Electrothermal Phenomena Research Articles

Numerical simulations of heterojunction GaN nanopillar light emitting diodes

The paper deals with numerical modeling of electrothermal phenomena in 3D GaN core-shell light-emitting diode (LED) structures that were developed in the frame of GECCO project.1 The simulations investigate the influence of pillar dimensions on the LED work conditions. The inherent feature of such a design is the discrepancy between the internal contact footprint current density JFP and the current density on the junction active area JAA, which, at the same contact current, decreases when the pillar is taller. The simulations indicate that the decrease of JAA results in significant changes in the LED parameters. At the same diode current, i.e., constant light emission, it leads to the voltage decrease leading to the reduction of power delivered to the diode and, consequently, to the increase of its efficiency.

Towards the prospection of an optimal thermal response of ZnO surge arrester in HV power system

In order to understand the thermal and electrical properties of surge arrester under standard climatic conditions, it would be useful to evaluate the heating progression in its different elements. These are constituted of heterogeneous materials, which have physical and electrical nonlinear properties along the surge arrester. Temperature predicting solutions for zinc oxide ceramic blocks provide fundamental elements of the electrothermal material characteristics in view of the lifetime estimation. Electrothermal phenomenon analysis of surge arresters is based on empirically-determined parameter models, using finite element method (FEM) simulated on COMSOL Multiphysics software which is a more precise approach compared to the existing models. In this paper, the behavioural model has been developed to study the surge arresters in both cases: the steady state and overvoltage state (lightning). Previous published works validate the first simulation results. This work is therefore extended to consider other operating states, which illustrate the dependence of the type of surge on the amplitudes and durations of electrothermal phenomena.

Thermal Modeling Approaches for a LiCoO2 Lithium-ion Battery—A Comparative Study with Experimental Validation

Temperature prediction of a battery plays a significant role in terms of energy efficiency and safety of electric vehicles, as well as several kinds of electric and electronic devices. In this regard, it is crucial to identify an adequate model to study the thermal behavior of a battery. This article reports a comparative study on thermal modeling approaches by using a LiCoO2 26650 lithium-ion battery, and provides a methodology to characterize electrothermal phenomena. Three approaches have been implemented numerically—a thermal lumped model, a 3D computational fluid dynamics model, and an electrochemical model based on Newman, Tiedemann, Gu and Kim formulation. The last two methods were solved using ANSYS Fluent software. Simulations were validated with experimental measurements of the cell surface temperature at constant current discharge and under a highway driving cycle. Results show that the three models are consistent with actual temperature measurements. The electrochemical method has the lower error at 0.5C. Nevertheless, this model provides the higher error ( 1.3∘C) at 1.5C, where the maximum temperature increase of the cell was 18.1∘C. Under the driving cycle, all the models are in the same order of error. Lumped model is suitable to simulate a wide range of battery operating conditions. Furthermore, this work was expanded to study heat generation, voltage and heat transfer coefficient under natural convection.

Quantum Transport in a Silicon Nanowire FET Transistor: Hot Electrons and Local Power Dissipation.

A review and perspective is presented of the classical, semi-classical and fully quantum routes to the simulation of electro-thermal phenomena in ultra-scaled silicon nanowire field-effect transistors. It is shown that the physics of ultra-scaled devices requires at least a coupled electron quantum transport semi-classical heat equation model outlined here. The importance of the local density of states (LDOS) is discussed from classical to fully quantum versions. It is shown that the minimal quantum approach requires self-consistency with the Poisson equation and that the electronic LDOS must be determined within at least the self-consistent Born approximation. To bring in this description and to provide the energy resolved local carrier distributions it is necessary to adopt the non-equilibrium Green function (NEGF) formalism, briefly surveyed here. The NEGF approach describes quantum coherent and dissipative transport, Pauli exclusion and non-equilibrium conditions inside the device. There are two extremes of NEGF used in the community. The most fundamental is based on coupled equations for the Green functions electrons and phonons that are computed at the atomically resolved level within the nanowire channel and into the surrounding device structure using a tight binding Hamiltonian. It has the advantage of treating both the non-equilibrium heat flow within the electron and phonon systems even when the phonon energy distributions are not described by a temperature model. The disadvantage is the grand challenge level of computational complexity. The second approach, that we focus on here, is more useful for fast multiple simulations of devices important for TCAD (Technology Computer Aided Design). It retains the fundamental quantum transport model for the electrons but subsumes the description of the energy distribution of the local phonon sub-system statistics into a semi-classical Fourier heat equation that is sourced by the local heat dissipation from the electron system. It is shown that this self-consistent approach retains the salient features of the full-scale approach. For focus, we outline our electro-thermal simulations for a typical narrow Si nanowire gate all-around field-effect transistor. The self-consistent Born approximation is used to describe electron-phonon scattering as the source of heat dissipation to the lattice. We calculated the effect of the device self-heating on the current voltage characteristics. Our fast and simpler methodology closely reproduces the results of a more fundamental compute-intensive calculations in which the phonon system is treated on the same footing as the electron system. We computed the local power dissipation and “local lattice temperature” profiles. We compared the self-heating using hot electron heating and the Joule heating, i.e., assuming the electron system was in local equilibrium with the potential. Our simulations show that at low bias the source region of the device has a tendency to cool down for the case of the hot electron heating but not for the case of Joule heating. Our methodology opens the possibility of studying thermoelectricity at nano-scales in an accurate and computationally efficient way. At nano-scales, coherence and hot electrons play a major role. It was found that the overall behaviour of the electron system is dominated by the local density of states and the scattering rate. Electrons leaving the simulated drain region were found to be far from equilibrium.

Electro-Thermal Model for Thermal Disturbance in Cross-Point Phase-Change Memory

We developed an electro-thermal model for cross-point phase-change memory (X-PCM) and compared the calculation values with the experimental results. In order to simulate the electro-thermal phenomenon such as thermal disturbance (TDB) of victim cells and reset the current ( ${I}_{{\text {RESET}}}$ ) of aggressor cells in a fully confined cross-point structure, a three by three mini-array was constructed with the finite-elemental method. Unlike the conventional thermal model, which only shows the temperature gradient for TDB, our new model can predict the crystallization behavior of victim cells by combining the crystallization model for nucleation and growth. This makes it possible to compare the calculation results with the experimental ones through the crystalline fraction of the victim cell. The simulation results clearly reveal that our new model can closely estimate the victim cell’s crystalline fraction for TDB, with the variation of both pulse height (PH) and pulse width (PW) during the RESET operation. Applying the confirmed model for devices of smaller dimensions shows that TDB increases as the device scales down. The increase of TDB is particularly severe as cells become smaller than 16 nm. We suggest that TDB can be reduced not only by decreasing the thermal conductivity ( $\kappa $ ) of the interlayer dielectric (ILD), but also by minimizing the PW of the aggressor.

AC electrokinetic induced non-Newtonian electrothermal blood flow in 3D microfluidic biosensor with ring electrodes for point-of-care diagnostics

Efficient pumping of whole blood is an essential task in biomedical engineering, especially for point-of-care diagnostics using lab-on-a-chip devices. Alternating current (AC) electrokinetics have been widely used for several different applications among which pumping fluids using the precisely controlled electric field without any moving mechanical parts is significant. Due to its high conductive characteristic, it is difficult to drive the blood flow using the AC electroosmosis phenomenon because the electric double layer is highly compressed. Fortunately, the AC electrothermal (ACET) phenomenon occurs due to the variation of temperature-dependent permittivity and conductivity caused by Joule heating effects or other heat sources making it powerful for driving high electrical conductivity physiological fluids in biomedical devices. Compared with Newtonian fluids like saline solutions or urine, the non-Newtonian rheological nature and AC frequency-dependent dielectric property of blood make its ACET driving mechanism more complicated and attractive. In this paper, ACET induced blood flow in the 3D microfluidic channel is modeled by the lattice Boltzmann method accelerated using graphics processor units. The Carreau-Yasuda model is applied to simulate the shear-thinning behavior of blood flow, and its electrothermal pumping efficiency is investigated with respect to the AC electrode configuration, AC voltage magnitude, and AC signal frequency by comparing it with the ACET pumping of Newtonian fluids using scaling law analysis. The results demonstrate that the ACET phenomenon is effective for pumping non-Newtonian whole blood flow in microfluidic devices with ring electrodes which will contribute to the point-of-care diagnostic of bacterial bloodstream infections or rapid detection of circulating tumor cells.

Electrothermal and Overload Performance of Metal-Oxide Varistors

The energy absorption capability of metal-oxide varistors, decidedly influenced by the electrothermal processes occurring within their microstructures, is a vital parameter for the effective surge protection. This paper proposes a numerical model for the simulation of the electrothermal phenomena occurring in metal-oxide varistors. The model is based on a 3D representation of the varistor's polycrystalline microstructure and the finite element method analysis. With the aid of the proposed model, the interdependence of the varistor's energy absorption capability, nonuniform current conduction, and the thermal behavior is investigated. Results are in good alignment with experimental data and discussed in the context of electrothermal properties of the varistor's microstructure. An unambiguous dependency of the varistor's energy handling capability on the applied overvoltage is shown to exist. A quantitative evaluation of the effective volume decrease due to the severe current localization at temporary overvoltages is demonstrated. The presented work analyses varistor's current conduction under temporary overvoltages and provides the means for extension of the varistors IV characteristics into the time domain so as to accurately predict their dynamic electrothermal behavior. The introduced dynamic (IV t) model may have significant implications in energy absorption requirements and design principles for surge protective devices.

Numerical investigation of the effect of the electrodes bed on the electrothermally induced fluid flow velocity inside a microchannel

Due to the development of technology, desire of using fluid pumping systems without movable components in micro scale is increased, which Electrokinetics is one of them. One of the sub-sets of this method is the electrothermal phenomenon. This phenomenon, being the focus of researchers because of its ability in pumping fluids with high electrical conductivity (more than 0.01S/m). There are different software for simulating this phenomenon, such as fluent and COMSOL Metaphysics, which could say cover most of them. In this paper, initially introduce new solver based of OpenFOAM for this phenomenon. Next, various geometry for the electrodes bed and their related parameters are investigated. At this part, initially the effect of different configuration of the electrodes bed on the fluid velocity inside a microchannel is investigated. Because the objective of this part is comparing different beds geometries for the electrode, only two pairs of electrodes are used. Results show that the bed with upward trapezoid, isosceles and acute configuration for electrodes bed have the best performance, which increase outlet velocity of the fluid 34.4% and 33% compared to flat mode, respectively. Secondly, the effect of the gap between the electrode pairs, the placement of acute trapezoidal configuration is investigated. By applying the best state of the examined parameters, fluid velocity in the trapezoidal channel with two and six pairs of electrodes increases up to 79.45% and 118% respectively. In the third section, a correlation between the fluid velocity at the outlet and ratio of various channel heights in respect to the trapezoid's height is derived. This Equation indicates direct relation between the outlet velocity and the channel height.

Bioparticle delivery in physiological conductivity solution using AC electrokinetic micropump with castellated electrodes

Alternating current (AC) electrokinetics have many applications in engineering fields, among which the efficient delivery of bioparticles in a continuous flow is significant for further biomedical manipulations. To avoid the cell lyse caused by low electrical conductivity environment, the AC electrothermal (ACET) phenomenon which effectively drives the physiological fluid with high electrical conductivity becomes attractive. In addition, negative dielectrophoresis (nDEP) is usually induced on the polarized bioparticles immersed in physiological fluid with a non-uniform electric field. In the current work, a novel AC electrokinetic micro-device with castellated electrodes is designed for delivering bioparticles in high electrical conductivity fluid without the use of a mechanical micropump. The ACET flow vortex could be eliminated by replacing the interdigitated electrodes with the castellated electrodes. Besides, by appropriately choosing the phases of AC voltages, a nDEP force is induced to repel the bioparticles from the electrode edges and the microchannel surfaces. Under this circumstance, the bioparticle transport efficiency is highly improved, and the cell adhension on the micropump surfaces is also reduced. The effects of AC voltage magnitudes, electrical conductivities of solution and bioparticles, bioparticle size, and bioparticle initial position on the multi-physical bioparticle transport process are investigated by immersed boundary-lattice Boltzmann method. The results demonstrate that the hybrid AC electrokinetics using castellated electrodes is an efficient technique of delivering bioparticles in a lab-on-a-chip device.

Design methodology for over-temperature and over-current protection of an LDO voltage regulator by using electro-thermal simulations

This paper presents a methodology for designing over-temperature and over-current protection (OTP and OCP) circuits for low drop-out voltage regulators (LDOs). The OTP monitors the die temperature developed within the LDO and disables its output stage when the temperature reaches a certain, user-defined, level (the OTP activation point). If the LDO output current reaches a set threshold (the OCP activation point), the OCP takes control of it, keeping the current value to an acceptable level. The proposed methodology involves running iteratively electrical, thermal and electro-thermal simulations. It addresses three major issues: first, it allows the designer to identify the suitable layout placement of the OTP and OCP sensors, based on the temperature distribution within the LDO power-stage. Second, the OTP and OCP activation points can be set accurately by taking into account coupled electro-thermal phenomena and the unavoidable differences between the temperature and current sensed by the protection circuits and those developed within the worst-case LDO section. Finally, the LDO design can be fine-tuned considering complex scenarios of real-life operation and test requirements. An LDO was designed using this methodology and the paper provides a direct comparison between the expected (simulated) results and measurements performed on the silicon implementation.

An Improved Electrical Switching and Phase-Transition Model for Scanning Probe Phase-Change Memory

Scanning probe phase-change memory (SPPCM) has been widely considered as one of the most promising candidates for next-generation data storage devices due to its fast switching time, low power consumption, and potential for ultra-high density. Development of a comprehensive model able to accurately describe all the physical processes involved in SPPCM operations is therefore vital to provide researchers with an effective route for device optimization. In this paper, we introduce a pseudo-three-dimensional model to simulate the electrothermal and phase-transition phenomena observed during the SPPCM writing process by simultaneously solving Laplace’s equation to model the electrical process, the classical heat transfer equation, and a rate equation to model phase transitions. The crystalline bit region of a typical probe system and the resulting current-voltage curve obtained from simulations of the writing process showed good agreement with experimental results obtained under an equivalent configuration, demonstrating the validity of the proposed model.

Simulation of thermomechanical and electrothermal hysteresis phenomena in porous nickel titanium

There is represented a general scheme of functioning and the corresponding model of a porous biomicrofluid matrix based on nickel titanium, which possesses a shape-memory effect, prepared by the method of selective laser sintering. The simulation of thermomechanical and electrothermal hysteresis phenomena composes the basis of the functioning of this MEMS. Such a model can be used for estimating the efficiency and controllability of drug delivery systems via the change in the volume of pores.

Investigation of various ac electrokinetics in a microniche system for cell separation

We investigate various AC electrokinetic (dielectrophoresis, electro-thermal and electro-osmosis) phenomena in a micro-niche system for negative-dielectrophoresis (n-dep) based cell separation. For that purpose, an experimental system is constructed. The system is comprised of a slit-type electrode array to apply voltage, a micro cam-scope to monitor cell movement, micro stages to adjust the focal length of the cam-scope properly, a function generator to apply voltage with various frequencies, and an illumination system. After the conductivity of the medium and the magnitude and frequency of the voltage are selected as experiment variables, cell movement is investigated. The ranges of each variable studied are 0.001 ∼ 0.2 S/m, 1 ∼ 7 Vpp, and 100 Hz ∼ 1 MHz respectively. For the target cells, k562 cells of a leukemia cell line were employed. In the experimental study, n-dep is most dominant over other electrokinetic phenomena with 5 Vpp of input voltage and in the range of 100 ∼ 500 kHz although the theoretical equation indicates that the higher voltage (7 Vpp) generates a higher n-dep force. This is because the higher voltage causes not only a higher n-dep force but also higher electro-thermal and electro-osmosis. Based on the obtained experimental results, we expect to establish a reliable n-dep force based cell sorting system in the near future.

Effect of solution conductivity and electrode shape on the deposition of carbon nanotubes from solution using dielectrophoresis

Dielectrophoresis (DEP) is a popular technique for fabricating carbon nanotube (CNT) devices. The electric current passing through the solution during DEP creates a temperature gradient, which results in electrothermal fluid flow because of the presence of the electric field. CNT solutions prepared with various methods can have different conductivities and the motion of the solution because of the electrothermal phenomenon can affect the DEP deposition differently in each case. We investigated the effect of this movement in solutions with various levels of conductivity through experiments as well as numerical modeling. Our results show that electrothermal motion in the solution can alter the deposition pattern of the nanotubes drastically for high conductivity solutions, while DEP remains the dominant force when a low conductivity (surfactant-free) solution is used. The extent of effectiveness of each force is discussed in the various cases and the fluid movement model is investigated using two- and three-dimensional finite element simulations.

Electrothermal phenomena in zinc oxide nanowires and contacts

Heat generation along nanowires and near their electrical contacts influences the feasibility of energy conversion devices. This work presents ZnO nanowire electrical resistivity data and models electrothermal transport accounting for heat generation at metal-semiconductor contacts, axial thermal conduction, and substrate heat losses. The current-voltage relationships and electron microscopy indicate that sample degradation is caused by the interplay of heat generation at contacts and within the nanowire volume. The model is used to interpret literature data for Si, GaN, and ZnO nanowires. This work assists with electrothermal nanowire measurements and highlights practical implications of utilizing solution-synthesized nanowires.

Electrical Resistance and Seebeck Coefficient in PbTe Nanowires

We address a physics-based simplified analytical formulation of the diffusive electrical resistance ( (Omega)) and Seebeck coefficient () in a PbTe nanowire dominated by acoustic phonon scattering under the presence of a low static longitudinal electric field. The use of a second-order nonparabolic electron energy band structure involving a geometry-dependent band gap has been selected in principle to demonstrate that the electron mean free path (MFP) in such a system can reach as low as about 8 nm at room temperature for a 10-nm-wide PbTe nanowire. This is followed by the formulation of the carrier back-scattering coefficient for determination of (Omega) and as functions of wire dimensions, temperature, and the field, respectively. The present analytical formulation agrees well with the available experimental data and may find extensive use in determination of various electrothermal transport phenomena in PbTe-based one-dimensional electron devices.

Monte Carlo study of self-heating in nanoscale devices

Progress in device miniaturization combined with the increase in integrated circuit packing density, as described by Moore's law, have been accompanied by an exponential increase in on-chip heat generation. In this context, there is an increasing demand for reliable electrothermal modeling techniques that accurately account for self-heating and allow a better understanding of thermal transport at the nanoscale. This paper presents a theoretical demonstration of the electrothermal phenomena in a variety of nanodevices, ranging from conventional Si- and III-V-based field-effect transistors (FETs) to nanowire devices. Simulation work relies on a very well-established Monte Carlo simulator considering self-heating using phonon statistics. The electrothermal simulator self-consistently couples a three-dimensional (3D) electronic trajectory simulation with the solution of the heat diffusion equation. The Monte Carlo technique is very suitable for the simulation of electronic transport in nanoscale semiconductor devices, as it is free from low-field near-equilibrium approximations. In addition, the method is well-suited for electrothermal modeling, since it allows a detailed microscopic description of electron-phonon scattering which provides an inherent and direct prediction of the spatial distribution of heat generation. The paper is divided into three parts. The first part includes a description of the computational approach to simulate electrothermal transport in FETs. The advantages of using the simulation method are demonstrated by presenting full results from the simulation of an InGaAs-channel high-electron mobility transistor (HEMT). The second part concerns electrothermal transport in conventional field-effect devices such as Si and III-V HEMTs. An investigation of self-heating effects in high-power devices, such as AlGaN/GaN HEMTs, and relevant Si-based FETs, e.g. Si/SiGe HEMTs, is presented. This part demonstrates how the analysis of self-heating effects may help us in understanding the electronic and thermal properties of nanoscale FETs. The third part of the paper consists of studying the electrothermal behavior of advanced structures. In this case, charge transport and self-heating effects are investigated in metal-insulator-semiconductor FETs (MISFETs) with a single InAs nanowire channel. Despite low heat dissipation, simulations predict significant local temperatures, due to the high current density levels and the poor thermal management in these nanowire structures.

Modelling the UCC3800 controller in SPICE with the electrothermal phenomena taken into account

A new electrothermal compact model of the current mode pulse width modulation controller—UCC3800—dedicated for SPICE is proposed in this paper. The form of this model is presented and described in detail. The results of the experimental verification of the model are also presented for the device operating in the test circuit. It is shown that self-heating influences essentially the characteristics of the considered device and the proposed model is of good accuracy. Copyright © 2011 John Wiley & Sons, Ltd.

Monte Carlo study of the electrothermal phenomenon in silicon-on-insulator and silicon-germanium-on-insulator metal-oxide field-effect transistors

Self-heating effects are investigated in silicon-on-insulator (SOI), silicon-germanium-on-insulator (SGOI), and strained-silicon-directly-on-insulator (SSDOI) metal-oxide field-effect transistors (MOSFETs), using a Monte Carlo simulator self-consistently coupled with the solution of the heat diffusion equation. Although the influence of thermal effects is in general higher in these structures, as compared to bulk Si MOSFETs, its impact is much more important in SGOI and SSDOI FET structures incorporating ultrathin Si channels, with SGOI FETs giving the worst thermal performance. A study of the dependence of the extent of self-heating on the buried-oxide thickness is also performed, showing that this parameter is important in designing SOI structures with better thermal management.

Electrothermal Phenomena About Current Limitation With Coated Conductors

The superconducting (SC) fault current limiter (FCL) is very attractive to enhance the security and the power quality of the electricity grid, so its great interest throughout the world. Most of SC FCL uses the quench of a SC element and the thermal phenomena are fundamental. We study them with two approaches: experiments and calculations. For the experiments, we have developed temperature sensors deposited directly on coated conductors to achieve the best thermal coupling and a very short time response. The sensors are micron size copper meanders. A very thin polymer layer provides the electrical isolation. The sensor performances are very satisfying both in terms of time response and accuracy. The measurements are in good agreement with temperature calculations, performed using the current data during limiting experiments. The recovery phase is very important and has been studied using the temperature measurements. The cooling conditions such as bath temperature play an important part for the recovery. Several tests carried out on different samples are presented and analysed. These works are important for a better understanding of the complex phenomena occurring during a limitation and for the design of a conductor suitable for FCL.