Ming-C. Cheng

Semiclassical Monte Carlo model for in-plane transport of spin-polarized electrons in III–V heterostructures

We study the in-plane transport of spin-polarized electrons in III–V semiconductor quantum wells. The spin dynamics is controlled by the spin-orbit interaction, which arises due to the bulk crystalline-structure asymmetry and quantum-well inversion asymmetry. This interaction, owing to its momentum dependence, causes rotation of the spin-polarization vector, and also produces effective spin dephasing. The density matrix approach is used to describe the evolution of the electron spin polarization, while the spatial motion of the electrons is treated semiclassically. Monte Carlo simulations have been carried out for temperatures in the range 77–300 K.

Modeling of thermal behavior in SOI structures

Several physics-based analytical steady-state heat flow models for silicon-on-insulator (SOI) devices are presented, offering approaches at different levels of accuracy and efficiency for prediction of temperature profiles induced by power dissipated in SOI MOSFETs. The approaches are verified with the rigorous device simulation based on the energy transport model coupled with the heat flow equation. The models describe the one-dimensional temperature profile in the silicon film of SOI structure and two-dimensional heat flow in FOX, accounting for heat loss to the substrate via BOX and FOX, heat loss to (or gain from) interconnects, and heat exchanges between devices. These models are applied to investigate thermal behavior in single SOI devices and two-device SOI structures.

Steady-state and dynamic thermal models for heat flow analysis of silicon-on-insulator MOSFETs

Abstract Self-heating in silicon-on-insulator (SOI) MOSFETs has become one of the vital issues for design, characterization, optimization and reliability prediction of SOI devices and integrated circuits due to the low thermal conductive buried oxide (BOX) and the continual increase in the microelectronic packaging density. Thermal models that are accurate and detailed enough to provide device temperature profiles and efficient enough for large scale electro-thermal simulation are therefore strongly desirable. This paper discusses the fundamental concepts for modeling of heat flow in semiconductor devices. A brief overview for the conventional approaches to thermal modeling of the SOI devices is given. Improved steady-state and dynamic SOI heat flow models based on the SOI film thermal resistance for efficient prediction of steady-state and dynamic temperature variations in SOI devices are presented. These improved models are applied to investigate temperature distributions and temporal evolution of the junction temperature in SOI nMOSFETs.

Monte Carlo modeling of spin FETs controlled by spin-orbit interaction

A method for Monte Carlo simulation of 2D spin-polarized electron transport in III-V semiconductor heterojunction (FETs) is presented. In the simulation, the dynamics of the electrons in coordinate and momentum space is treated semiclassically. The density matrix description of the spin is incorporated in the Monte Carlo method to account for the spin polarization dynamics. The spin-orbit interaction in the spin FET leads to both coherent evolution and dephasing of the electron spin polarization. Spin-independent scattering mechanisms, including optical phonons, acoustic phonons and ionized impurities, are implemented in the simulation. The electric field is determined self-consistently from the charge distribution resulting from the electron motion. Description of the Monte Carlo scheme is given and simulation results are reported for temperatures in the range 77-300 K.

Magnetic Characteristics and Core Losses in Machine Laminations: High-Frequency Loss Prediction From Low-Frequency Measurements

To study the fundamental essence of core losses and to achieve an accurate core loss separation formula, a dynamic finite-element model for the nonlinear hysteresis loop of laminations has been established. In the model, Maxwells equations are solved for the hysteresis character in the magnetic lamination, using the Galerkin finite-element method, where the hysteresis is represented by an energetic hysteresis model. Based on the simulation results, the magnetic characteristics, skin effect, time delay, and magnetic field distribution are discussed. Then, core losses, particularly excess losses, affected by the magnetic characteristics are carefully examined. It is concluded that excess current loss formula is only applicable for the cases where skin effect is negligible and the sum of hysteresis losses and eddy current losses can more generally represent total losses.

Magnetic Characteristics and Excess Eddy Current Losses

To study the fundamental essence of excess losses and to achieve an accurate core loss separation formula, a dynamic finite element model for the non-linear hysteresis loop of laminations has been established. In the model, Maxwells Equations are solved for the hysteresis character in the magnetic lamination, using the Galerkin finite element method, where the hysteresis is represented by an energetic hysteresis model. Based on the simulation results, the magnetic characteristics, skin effect, time delay and magnetic field distribution are discussed. Then core losses, especially excess losses, affected by the magnetic characteristics are carefully examined. It is concluded that excess current losses are due to the non-uniform magnetic field distribution resulting from the skin effect and the non-linear diffusion of magnetic flux from the boundary to the inside of the lamination.

High order finite element model for core loss assessment in a hysteresis magnetic lamination

A dynamic model for evaluating core losses in a hysteretic magnetic lamination is developed and then solved using a high-order finite element method that includes time-history effects. It is demonstrated that the dynamic hysteresis effect, previously used to explain the frequency dependence of B−H loops, is not a fundamental phenomenon of magnetic materials but originates from the skin effect. It arises because the measured flux density is an averaged value over the lamination thickness, and this value is influenced strongly by the skin effect. The study verifies that, unlike the observed dynamic hysteresis effect, the local B−H loop is in fact frequency independent. The developed dynamic core loss model is thus derived based on the frequency-independent B−H loop. It is shown that the developed model can accurately evaluate the losses for different frequencies and thicknesses based on only one set of inputs of an experimental B−H loop at one low frequency without a huge database of experimental losses.

Monte Carlo modeling of spin injection through a Schottky barrier and spin transport in a semiconductor quantum well

We develop a Monte Carlo model to study injection of spin-polarized electrons through a Schottky barrier from a ferromagnetic metal contact into a nonmagnetic low-dimensional semiconductor structure. Both mechanisms of thermionic emission and tunneling injection are included in the model. Due to the barrier shape, the injected electrons are nonthermalized. Spin dynamics in the semiconductor heterostructure is controlled by the Rashba and Dresselhaus spin-orbit interactions and described by a single electron spin density matrix formalism. In addition to the linear term, the third-order term in momentum for the Dresselhaus interaction is included. Effect of the Schottky potential on the spin dynamics in a two-dimensional semiconductor device channel is studied. It is found that the injected current can maintain substantial spin polarization to a length scale in the order of 1μm at room temperature without external magnetic fields.

Proper-Orthogonal-Decomposition Based Thermal Modeling of Semiconductor Structures

A thermal model of a semiconductor structure is developed using a hierarchical function space, rather than physical space. The thermal model is derived using the proper orthogonal decomposition (POD) and does not require any assumptions about the physical geometry, dimensions, or heat flow paths, as is usually necessary for compact/lumped thermal models. The approach can be applied to complex geometries and provides detailed thermal information at a computational cost comparable to that of lumped thermal models. The POD thermal model is applied to steady thermal simulations of a 2-D silicon-on-insulator (SOI) device structure and validated at various power levels against detailed numerical simulation (DNS) data. It is shown that a POD thermal model using only four POD modes can duplicate the temperature solution derived from DNS, including the hot spot and temperature gradients along the device island. In addition, an unsteady POD model of the SOI device structure is constructed. A POD model incorporating ten modes yielded a virtually identical solution when compared to corresponding unsteady DNS results.

General core loss models on a magnetic lamination

An analytical core loss formula and a dynamic hysteresis core loss model have been developed. Experimental data for different lamination thicknesses have been measured and analyzed at various frequencies. Based on the analysis, an analytical eddy current loss model considering skin effect and a dynamic hysteresis finite element core loss model have been established and validated by experiments. The developed models are simple and efficient, and have been shown to be very accurate compared with the experiments. Moreover, the models can calculate core losses based on the input parameters obtained from experimental measurements at only one single frequency in a thin lamination. The models, to our best knowledge, are the first ones that are capable of calculating core losses for different thicknesses of materials and different operating frequencies, without a massive experimental database.