Electro-thermal Simulation Research Articles

Modeling heterogeneous melting in phase change memory devices

We present thermodynamic crystallization and melting models and calculate phase change velocities in Ge2Sb2Te5 based on kinetic and thermodynamic parameters with a focus on the impacts of grain boundary melting. The calculated phase change velocities are strong functions of grain size, with smaller grains beginning to melt at lower temperatures. Phase change velocities are continuous functions of temperature which determine crystallization and melting rates. Hence, set and reset times as well as power and peak current requirements for switching are strong functions of grain size. Grain boundary amorphization can lead to a sufficient increase in cell resistance for small-grain phase change materials even if the whole active region does not completely amorphize. Isolated grains left in the amorphous regions, the quenched-in nuclei, facilitate templated crystal growth and significantly reduce set times for phase change memory cells. We demonstrate the significance of heterogeneous melting through 2-D electrothermal simulations coupled with a dynamic material phase change model. Our results show reset and set times on the order of ∼1 ns for 30 nm wide confined nanocrystalline (7.5 nm–25 nm radius crystals) phase change memory cells.

Inhibition of peak electric field shifting on current filaments of high voltage diode

The buffer layers can improve significantly the ruggedness of a high voltage diode, but the inhibition mechanism on current filaments is unclear. In this paper, an analysis of the electric field gradient analytical model is used to explain the peak electric field shifting of high voltage diode with buffer layers during reverse recovery, and the influence of the buffer layers on current filament during reverse recovery is analyzed. Electrothermal simulations show that the peak electric field shifting even occurs before dynamic avalanche, leading to the appearance of the positive differential resistance. A self-stabilizing mechanism that the increase of the electric field strength in buffer layers is restrained after the peak electric field shifting is found. This provokes the weak dynamic avalanche inside high voltage diode, and the current filaments are inhibited.

Application of CFD and vapor bubble dynamics for an efficient electro-thermal simulation of EDM: an integrated approach

Micro-chip removal and heat dissipation are two important and essential utilities of dielectric medium along with its vital contribution in plasma channel formation, which has a significant influence in metal removal, tool wear, and surface integrity. The vapor bubbles, which are formed after every progressive spark, expand and collapse in the electric field and finally condensed inside the dielectric medium. As the localized heat and excess temperature of the eroded spot increases, the dynamic properties of the bubbles accelerate and develop a lower pressure region, which is responsible for dielectric flushing. In this paper, the integrated approach of vapor bubble dynamics and computational fluid dynamics is used to study the dynamic behavior and heat dissipation of dielectric fluid through the plasma channel. At the very beginning of machining, the excess temperature is comparatively low; so, the detached bubbles are condensed inside the fluid medium and are not able to move above the free surface. However, as the temperature of the machined zone rises, excess temperature also increases and bubbles are able to carry the heat up to the free surface of the dielectric medium and finally condense. This phenomenon is very much analogous to the boiling process. Earlier, many simulation processes are used by researchers to investigate heat flux and heat dissipation rate of the dielectric fluid, but, here, vapor bubble dynamics and periodic heating are introduced to consider heat absorption by vapor bubbles, which gives more accurate heat transfer analysis of electric discharge machining. It is observed that the adopted model has the accuracy level of more than 95% with 4.5–4.6% of average prediction error and 1.5–2% more efficient than the existing models.

An Insight Into Self-Heating Effects and Its Implications on Hot Carrier Degradation for Silicon-Nanotube-Based Double Gate-All-Around (DGAA) MOSFETs

Silicon-Nanotube-based ultra-thin DGAA MOSFETs have been extensively studied for their superior immunity to short channel effects (SCEs) and better drive current capability; however, the reliability issues owing to self-heating effects (SHEs) and hot carrier injection (HCI) degradation are yet to be investigated systematically. In advanced non-planar device structures, an increase in power density due to ultra-scaled device dimensions can aggravate both the carrier heating as well as lattice heating. In this paper, 3-dimensional (3-D) electrothermal (ET) simulations using coupled hydrodynamic and thermodynamic transport models are performed to analyze the electrothermal behavior and SHEs in ultra-thin DGAA MOSFET. 3-D TCAD simulation parameters are calibrated with the data obtained from the literature. Through advanced 3-D ET simulations, we demonstrate that the device thermal contact resistance adversely influences both the carrier temperature as well as lattice temperature. The implication of SHE on the device output drive current reduction is also analyzed. The effective drive current method is used to observe the impact of SHE on the intrinsic delay of the device. Further, the performance of the device due to HCI is also highlighted. HCI significantly degrades the overall device performance leading to increased gate leakage current. Finally, the reliability issues induced by SHEs with on-chip ambient temperature variations have also been interpreted using Sentauras based TCAD simulator.

SPICE Modeling of Insulator Metal Transition: Model of the Critical Temperature

This paper proposes a compact SPICE phenomenological model for insulator metal transition (IMT) devices. The proposed model captures the interplay of electric field and Joule heating to effect a transition from a high resistance insulating state to a low resistance metallic state. The model is corroborated against experimental results and electrothermal simulations available in the literature. The proposed model is implemented in Verilog-A and is fully compatible with commercial SPICE simulators such as Spectre from Cadence, used in this paper. An IMT-based artificial neuron is then designed and simulated using the proposed IMT compact model and design expressions for the operation of the proposed neuron are derived. The simulation results agree with the expected neuron behavior as well as the simulation results of other similar neurons proposed in the literature. This paper will enable circuit designers to design and simulate IMT-based systems and help them explore the full potential of such novel devices.

铝电解槽电热场仿真的研究现状与进展

铝电解槽电热场仿真的研究现状与进展

Heavy Ion Transport Modeling for Single-Event Burnout in SiC-Based Power Devices

SiC power devices have been found experimentally to burn out from heavy ion strikes with linear energy transfers as low as 2.0 MeV $\cdot $ cm2/mg. In this paper, to better understand the failure mechanisms, we study the single-event burnout (SEB) phenomenon using a unified physics model between heavy ion radiation transport and device response. High-fidelity radiation data, generated from a general purpose Monte Carlo N-particle transport code (MCNP6.2), were modeled using a double Gaussian function to take into account both the heavy ion and the delta ray contributions. SiC junction barrier Schottky (JBS) diodes underwent 3-D TCAD electrothermal simulations using the double Gaussian model. This model was compared against other heavy ion models to determine the behavior of the thermal response from a heavy ion strike. The results reveal that there is a more rapid thermal response from models using high-fidelity heavy ion radiation data than approximated models provided by TCAD simulators. Peak temperature results from high-voltage SiC JBS diodes, which agree with the experimental observation of SEBs in SiC power devices.

Electrothermal Optimization of Field Grading Systems of Station Class Surge Arresters

An electrothermal simulation based procedure for the optimization of the field grading systems of station class surge arresters is presented. The main difficulty consists in the numerical solution of the three-dimensional (3-D) field grading system, and the transient electric field problem that arises when a field grading system is introduced. Therefore, we first develop an equivalent axisymmetric arrester model that can reproduce the electric field stress in the resistor column with high accuracy. The model employs a special virtual electrode geometry whose shape and position are determined by a multi-parametric optimization procedure. On the basis thereof, a numerical optimization of the 3-D grading system for an arbitrarily large arrester can be efficiently performed on the basis of transient, electroquasistatic simulations of its axisymmetric equivalent. This procedure is demonstrated for the standard International Electrotechnical Commission (IEC) surge arrester. The detailed electrothermal analysis of this arrester shows that an immense improvement of the field and thermal stress distribution in the resistor column can be obtained.

Electro-thermal assessment of heterojunction tunnel-FET for low-power digital circuits

To overcome the fundamental limitations of conventional MOSFETs, tunnel field effect transistors (TFETs) with strained-SiGe channel (via heterogeneous integration) may be used and is demonstrated using TCAD simulations. We mainly focus on the design and implementation of silicon-germanium (SiGe)-based tunnel field effect transistor, aiming to reduce the device operation voltage down to below 0.5 V. Physics-based electro-thermal simulations are performed for evaluating the self-heating (temperature rise) in the devices. We present the results of the electro-thermal analysis supported by effective 2D and 3D device simulations. Performance improvement in drain current as high as 200% has been achieved.

An Improved Algorithm for Drift Diffusion Transport and Its Application on Large Scale Parallel Simulation of Resistive Random Access Memory Arrays

A hybrid finite volume–finite element method which can avoid overestimation of volume shared by each vertex in the meshing grid is proposed to solve the diffusive transport governed continuity equation. The simulation results demonstrate that the improved algorithm can eliminate unphysical distortions as compared to the conventional Scharfetter Gummel method. Based on the proposed algorithm, a parallel-computation simulator is developed for large-scale electrothermal simulation of resistive random access memory (RRAM) arrays, in which the domain decomposition method and J parallel adaptive unstructured mesh applications infrastructure are adopted. The validity, speedup, and scalability of the parallel simulator are investigated on the TianHe-2 supercomputer. Based on the simulated results, the electrothermal characteristics and reliability analysis of large-scale RRAM arrays are investigated in detail.

Online Fatigue Estimation and Prediction of Switching Device in Urban Railway Traction Converter Based on Current Recognition and Gray Model

It is difficult to estimate and predict the fatigue state of switchgear in traction converter of urban Electric Multiple Unit (EMU). At the same time, this is just the purpose of this paper. Firstly, the power loss and junction temperature of the device are analyzed by using the results of electro-thermal simulation and actual working conditions. Then, a novel Bi-directional accelerated fatigue test (BAFT) is proposed, and the data obtained from BAFT are used to establish a fitting fatigue model for a specific switch device. BAFT data enable the fatigue interaction between Insulated Gate Bipolar Transistor (IGBT) and Free Wheeling Diode (FWD) to be displayed in the common switching device module, and the interaction is represented by an acceleration factor. In the fatigue model, the service life and fatigue state of the equipment is related to the equivalent fatigue current which is used to generate the fatigue. The equivalent fatigue current is observed by the proposed analysis identification model. The historical data of fatigue values are subjected to Gray Model (GM) (2, 1) for trend prediction, and then the service time of the switch device is calculated using the predicted fatigue values from GM (2, 1). The examples presented in this paper are based on recorded field data.

Fully Coupled Electrothermal Simulation of Large RRAM Arrays in the “Thermal-House”

Thermal crosstalk in a highly integrated RRAM array due to the self-heating effect is one of the most critical issues affecting device reliability. In this paper, two types of “thermal-house” structures are proposed to optimize the thermal management of the RRAM array. An in-house developed parallel simulator is employed to study the performance of the proposed thermal house structures in terms of resistance ratio and crosstalk temperature. It is demonstrated that the proposed thermal house structures can help to reduce thermal crosstalk in high-density RRAM arrays. Some suggestions are also provided for further improving the thermal management capability of the thermal houses as well.

Electro-thermal assessment of heterojunction tunnel-FET for low-power digital circuits

To overcome the fundamental limitations of conventional MOSFETs, tunnel field effect transistors (TFETs) with strained-SiGe channel (via heterogeneous integration) may be used and is demonstrated using TCAD simulations. We mainly focus on the design and implementation of silicon-germanium (SiGe)-based tunnel field effect transistor, aiming to reduce the device operation voltage down to below 0.5 V. Physics-based electro-thermal simulations are performed for evaluating the self-heating (temperature rise) in the devices. We present the results of the electro-thermal analysis supported by effective 2D and 3D device simulations. Performance improvement in drain current as high as 200% has been achieved.

Thermal performance analysis of GaN nanowire and fin-shaped power transistors based on self-consistent electrothermal simulations

We present self-consistent electrothermal simulations of the GaN nanowire-based field-effect transistor (NWFET) and vertical fin field-effect transistor (FinFET) by taking into account all major heat flux paths. Simulation results of a NWFET validated by experimental data are compared to the results of a vertical FinFET designed with same sizes that ensures a fair comparison of their thermal performance. It is found that the peak temperature in the NWFET is close to the uppermost contact, which facilitates heat removal from top. As a result, NWFETs have the potential to achieve a higher power density at a temperature limit compared with the FinFETs, especially when the heat removal from the top contact is eased. The impact of the thermal surface resistance of the top contact and substrate thinning on the thermal performance of these two vertical structures is also investigated.

Static and Dynamic Effects of the Incomplete Ionization in Superjunction Devices

The incomplete ionization of impurity atoms affects the free carrier concentration of several wide-bandgap (WBG) semiconductor materials even at room temperature, thus modifying the electrical properties of power devices. In this paper, the influence of the partial ionization of the dopants on the static and dynamic behaviors of WBG semiconductor-based superjunction (SJ) devices has been numerically investigated through extensive 2-D finite-element simulations. While this physical effect has only a minor impact on the static device characteristics, if a reverse bias pulse with a rise time comparable or smaller than the ionization time constant is applied to the structure, a “dynamic ionization” phenomenon can take place. The onset of this time-dependent ionization is the cause of charge unbalance effects in the device structure, due to the temporal dependence of the activated number of dopants. Electrothermal simulations, which have been carried out for a 4H-silicon carbide SJ diode, show a nonuniform temperature distribution during the transients which leads, in turn, to the self-heating of the device. Through an accurate redesign of the cross-sectional view of the device, the drawbacks of the incomplete ionization have been mitigated and the device performance enhanced.

Energy efficient spark plasma sintering: Breaking the threshold of large dimension tooling energy consumption

AbstractAn energy efficient spark plasma sintering method enabling the densification of large size samples assisted by very low electric current levels is developed. In this method, the electric current is concentrated in the graphite foils around the sample. High‐energy dissipation is then achieved in this area enabling the heating and full densification of large (alumina) parts (Ø 40 mm) at relatively low currents (800 A). The electrothermal mechanical simulation reveals that the electric current needed to heat the large samples is 70% lower in the energy efficient configuration compared to the traditional configuration. The presence of thermal and densification gradients is also revealed for the larger size samples. Potential solutions for this problem are discussed. The experiments confirm the possibility of full densification (96%‐99%) of large alumina samples. This approach allows using small (and low cost) SPS devices (generally limited to 10‐15 mm samples) for large‐size samples (40‐50 mm). The developed technique enables also an optimized energy consumption by large‐scale SPS systems.

Understanding Electromigration in Cu-CNT Composite Interconnects: A Multiscale Electrothermal Simulation Study

In this paper, we report a hierarchical simulation study of the electromigration (EM) problem in Cu-carbon nanotube (CNT) composite interconnects. This paper is based on the investigation of the activation energy and self-heating temperature using a multiscale electrothermal simulation framework. We first investigate the electrical and thermal properties of Cu-CNT composites, including contact resistances, using the density functional theory and reactive force field approaches, respectively. The corresponding results are employed in macroscopic electrothermal simulations taking into account the self-heating phenomenon. Our simulations show that although Cu atoms have similar activation energies in both bulk Cu and Cu-CNT composites, Cu-CNT composite interconnects are more resistant to EM thanks to the large Lorenz number of the CNTs. Moreover, we found that a large and homogenous conductivity along the transport direction in interconnects is one of the most important design rules to minimize the EM.

A new method for estimating the conductive filament temperature in OxRAM devices based on escape rate theory

Because of the atomic nature of the system under study, an estimation of the temperature of the conductive filament (CF) in OxRAM devices as a function of the applied bias can only be obtained by means of indirect methods, usually electrothermal simulations. In this paper, a heuristic approach that combines time-dependent dielectric breakdown (TDDB) statistics for the electroformed device with field and temperature-assisted ionic transport within the framework of escape rate theory is presented. Extended expressions for the time-to-failure acceleration law (E-model) and for the Kramers' rate compatible both with the standard models at moderate/high biases and with the principle of detailed balance at equilibrium are proposed. An approximate expression for the CF temperature is reported. For the investigated stress voltage range (0.30 V–0.65 V), the estimated CF temperature at the SET condition is found to be in the range 350 K–600 K.

Voltage oscillations during surge pulses induced by self-extinguishing non-destructive second breakdown in pn-junction diodes

PN-junction diodes with different breakdown voltages have been subjected to surge pulses per the standard IEC 61000-4-5 and their transient behaviour has been studied. For medium breakdown voltages (20–40 V) at high surge currents large transient oscillations in the voltage drop across the diodes are observed. After such an event, the devices are still operational. 3D electro-thermal TCAD simulations have been done to understand the phenomenon. A comparison between measurement and simulation reveals that the periodic voltage drop is caused by non-destructive second breakdown.

Measurement and simulation of charge accumulation on a disc spacer with electro-thermal stress in SF6 gas

The problem of surface charge accumulation on a gas-spacer interface is one of the critical factors that limits the application of DC gas-insulated transmission lines (DC GIL). In previous works, the models of surface charge accumulation under different temperatures on a spacer were investigated. However, these simulation models lacked experimental verification. In this paper, a coupled electro-thermal field simulation model for charge accumulation on a spacer under DC stress is introduced, and an experimental platform with the ability to load the thermal field on a spacer is built. Then, the effect of the thermal gradient on the surface charge of a spacer is investigated by measurements and simulations. The results indicate that for the spacer sample in the experiment, the average surface potential increased 277 %, and the variation range of that increased 309 % as the conductor temperature increased from 303.2 to 343.0 K. The coupled electro-thermal field simulation model of charge accumulation on a spacer under DC stress was verified by experimental results. The simulation model will hopefully be helpful for the insulation design of DC-GIL.