux is found to be dominant in the high-field region     

A computationally efficient model of single electron transistor for analog IC simulation

A compact analytical single electron transistor (SET) model is proposed. This model is based on the “orthodox theory” of single electron tunneling, valid for unlimited range of drain to source voltage, valid for single or multi-gate, symmetric or asymmetric devices and takes the background charge effect into account. This model is computationally efficient in comparison with existing models. SET characteristics produced by the proposed model have been verified against Monte Carlo simulator SIMON and show good agreement. This model has been implemented in HSPICE simulator through its Verilog-A interface to enable simulation with conventional MOS devices and single electron inverter has been simulated and verified with SIMON results. At high operating temperature, the thermionic current is taken into account.     

Two-dimensional quantum mechanical simulation of chargedistribution 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     

Simulation of Schottky barrier MOSFETs with a coupled quantuminjection/Monte Carlo technique

A full-band Monte Carlo device simulator has been used to analyze the performance of sub-0.1 μm Schottky barrier MOSFETs. In these devices, the source and drain contacts are realized with metal silicide, and the injection of carriers is controlled by gate voltage modulation of tunneling through the source barrier. A simple model treating the silicide regions as metals, coupled with an Airy function approach for tunneling through the barrier, provides injecting boundary conditions for the Monte Carlo procedure. Simulations were carried out considering a p-channel device with 270 Å gate length for which measurements are available. Our results show that in these structures there is not a strong interaction with the oxide interface as in conventional MOS devices and carriers are injected at fairly wide angles from the source into the bulk of the device. The Monte Carlo simulations not only give good agreement with current-voltage (I-V) curves, but also easily reproduce the subthreshold behavior since all the computational power is devoted to simulation of channel particles. The simulations also clarify why these structures exhibit a large amount of leakage in subthreshold regime, due to both thermionic and tunneling emission. Computational experiments suggest ways to modify the doping profile to reduce to some extent the leakage     

Quantum mechanical effects on noise properties of nanoelectronic devices: application to Monte Carlo simulation

A discussion about the quantum mechanical effects on noise properties of ballistic (phase-coherent) nanoscale devices is presented. It is shown that quantum noise can be understood in terms of quantum trajectories. This interpretation provides a simple and intuitive explanation of the origin of quantum noise that can be very salutary for nanoelectronic engineers. In particular, an injection model is presented that, coupled with a standard Monte Carlo algorithm, provides an accurate modeling of quantum noise. As a test, the standard results of noise in tunneling junction devices are reproduced within this approach.     

Comparison of semiconductor transport models using a Monte Carloconsistency test

This paper proposes a scheme to compare different transport models which are used to simulate submicron semiconductor devices. The procedure requires self-consistent Monte Carlo simulation data for a particular test device. We have compared four different hydrodynamic transport models which have been proposed recently. All four sets of hydrodynamic equations can be cast into a single form by selecting appropriate models for various transport parameters. The advantage is that we can use the same discretized set of equations to implement different transport models. We have also compared the results obtained from the Monte Carlo consistency test with those obtained by the hydrodynamic equation solver. The consistency test has been used to highlight the merits and demerits of the transport models on a common platform     

CASTAM: A process variation analysis simulator for MOS LSI's

A simulator named CASTAM, which includes both process and device models, has been developed to predict MOS process variations through the analysis of variations in electrical characteristics of fabricated MOS devices using the Monte Carlo method. Analysis accuracy using the simulator is examined. Investigation shows that process parameter variations can be estimated with an error of less than 10 percent if an appropriate set of device characteristic items is chosen. Wafer inspection data for a CMOS pilot line can be analyzed with this simulator, and the main cause of threshold voltage variation pinpointed. Predictions derived from the analyzed results have been confirmed using experimental data. This shows that analysis using CASTAM is sufficiently reliable.     

Monte Carlo surface scattering simulation in MOSFET structures

The Monte Carlo method has been applied to MOSFET devices with the gate lengths less than 1 µm. The electric field in the channel was obtained by an analytical approach. Since the classical situation is approached in the submicrometer gate device, the partial diffusive model is employed for surface scattering process. Transient phenomena such as velocity overshoot have been predicted with drain biases causing a large field gradient in the channel. Comparison of the results of the Monte Carlo simulation with those obtained by an analytical approach based on static mobility shows that the carrier transit time in the channel is shorter (as much as two times) than that predicted by the analytical approach for a 0.3 µm gate device.     

Physics of Hole Transport in Strained Silicon MOSFET Inversion Layers

A comprehensive quantum anisotropic transport model for holes was used to study silicon PMOS inversion layer transport under arbitrary stress. The anisotropic band structures of bulk silicon and silicon under field confinement as a twodimensional quantum gas are computed using the pseudopotential method and a six-band stress-dependent k.p Hamiltonian. Anisotropic scattering is included in the momentum-dependent scattering rate calculation. Mobility is obtained from the Kubo–Greenwood formula at low lateral field and from the fullband Monte Carlo simulation at high lateral field. Using these methods, a comprehensive study has been performed for both uniaxial and biaxial stresses. The results are compared with device bending data and piezoresistance data for uniaxial stress, and device data from strained Si channel on relaxed SiGe substrate devices for biaxial tensile stress. All comparisons show a very good agreement with simulation. It is found that the hole band structure is dominated by 12 “wings,” where mechanical stress, as well as the vertical field under certain stress conditions, can alter the energies of the few lowest hole subbands, changing the transport effective mass, density-of-states, and scattering rates, and thus affecting the mobility.     

The effect of potential obstacles on charge transfer in imagesensors

Numerical methods are presented to investigate charge transfer in charge-coupled devices (CCDs) when potential barriers or wells occur. A Monte Carlo simulation of electron thermal diffusion and field-aided drift is used to determine the time scale for charge transfer. The Monte Carlo approach is useful for exploring new problems, but it requires considerable amounts of computer time. A quicker technique, that of the mean first passage time, is introduced. This method reduces the solution of the carrier continuity equation for charge transfer to the evaluation of a double integral that yields the characteristic time τ for e^-tτ/. This provides the leading or dominant time dependence of the carrier continuity equation's solution. Numerical examples are presented to show how τ varies with the size and location of the potential obstacle. The mean first passage time approach permits rapid estimates of the effects of potential obstacles on charge transfer in CCD's. These estimates are in excellent agreement with the results of the Monte Carlo simulations     

Nanoscale electrothermal co-simulation: compact dynamic models of hyperbolic heat transport and self-consistent device Monte Carlo

Two problems in the self-consistent, electrothermal co-simulation of nanoscale devices, are discussed. It is shown that the construction of dynamic compact thermal models for nanoscale devices, based on solution of the hyperbolic (wavelike) heat transport equation, can follow essentially the same approach as the authors' analytical thermal impedance matrix method for the parabolic (diffusive) equation. The physicality of the hyperbolic equation is discussed in the light of calculated results. The analytical impedance matrix method for the time-independent case is employed in a thermally self-consistent device Monte Carlo simulation, illustrating the potential for detailed study of nanoscale electrothermal effects.     

65nm n沟MOSFET的重离子辐照径迹效应研究

重离子在SiO2中能产生永久径迹,因此它可能对MOS器件电学特性产生影响。文章用Geant4软件对Au和Sn两种离子进行蒙特卡洛模拟,重点分析高能粒子在SiO2中的能量沉积及径迹。基于模拟分析,对专门设计的65 nm n沟MOSFET器件进行Sn离子辐照实验,发现辐照后Ids和Ig明显增大,分析器件辐照前后阈值电压、跨导、沟道电流以及栅漏电流等特性参数变化的原因。     

Electromagnetic radiation from hot carriers in FET-devices

We report on the emission of light from si MOS and GaAs MES devices. Processes involving band-to-band transitions and a Bremsstrahlung-continuum below the band-gap are shown to exist. Spatially nonuniform emission from the MESFETs is observed. The GaAs results are compared with Monte Carlo simulations.