Hierarchical 2-D DD and HD noise simulations of Si and SiGedevices. I. Theory

Langevin-type two-dimensional (2-D) bipolar drift-diffusion (DD) and hydrodynamic (HD) noise models are presented for Si and SiGe devices, which are based on the new concept of modified Langevin forces, which ensure that the DD and HD models exactly reproduce the fluctuations of the full-band Monte Carlo (MC) model under homogeneous bulk conditions. All transport and noise parameters are generated by MC bulk simulations and stored in lookup tables, which must be built only once. As a consequence, the accuracy of the DD and HD models is improved without an increase in CPU time compared to models with analytical expressions for the parameters. Considering the full-band structure, a remarkably strong dependence of noise on crystal orientation is found     

On the accuracy and efficiency of substrate current calculationsfor sub-μm n-MOSFET's

The accuracy and efficiency of the self-consistent (regarding the electric field) Monte Carlo model, nonself-consistent Monte Carlo model, and the soft-threshold lucky electron model (LEM) for the calculation of substrate currents in deep sub-μm n-MOSFET's are investigated. While the two Monte Carlo models are in good agreement with the experiment, the simpler LEM model still gives reasonable results even for a 0.16 μm n-MOSFET. On the other hand, huge differences in the CPU time consumption are found and the LEM is about four orders of magnitude faster than the self-consistent Monte Carlo simulations. The nonself-consistent calculations are only one order of magnitude slower than the LEM. The good agreement with the experiment is obtained without considering the so-called surface impact ionization or any fitting of parameters on the device level     

High-mobility low band-to-band-tunneling strained-Germanium double-gate heterostructure FETs: Simulations

Large band-to-band tunneling (BTBT) leakage currents can ultimately limit the scalability of high-mobility (small-bandgap) materials. This paper presents a novel heterostructure double-gate FET (DGFET) that can significantly reduce BTBT leakage currents while retaining its high mobility, making it suitable for scaling into the sub-20-nm regime. In particular, through one-dimensional Poisson-Schrodinger, full-band Monte Carlo, and detailed BTBT simulations, the tradeoffs between carrier transport, electrostatics, and BTBT leakage in high-mobility sub-20-nm Si-strained SiGe-Si (high germanium concentration) heterostructure PMOS DGFETs are thoroughly analyzed. The results show a dramatic (>100/spl times/) reduction in BTBT and an excellent electrostatic control of the channel while maintaining very high drive currents and switching frequencies in these nanoscale transistors.     

An analytical approach for physical modeling of hot-carrier induced degradation

We develop an analytical model for hot-carrier degradation based on a rigorous physics-based TCAD model. The model employs an analytical approximation of the carrier acceleration integral (calculated with our TCAD approach) by a fitting formula. The essential features of hot-carrier degradation such as the interplay between single-and multiple-electron components of Si–H bond dissociation, mobility degradation during interface state build-up, as well as saturation of degradation at long stress times are inherited. As a result, the change of the linear drain current can be represented by the analytical expression over a wide range of stress conditions. The analytical model can be used to study the impact of device geometric parameters on hot-carrier degradation.     

A deterministic method study of the impact of the Pauli principle in double-gate MOSFETs

The Pauli principle is included in a multisubband deterministic solver for two-dimensional devices without approximations.The nonlinear Boltzmann equations are treated properly without compromising on accuracy,convergence,or CPU time.The simulation results indicate the significant impact of the Pauli principle on the transport properties of the quasi-2D electron gas,especially for the on state.     

Efficient simulation of hole transport in strained Si and SiGe on insulator inversion layers

Linear and non-linear transport of holes in strained Si and SiGe on insulator inversion layers has been simulated. A deterministic method based on the Fourier expansion of the distribution function is proposed to solve the BTE. The model takes into account the fully anisotropic transition rates of the scattering mechanisms and the Pauli principle. The simulated low-field mobility and drift velocity reproduce experimental data for different MOS structures.     

Physics-Based Modeling of Hole Inversion-Layer Mobility in Strained-SiGe-on-Insulator

The hole inversion-layer mobility of strained-SiGe homo- and heterostructure-on-insulator in ultrathin-body MOSFETs is modeled by a microscopic approach. The subband structure of the quasi-2-D hole gas is calculated by solving the 6times6koarrldrpoarr Schrodinger equation self-consistently with the electrostatic potential. The model includes four important scattering mechanisms: optical phonon scattering, acoustic phonon scattering, alloy scattering, and surface-roughness scattering. The model parameters are calibrated by matching the measured low-field mobility of two particularly selected long-channel pMOSFET cases. The calibrated model reproduces available channel-mobility measurements for many different strained-SiGe-on-insulator structures. For the silicon-on-insulator MOS structures with unstrained-Si channels, the silicon-thickness dependence resulting from our model for the low-field channel mobility agrees with previous publications.     

Failure of moments-based transport models in nanoscale devices near equilibrium

It is shown that the conductance in nanoscale devices near equilibrium strongly depends on the choice of the transport model. Errors larger than a factor of two can be encountered, if the drift-diffusion (DD) model is used instead of a model based on the full Boltzmann equation. This effect is due to a fundamental difference in carrier heating between bulk systems and devices. Although carrier heating is included in hydrodynamic models, this effect is captured only partially by these models due to the model inherent approximations. A direct consequence of the failure of the DD approximation is that the usual method for inversion layer mobility extraction from measurements in the linear regime becomes inaccurate for short gate lengths and the extracted mobilities might be too small. This error has also an impact on the modeling accuracy at strong nonequilibrium. In the case of the DD model, the overestimation of the conductivity in the linear regime can partly compensate the underestimation of the current at high bias, and the model accidentally appears to be more accurate than expected.     

Hierarchical 2-D DD and HD noise simulations of Si and SiGe devicesII. Results

For pt. I see ibid., vol. 49, pp. 1250-1257 (2002). Terminal current noise is investigated with Langevin-type drift-diffusion (DD) and hydrodynamic (HD) noise models for one-dimensional (1-D) N⁺ NN⁺ and P⁺ PP⁺ structures and a realistic two-dimensional (2-D) SiGe NPN HBT. The new noise models, which are suitable for technology computer aided design (TCAD), are validated by comparison with Monte Carlo (MC) device simulations for the 1-D structures including noise due to particle scattering and generation of secondary particles by impact ionization (II). It is shown that the accuracy of the usual approach based on the DD model in conjunction with the Einstein relation degrades under nonequilibrium conditions. 2-D MC noise simulations are found to be feasible only if the current correlation functions decay on a subpicosecond scale, what is not always the case