Electrothermal Effects Research Articles

(Invited) Towards 0.7 Terahertz Silicon Germanium Heterojunction Bipolar Technology – The DOTSEVEN Project

DOTSEVEN is a very ambitious 3.5 year R&D project targeting the development of Silicon Germanium (SiGe) Heterojunction Bipolar Transistor (HBT) technologies with cut-off frequencies (fmax) of around 700 GHz. The project is publically funded within the European Union's Seventh Framework Program for Research (FP7). It started in October 2012 and is now at the beginning of its second year. 10 out of the 12 DOTSEVEN participants were already partnering in the predecessor FP7 project DOTFIVE which succeeded for the first time to push fmax of SiGe HBTs (at room temperature) into the 500 GHz region thus setting a new world-wide benchmark.For a given lithography node, SiGe HBTs provide much higher cut-off frequencies compared to CMOS transistors while offering higher power density and better analog performance. Although ITRS roadmaps [1] predict ever increasing cut-off frequencies for CMOS transistors up to the THz region, there are indications [2,3] that at least fmax will saturate around the 28 nm node at 300-400 GHz (depending on the extraction methodology) and will even drop beyond that node due to parasitic gate resistance [Figure 1]. Thus SiGe HBT technology and SiGe HBT enhanced CMOS (SiGe BiCMOS) will be key enablers for demanding mm-wave systems requiring more than a few µWatts of RF output power (which limits the applicability of even very advanced CMOS) and also offer high integration levels at low cost which precludes expensive and less integrated III-V solutions. Physics based simulations predict “balanced” fT/fmax limits of well above 1 THz for SiGe HBTs [4]. In DOTFIVE the practical performance limits of lateral and vertical scaling of standard HBT topologies which are in use in 200-300 GHz production processes at STM and IFX have shown up clearly. With these conventional DPSA (double poly self aligned) emitter / base structures incorporating an external poly-silicon base-contact which is vertically linked to the SEG (selectively grown) SiGe:C base layer, we were not able to surpass a “400 GHz” limit of fmax significantly [5]. Reaching 500 GHz was only possible with two new transistor architectures realized by IHP employing novel “lateral base-link” [6] or “epitaxial external base-contact” concepts [7] which will be utilized in DOTSEVEN as starting point as they have potential to overcome a basic weakness of conventional DPSA structures: their relatively high-resistive external base-link region [8,9].Within the project the capabilities and benefits of >500 GHz SiGe HBT technology will be demonstrated by benchmark circuits and advanced system applications in the 0.1 to 1 THz range like THz imaging and sensing, wireless Gb/s communications and millimeter-wave radar. Process development will be strongly supported by TCAD and physics-based modeling including 3D, nanoscale and thermal effects where as "first-time-right" circuit design will be enabled by accurate compact device modeling including electro-thermal, substrate and other parasitic effects. Of course these models have to be based on accurately extracted parameter sets for which in turn accurate and reliable characterization and extraction methodologies are mandatory. High current densities, excessive self-heating and operation at collector-emitter voltages above the open-base VCE breakdown limit might impose considerable reliability risks in downscaled HBTs and limit their practical application. Therefore in-depth reliability modeling and investigations will be undertaken in the project too.

Electrothermal effects of advanced electromagnetic materials on electrical loss and thermal characteristics of synchronous rotational machines

Electrothermal characteristics of high speed synchronous rotational machine were studied experimentally and theoretically. Electrical rotating machines with three types of electromagnetic materials were fabricated to identify core losses in the iron cores of high speed synchronous machine. Dummy and isolated rotational machines were devised to measure pure mechanical losses and to examine conductive thermal resistances in stators. Results showed that mechanical losses are linearly proportional to driving speeds of electrical rotating machines, and that viscosity-induced mechanical losses in air are relatively negligible in comparison with friction-induced mechanical losses. It was found that an electrical rotating machine with 15HTH1000 electromagnetic material has relatively lower core and coil losses than the machines with 35PN440, 35PN250, thereby making it applicable for advanced high speed electric vehicular applications. In addition, core materials had a negligible effect on torque constants of the electrical rotating machines at a lower non-dimensional rotational speed of 0.2. However, the rotating machine with 15HTH1000 showed an 8.0% higher input current than did that with 35PN440 at a higher rotational speed of 0.9. Finally, thermal resistances in between iron cores and stators could be accurately characterized by a nonlinear empirical formula with the minimum R2 value of curve fittings equal to 0.997 which reflects the convection effects on heat transfer enhancement.

Calculation of Power Transfer Limit Considering Electro-Thermal Coupling of Overhead Transmission Line

In this paper, new formulations of the power flow and continuation power flow that allow for electro-thermal coupling in transmission lines have been proposed. The new formulations capture the overhead lines' electro-thermal coupling effects by treating their series resistances as temperature dependent variables. They generate results that can differ from the results of conventional formulations markedly, particularly for problems centring on line impedances. The paper demonstrates this by applying the new formulations to the power transfer limit calculation. Generally, power transfer limits are defined either by encountering a line's thermal limit or detecting the onset of voltage collapse in the system. Studies based on 2-bus and 14-bus test systems are used to demonstrate the efficacy of the new formulations for both situations. For these studies, specific point-to-point power transfer limits are calculated with and without the lines' electro-thermal coupling effects and the results are compared.

Spurious phenomena occurring during current measurement on ultra-thin dielectric layers: From electro-thermal effects to surface damage

In this paper, the conduction properties of dielectric ultra-thin layers are studied using atomic force microscopy. Especially, the conductive-atomic force microscope allows to measure the leakage current at the nanoscale and to study the degradation mechanisms locally. Nonetheless, the dielectric layer seems to be damaged by a technique's specific phenomenon: hillocks appear when a positive tip bias is applied on different dielectrics. In this paper, the formation of these hillocks is studied. Contrary to what is observed during the dielectric breakdown, the conductivity is reduced after hillocks formation which occurs after the dielectric breakdown. Moreover, we have observed the formation of cavities in the silicon substrate linked to the formation of hillocks, which is not compatible with a swelling process (as dielectric breakdown induced epitaxy). We propose that these results may be explained by an electro-thermal effect due to the large dissipated energy, maybe combined with the oxidation of the substrate. Finally, the interdependence of measurements is demonstrated during serial acquisition.

Numerical Modeling of Thermoelectric Thomson Effect in Phase Change Memory Bridge Structures

Phase change memory is a non-volatile memory technology that utilizes the electrical resistivity contrast between resistive amorphous and conductive crystalline phases of phase change materials. These devices operate at high current densities and high temperature gradients which lead to significant thermoelectric effects. We have performed numerical modeling of electrothermal effects in p-type Ge2Sb2Te5 phase change memory structures suspended on TiN contact pads using COMSOL Multiphysics. Temperature dependent material parameters are used in the model. Strong asymmetry is observed in temperature profiles in all cases: the hottest spot appears closer to the higher potential end suggesting that the thermal profile can be significantly altered by the thermoelectric effects during device operation. Hence, thermoelectric effects need to be considered for device designs for lower power and higher reliability devices.

Accurate simulation of thermoelectric power generating systems

Recent interest in the use of thermoelectric generators (TEGs) to recover waste heat in large-scale applications calls for precise simulation to appropriately design complicated and dynamic systems. The aim of this work is to develop a computer tool to accurately simulate the thermal and electrical dynamics of a real thermoelectric (TE) power generating system.The computer-aided model presented here is able to accurately simulate the non-linear electro-thermal coupled effects which occur during changes in the operating conditions, e.g. temperature or load changes.Simulation results are compared to experimental data obtained from a real TE system. The comparison shows great accuracy both during transients and in the steady-state, thus validating the model as a reliable tool to simulate TE generating systems.

Analysis of the UIS behavior of power devices by means of SPICE-based electrothermal simulations

This paper presents an accurate, yet computationally effective, 3-D simulation strategy devised for the UIS analysis of multicellular power transistors, which accounts for electrothermal effects and is based on a circuit representation of the whole device under test. The approach is exploited to examine the correlation between the shape of the avalanche curve of IGBTs and the physical mechanisms occurring during UIS discharging. In particular, an in-depth investigation is performed on a current hopping phenomenon that – although not usually destructive – entails a reduction in both ruggedness and reliability of the device.

Ultra Dielectrophoresis Using Atomic Layer Deposited Films: Electrothermal Analysis

By increasing the ability to apply high ac voltages at interdigitated electrodes, and thus significantly improving the strength of negative dielectrophoresis (nDEP) force, we demonstrate a significantly enhanced electrokinetic actuation microsystem. A pinhole-free, nanometer-scale, SiO2 film was deposited on electrodes, using atomic layer deposition, as a protective layer to protect the electrodes from corrosion, when applying high ac voltages (>10 V). The electrodes were capacitively coupled to the electrolyte buffer by the application of a high frequency ac voltage signal, thus avoiding electric field degradation and the consequent reduction in DEP force due to the presence of the insulating oxide layer. Additionally, temperature profile of this high voltage system was simulated to ensure that our system is not disturbed by electrothermal effects. To illustrate an application, we used this ultra-DEP (uDEP) device to demonstrate 100% detachment of anti- IgG and IgG bound beads from the surface.

Direct two-dimensional electrochemical impedance spectra simulation for solid oxide fuel cell

A two-dimensional (2D) EIS simulation approach is developed by solving a SOFC unit cell model with imposed sinusoidal voltage perturbations at different frequencies. The transient SOFC unit cell model describes the intricate interdependency among the ionic/electronic conduction, multi-component species transport, electrochemical reaction processes and electrode microstructure as well as the coupling processes of mass, energy, momentum transport within flow channels. The model calculates the local transient response and impedance spectra as a function of channel position. The effects of the reaction depletion, product accumulation as well as the temperature variation along the flow channels on the EIS spectra are numerically simulated with a counter-flow mode. The results show that the convection-diffusion process along the flow channel has significant effects on the low frequency half circle of the impedance spectra. The temperature oscillations accumulate along the flow channels, and then affect the current responses which probably lead to an electro-thermal impedance effects.

Ultrahigh Frequency Voltammetry: Effect of Electrode Material and Frequency of Alternating Potential Modulation on Mass Transport at Hot-Disk Microelectrodes

Ultrahigh frequency voltammetry involves low scan rate voltammetric measurements with microelectrodes polarized by high-frequency large-amplitude alternating potential. The method provides a simple means for studying electrothermal and dielectrophoretic effects, which are important in micro and nanofluidic systems. The method also allows for indirect measurements of electrode impedance at gigahertz frequencies. This increases the upper frequency limit in impedance measurements about 1000 times. In this work we demonstrated, for the first time, that the effect of dielectric relaxation of water can be observed in a simple voltammetric experiment. The paper focuses on the description of electrothermal convection at ac heated disk microelectrodes as a function of frequency and provides a comparison of numerical simulations with experimental results.

Electrothermally driven current vortices in inhomogeneous bipolar semiconductors

We report an effect that occurs in semiconductors where internal electrical fields interact with a temperature gradient. Steady current vortices and a magnetic field develop in the system, even without external carrier injection. The effect is electrodynamic, energy dissipative, and fundamentally distinct from any previously described electrothermal effects. In bipolar structures the effective thermopower can be significantly modified by the vortices. Joule heating arising from the vortices reduces the thermal conductivity by an amount comparable to the electronic thermal conductivity.

Analysis of Electrothermal Effects in Bipolar Differential Pairs

An extensive experimental and theoretical analysis of bipolar differential pairs subject to radical electrothermal feedback is presented. Measurements demonstrate that considerable thermally-induced degradation of circuit characteristics may occur, eventually turning into the full disappearance of a linear region, which is replaced by a hysteresis behavior under voltage-controlled conditions. An analytical model is derived for a simple yet reliable prediction of the distortion of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$I$</tex></formula> – <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$V$</tex></formula> curves. A more elaborated circuit approach is employed to accurately quantify the concurrent destabilizing action of electrothermal and impact ionization effects, as well as to evaluate the impact of layout asymmetries and examine the beneficial influence of emitter degeneration resistors. Simulation results are found to compare favorably with experiments performed on silicon-on-glass test structures with various layouts and isolation schemes, from which the benefits of thermally coupling the two devices become evident.

Electrothermal Access Resistance Model for GaN-Based HEMTs

The high-power density in GaN-based high-electron-mobility transistors (HEMTs) increases demands on the accurate extraction and modeling of electrothermal effects such as self-heating. This paper presents a new electrothermal model of the access resistances in GaN HEMTs, taking into account both self heating and bias dependence. A coplanar ungated transfer length method (TLM) structure has been used to extract the resistance versus temperature, bias, and RF power. The temperature dependence is extracted from dc measurements at ambient temperatures between 293 and 443 K. Small-signal measurements are used to extract the time constants in the thermal impedance. The bias dependence of the current is characterized by isothermal large-signal RF measurements between 1 and 6 GHz. A new method for extracting the thermal resistance from the large-signal measurements together with temperature-dependent dc measurements is also presented.

ELECTROTHERMAL EFFECTS IN HIGH DENSITY THROUGH SILICON VIA (TSV) ARRAYS

Electrothermal effects in various through silicon via (TSV) arrays are investigated in this paper. An equivalent lumped-element circuit model of a TSV pair is derived. The temperature-dependent TSV capacitance, silicon substrate capacitance and conductance are examined for low-, medium-, and high-resistivity silicon substrates, respectively. The partial-element equivalent-circuit (PEEC) method is employed for calculating per-unit-length (p.u.l.) resistance, inductance, insertion loss and characteristic impedances of copper and polycrystalline silicon (poly-Si) TSV arrays, and their frequencyand temperature-dependent characteristics are treated rigorously. The modified time-domain finite-element method (TD-FEM), in the presence of a set of periodic differential-mode voltage pulses, is also employed for studying transient electrothermal responses of 4and 5-TSV arrays made of different materials, with their maximum temperatures and thermal crosstalk characterized thoroughly.

Electrothermal Fluid Manipulation of High-Conductivity Samples for Laboratory Automation Applications

Electrothermal flow is a promising technique in microfluidic manipulation toward laboratory automation applications, such as clinical diagnostics and high throughput drug screening. Despite the potential of electrothermal flow in biomedical applications, relative little is known about electrothermal manipulation of highly conductive samples, such as physiological fluids and buffer solutions. In this study, the characteristics and challenges of electrothermal manipulation of fluid samples with different conductivities were investigated systematically. Electrothermal flow was shown to create fluid motion for samples with a wide range of conductivity when the driving frequency was above 100 kHz. For samples with low conductivities (below 1 S/m), the characteristics of the electrothermal fluid motions were in quantitative agreement with the theory. For samples with high conductivities (above 1 S/m), the fluid motion appeared to deviate from the model as a result of potential electrochemical reactions and other electrothermal effects. These effects should be taken into consideration for electrothermal manipulation of biological samples with high conductivities. This study will provide insights in designing microfluidic devices for electrokinetic manipulation of biological samples toward laboratory automation applications in the future.

Electrothermal flow effects in insulating (electrodeless) dielectrophoresis systems

We simulate electrothermally induced flow in polymeric, insulator-based dielectrophoresis (iDEP) systems with DC-offset, AC electric fields at finite thermal Péclet number, and we identify key regimes where electrothermal (ET) effects enhance particle deflection and trapping. We study a single, two-dimensional constriction in channel depth with parametric variations in electric field, channel geometry, fluid conductivity, particle electrophoretic (EP) mobility, and channel electroosmotic (EO) mobility. We report the effects of increasing particle EP mobility, channel EO mobility, and AC and DC field magnitudes on the mean constriction temperature and particle behavior. Specifically, we quantify particle deflection and trapping, referring to the deviation of particles from their pathlines due to dielectrophoresis as they pass a constriction and the stagnation of particles due to negative dielectrophoresis near a constriction, respectively. This work includes the coupling between fluid, heat, and electromagnetic phenomena via temperature-dependent physical parameters. Results indicate that the temperature distribution depends strongly on the fluid conductivity and electric field magnitude, and particle deflection and trapping depend strongly on the channel geometry. Electrothermal (ET) effects perturb the EO flow field, creating vorticity near the channel constriction and enhancing the deflection and trapping effects. ET effects alter particle deflection and trapping responses in insulator-based dielectrophoresis devices, especially at intermediate device aspect ratios (2 ≤ r ≤ 7) in solutions of higher conductivity (σ m ≥ 1 × 10(-3)S/m). The impact of ET effects on particle deflection and trapping are diminished when particle EP mobility or channel EO mobility is high. In almost all cases, ET effects enhance negative dielectrophoretic particle deflection and trapping phenomena.

Cover Picture: Electrophoresis 21'2010

AbstractIssue no. 22 is a special issue on “Miniaturization” and comprises 21 contributions 3 of them are Fast Track contributions. The other 18 manuscripts are distributed over 3 distinct parts. Part I is on Fundamentals and has 5 research articles on various aspects of fundamentals including gradient elution moving boundary electrophoresis, influence of cross‐sectional area and zeta potential on separations in hybrid‐chip‐based electrophoresis, collision of DNA with nanowires, and retention modeling for solid microfabricated pillar columns. Part II is on New Technology, and contains 9 full research articles. Various innovative applications are described in 4 research articles making up Part III of the issue.FAST TRACK articles:Electrothermal flow effects in insulating (electrodeless) dielectrophoresis systems (10.1002/elps.201000429))Characterization of particle capture in a sawtooth patterned insulating electrokinetic microfluidic device (10.1002/elps.201000438))Analysis of carbonaceous biomarkers with the Mars Organic Analyzer microchip capillary electrophoresis system: Aldehydes and ketones (10.1002/elps.201000424))

Real-Time Compact Thermal Models for Health Management of Power Electronics

Implementation of real-time health assessment and thermal management of power electronic devices require real-time electrothermal models that can be used to predict the temperatures of device junctions, interfaces, etc., which cannot ordinarily be measured during service. This paper presents a real-time reduced-order compact thermal model, which is incorporated in a pulsewidth modulation and current controlled full bridge. An accurate representation of the dynamic thermal behavior was obtained experimentally and converted into a simplified multiexponential form then combined with lookup tables that provide estimates of the device losses based on measured values of the phase current. For interfaces away from the surface, such as solder layers, a validated Flotherm model is used to predict the temperatures of the hidden layers. Comparison of the real-time temperature estimates with IR measured values obtained from a high-speed IR camera showed that the reduced-order model was capable in estimating the module's temperatures over a range of modulation conditions. This real-time model is well-suited to the continuous monitoring of the internal behavior of the electrothermal effects within power electronic modules and can thus be used as part of a prognostic tool to provide knowledge through thermal cycling by combining with thermomechanical wear out models for health management of power electronics.

Comprehensive Compact Models for the Circuit Simulation of Multichip Power Modules

This paper proposes the model development of a packaged semiconductor power module, for use in a circuit simulation environment. Focusing on railway traction applications, it considers a state-of-the-art 6.5 kV insulated gate bipolar transistor (IGBT) module, taking into account the antiparallel connection of IGBTs and free-wheeling diodes, and including all main electrothermal and electromagnetic effects associated with the multichip structure. The description of semiconductor physics is coupled with self-heating effects; electromagnetic phenomena associated with the packaging, layout, and interconnections are also taken into account. To optimize the compromise between accuracy and computational effort, the device models are based on a mixed physical and behavioral description, and are scalable to be representative of a desired number of parallel devices and to allow for the introduction of parasitic elements, as required. Electromagnetic effects are modeled by means of equivalent lumped elements, extracted by numerical simulation of an accurate 3-D structural assembly model. The description of transient thermal phenomena relies on a finite-difference approach that considers the use of both essentially 1-D lumped equivalent models and fully 3-D distributed description. The model is validated statically and dynamically, against both data-sheet information and measurements; a selection of simulation examples demonstrates its usefulness and versatility. Although developed for a specific application scenario, the proposed approach is of general validity.

An Investigation of Electrothermal Characteristics on Low-Temperature Polycrystalline-Silicon Thin-Film Transistors

In this paper, theoretical and experimental analyses have been performed to explore the electrothermal characteristics in low-temperature polycrystalline-silicon thin-film transistors (LTPS TFTs). A theoretical simulation reveals that electrothermal effects strongly influence the performance of LTPS TFTs, particularly under high-current conditions. Measurements show that, under extremely high currents, LTPS devices demonstrate an open-circuit behavior, while the behavior of devices based on single-crystalline silicon films using an SOI CMOS process results in a short-circuit model. In summary, both the simulation and measurements indicate that grain boundaries will degrade the electrothermal properties of the LTPS thin film and suppress reliability in TFT design.