Engineering Commons^™

Human Body–Electrode Interfaces For Wide-Frequency Sensing And Communication: A Review, Kurian Polachan, Baibhab Chatterjee, Scott Weigand, Shreyas Sen

Department of Electrical and Computer Engineering Faculty Publications

Several on-body sensing and communication applications use electrodes in contact with the human body. Body–electrode interfaces in these cases act as a transducer, converting ionic current in the body to electronic current in the sensing and communication circuits and vice versa. An ideal body–electrode interface should have the characteristics of an electrical short, i.e., the transfer of ionic currents and electronic currents across the interface should happen without any hindrance. However, practical body–electrode interfaces often have definite impedances and potentials that hinder the free flow of currents, affecting the application’s performance. Minimizing the impact of body–electrode interfaces on the application’s …

Limitations Of Zt As A Figure Of Merit For Nanostructured Thermoelectric Materials, Xufeng Wang, Mark Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

Thermoelectric properties of nanocomposites are numerically studied as a function of average grain size or nanoparticle density by simulating the measurements as they would be done experimentally. In accordance with previous theoretical and experimental results, we find that the Seebeck coefficient, power factor and figure of merit, zT, can be increased by nanostructuring when energy barriers exist around the grain boundaries or embedded nanoparticles. When we simulate the performance of a thermoelectric cooler with the same material, however, we find that the maximum temperature difference is much less than expected from the given zT. This occurs because the …

Thermodynamic Efficiency Limits Of Classical And Bifacial Multi-Junction Tandem Solar Cells: An Analytical Approach, Muhammad A. Alam, Mohammad Ryyan Khan

Department of Electrical and Computer Engineering Faculty Publications

Bifacial tandem cells promise to reduce three fundamental losses (i.e., above-bandgap, below bandgap, and the uncollected light between panels) inherent in classical single junction photovoltaic (PV) systems. The successive filtering of light through the bandgapcascade and the requirement of current continuity make optimization of tandem cellsdifficult and accessible only to numerical solution through computer modeling. The challenge is even more complicated for bifacial design. In this paper, we use an elegantly simple analytical approach to show that the essential physics of optimization is intuitively obvious, and deeply insightful results can be obtained with a few lines of algebra. This powerful …

Bifacial Si Heterojunction-Perovskite Organic-Inorganic Tandem To Produce Highly Efficient (Η T * ~ 33%) Solar Cell, Reza Asadpour, Raghu Vamsi Krishna Chavali, Mohammad Ryyan Khan, Muhammad Ashraful Alam

Department of Electrical and Computer Engineering Faculty Publications

As single junction photovoltaic (PV) technologies both Si heterojunction (HIT) and perovskite based solar cells promise high efficiencies at low cost. Intuitively a traditional tandem cell design with these cells connected in series is expected to improve the efficiency further. Using a self-consistent numerical modeling of optical and transport characteristics however we find that a traditional series connected tandem design suffers from low JSC due to band-gap mismatch and current matching constraints. Specifically a traditional tandem cell with state-of-the-art HIT ( η=24% ) and perovskite ( η=20% ) sub-cells provides only a modest tandem efficiency of η_T~ 25%. …

A Simple Boltzmann Transport Equation For Ballistic To Diffusive Transient Heat Transport, Mark S. Lundstrom, Jesse Maassen

Department of Electrical and Computer Engineering Faculty Publications

Developing simplified, but accurate, theoretical approaches to treat heat transport on all length and time scales is needed to further enable scientific insight and technology innovation. Using a simplified form of the Boltzmann transport equation (BTE), originally developed for electron transport, we demonstrate how ballistic phonon effects and finite-velocity propagation are easily and naturally captured. We show how this approach compares well to the phonon BTE, and readily handles a full phonon dispersion and energy-dependent mean-free-path. This study of transient heat transport shows (i) how fundamental temperature jumps at the contacts depend simply on the ballistic thermal resistance, (ii) that …

Computational Electronics For The 21st Century: Reflections On The Past, Present, And Future, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The author’s career has coincided with the development of numerical simulation into an essential component of semiconductor device technology research and development. We now have a sophisticated suite of simulation capabilities along with new challenges for 21st Century electronics. This talk presents a short history of the field and a description of the current state of the art, but it concentrates on lessons learned and thoughts about how computational electronics can continue to contribute effectively to the development of new electronic device technologies. The author will argue that electronics is changing, and that computational electronics can play a key role …

Steady-State Heat Transport: Ballistic-To-Diffusive With Fourier's Law, Jesse Maassen, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

It is generally understood that Fourier's law does not describe ballistic phonon transport, which is important when the length of a material is similar to the phonon mean-free-path. Using an approach adapted from electron transport, we demonstrate that Fourier's law and the heat equation do capture ballistic effects, including temperature jumps at ideal contacts, and are thus applicable on all length scales. Local thermal equilibrium is not assumed, because allowing the phonon distribution to be out-of-equilibrium is important for ballistic and quasi-ballistic transport. The key to including the non-equilibrium nature of the phonon population is to apply the proper boundary …

Photovoltaic Material Characterization With Steady-State And Transient Photoluminescence, Xufeng Wang, Jayprakash Bhosale, James Moore, Rehan Kapadia, Peter Bermel, Ali Javey, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

In this study, we develop an approach to characterize the surface and bulk properties for thin films of photovoltaic materials by combining two experimental photoluminescence (PL) techniques with one multiphysics simulation. This contactless, in-line characterization technique allows reliable extraction of key lifetime parameters. In this study, we first discuss the strengths and weaknesses of both steady-state and transient PL techniques (specifically, steady-state PL excitation spectroscopy and time-resolved PL) and show that combining them with numerical simulation can be used to extract surface and bulk lifetimes self consistently. The method is applied to InP thin films grown with a novel vapor-liquid-solid …

Emission-Diffusion Theory Of The Mosfet, Mark S. Lundstrom, Supriyo Datta, Xingshu Sun

Department of Electrical and Computer Engineering Faculty Publications

An emission-diffusion theory that describes MOSFETS from the ballistic to diffusive limits is developed. The approach extends the Crowell-Sze treatment of metalsemiconductor junctions to MOSFETs and is equivalent to the scattering/transmission model of the MOSFET. The paper demonstrates that the results of the transmission model can be obtained from a traditional, drift-diffusion analysis when the boundary conditions are properly specified, which suggests that traditional drift-diffusion MOSFET models can also be extended to comprehend ballistic limits.

Mass Media And The Contagion Of Fear: The Case Of Ebola In America, Sherry M J Towers, Shehzad Afzal, Gilbert Bernal, Nadya T. Bliss, Baltazar Espinoza, Jasmine Jackson, Julia Judson-Garcia, Maryam Khan, Michael Lin, Robert Mamada, Victor M. Moreno, Fereshteh Nazari, Kamaldeen Okumeye, Mary L. Ross, Claudia Rodriguez, Jan Medlock, David S. Ebert, Carlos Castillo-Chavez,

Department of Electrical and Computer Engineering Faculty Publications

Background: In the weeks following the first imported case of Ebola in the U. S. on September 29, 2014, coverage of the very limited outbreak dominated the news media, in a manner quite disproportionate to the actual threat to national public health; by the end of October, 2014, there were only four laboratory confirmed cases of Ebola in the entire nation. Public interest in these events was high, as reflected in the millions of Ebola-related Internet searches and tweets performed in the month following the first confirmed case. Use of trending Internet searches and tweets has been proposed in the …

Performance-Limiting Factors For Gaas-Based Single Nanowire Photovoltaics, Xufeng Wang, Mohammad Ryyan Khan, Mark Lundstrom, Peter Bermel

Department of Electrical and Computer Engineering Faculty Publications

GaAs nanowires (NWs) offer the possibility of decoupling light absorption from charge transport for high-performance photovoltaic (PV) devices. However, it is still an open question as to whether these devices can exceed the Shockley-Queisser efficiency limit for single-junction PV. In this work, single standing GaAs-based nanowire solar cells in both radial and vertical junction configurations is analyzed and compared to a planar thin-film design. By using a self-consistent, electrical-optically coupled 3D simulator, we show the design principles for nanowire and planar solar cells are significantly different; nanowire solar cells are vulnerable to surface and contact recombination, while planar solar cells …

Thermoelectric Properties Of Epitaxial Scn Films Deposited By Reactive Magnetron Sputtering Onto Mgo(001) Substrates, Polina V. Burmistrova, Jesse Maassen, Tela Favaloro, Bivas Saha, Shuaib Salamat, Yee Rui Koh, Mark S. Lundstrom, Ali Shakouri, Timothy D. Sands

Department of Electrical and Computer Engineering Faculty Publications

Epitaxial ScN(001) thin films were grown on MgO(001) substrates by dc reactive magnetron sputtering.The deposition was performed in an Ar/N2 atmosphere at 2 x 10−3 Torr at a substrate temperature of 850 °C in a high vacuum chamber with a base pressure of 10−8 Torr. In spite of oxygen contamination of 1.6 +/- 1 at. %, the electrical resistivity, electron mobility, and carrier concentration obtained from a typical filmgrown under these conditions by room temperature Hall measurements are 0.22 mΩ cm, 106 cm2 V−1 s−1, and 2.5 x 1020 cm−3, respectively. These films exhibit remarkable thermoelectric power factors of 3.3–3.5 …

A Computational Study Of The Thermoelectric Performance Of Ultrathin Bi2te3 Films, Jesse Maassen, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The ballistic thermoelectric performance of ultrathin films of Bi2Te3, ranging in thickness from 1 to 6 quintuple layers, is analyzed using density functional theory combined with the Landauer approach. Our results show that the thinnest film, corresponding to a single quintuple layer, has an intrinsic advantage originating from the particular shape of its valence band, leading to a large power factor and figure-of-merit exceeding bulk Bi2Te3. The interaction between the top and bottom topological surface states is key. The thinnest film yields a six-fold increase in power factor compared to bulk.

On The Use Of Rau’S Reciprocity To Deduce External Radiative Efficiency In Solar Cells, Xufeng Wang, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

Rau’s reciprocity relation has been used to deduce the external radiative efficiency of a wide variety of solar cells using just standard solar cell measurements, but it is based on a number of assumptions, some of which may not be valid for typical thin-film solar cells. In this paper, we use rigorous optical simulations coupled with carrier transport simulations to examine some common thin film solar cells. The results provide guidance on when the Rau relation can be used, why it can fail, and on the magnitude of the errors that can be expected in practice.

Compact Models And The Physics Of Nanoscale Fets, Mark S. Lundstrom, Dimitri A. Antoniadis

Department of Electrical and Computer Engineering Faculty Publications

The device physics of nanoscale MOSFETs is reviewed and related to traditional compact models. Beginning with the Virtual Source model, a model for nanoscale MOSFETs expressed in traditional form, we show how a Landauer approach gives a clear, physical interpretation to the parameters in the model. The analysis shows that transport in the channel is limited by diffusion near the virtual source both below and above threshold, that current saturation is determined by velocity saturation near the source, not by the maximum velocity in the channel, and that the channel resistance approaches a finite value as the channel length approaches …

Thermal Conductivity Of Bulk And Thin-Film Silicon: A Landauer Approach, Changwook Jeong, Supriyo Datta, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The question of what fraction of the total heat flow is transported by phonons with different mean-free-paths is addressed using a Landauer approach with a full dispersion description of phonons to evaluate the thermal conductivities of bulk and thin film silicon. For bulk Si, the results reproduce those of a recent molecular dynamic treatment showing that about 50% of the heat conduction is carried by phonons with a mean-free-path greater than about 1 μm. For the in-plane thermal conductivity of thin Si films, we find that about 50% of the heat is carried by phonons with mean-free-paths shorter …

Can Morphology Tailoring Improve The Open Circuit Voltage Of Organic Solar Cells?, Biswajit Ray, Mark S. Lundstrom, Muhammad A. Alam

Department of Electrical and Computer Engineering Faculty Publications

While the effect of interfacial morphology on the short circuit current (ISC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (VOC) and fill-factor (FF) are less clear. Since the output power of a solar cell Pout 1/4 ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve VOC by tailoring bulk heterojunction morphology is futile, because any increase in ISC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of …

Computational Study Of Energy Filtering Effects In One-Dimensional Composite Nano-Structures, Raseong Kim, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

Possibilities to improve the Seebeck coefficient S versus electrical conductance G trade-off of diffusive composite nano-structures are explored using an electro-thermal simulation framework based on the non-equilibrium Green’s function method for quantum electron transport and the lattice heat diffusion equation. We examine the role of the grain size d, potential barrier height UB, grain doping, and the lattice thermal conductivity jL using a one-dimensional model structure. For a uniform jL, simulation results show that the power factor of a composite structure may be improved over bulk with the optimum UB being about kBT, where kB and T are the Boltzmann …

On The Best Bandstructure For Thermoelectric Performance: A Landauer Perspective, Changwook Jeong, Raseong Kim, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The question of what bandstructure produces the best thermoelectric device performance is revisited from a Landauer perspective. We find that a delta-function transport distribution function (TDF) results in operation at the Mahan-Sofo upper limit for the thermoelectric figure-of-merit, ZT. We show, however, the Mahan-Sofo upper limit itself depends on the bandwidth (BW) of the dispersion, and therefore, a finite BW dispersion produces a higher ZT when the lattice thermal conductivity is finite. Including a realistic model for scattering profoundly changes the results. Instead of a narrow band, we find that a broad BW is best. The prospects of increasing ZT …

Computational Study Of The Seebeck Coefficient Of One-Dimensional Composite Nanostructures, Raseong Kim, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The Seebeck coefficient (S) of composite nano-structures is theoretically explored within a self-consistent electro-thermal transport simulation framework using the non-equilibrium Green’s function method and a heat diffusion equation. Seebeck coefficients are determined using numerical techniques that mimic experimental measurements. Simulation results show that, without energy relaxing scattering, the overall S of a composite structure is determined by the highest barrier within the device. For a diffusive, composite structure with energy relaxation due to electron-phonon scattering, however, the measured Sis an average of the position-dependent values with the weighting factor being the lattice temperature gradient. The results stress …

Full Dispersion Vs. Debye Model Evaluation Of Lattice Thermal Conductivity With A Landauer Approach, Changwook Jeong, Supriyo Datta, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

Using a full dispersion description of phonons, the thermal conductivities of bulk Si and Bi2Te3 are evaluated using a Landauer approach and related to the conventional approach based on the Boltzmann transport equation. A procedure to extract a well-defined average phonon mean-free-path from the measured thermal conductivity and given phonon-dispersion is presented. The extracted mean-free-path has strong physical significance and differs greatly from simple estimates. The use of simplified dispersion models for phonons is discussed, and it is shown that two different Debye temperatures must be used to treat the specific heat and thermal conductivity (analogous to the two different …

On Momentum Conservation And Thermionic Emission Cooling, Raseong Kim, Changwook Jeong, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The possibility of increasing the performance of thermionic cooling devices by relaxing lateral momentum conservation is examined. Upper limits for the ballistic emission current are established. It is then shown that for most cases, nonconserved lateral momentum model produces a current that exceeds this upper limit. For the case of heterojunctions with a much heavier effective mass in the barrier and with a low barrier height, however, relaxing lateral momentum may increase the current. These results can be simply understood from the general principle that the current is limited by the location, well or barrier, with the smallest number of …

On Landauer Versus Boltzmann And Full Band Versus Effective Mass Evaluation Of Thermoelectric Transport Coefficients, Changwook Jeong, Raseong Kim, Mathieu Luisier, Supriyo Datta

Department of Electrical and Computer Engineering Faculty Publications

Using a full band description of electronic bandstructure, the Landauer approach to diffusive transport is mathematically related to the solution of the Boltzmann transport equation, and expressions for the thermoelectric parameters in both formalisms are presented. Quantum mechanical and semiclassical techniques to obtain from a full description of the bandstructure, E(k)E(k), the density of modes in the Landauer approach or the transport distribution in the Boltzmann solution are compared and thermoelectric transport coefficients are evaluated. Several example calculations for representative bulk materials are presented and the full band results are related to the more common effective mass formalism. Finally, given …

Simulation Of Spin Field Effect Transistors: Effects Of Tunneling And Spin Relaxation On Performance, Yunfei Gao, Tony Low, Mark S. Lundstrom, Dmitri E. Nikonov

Department of Electrical and Computer Engineering Faculty Publications

numerical simulation of spin-dependent quantum transport for a spin field effect transistor is implemented in a widely used simulator, nanoMOS. This method includes the effect of both spin scattering in the channel and the tunneling barrier between the source/drain and the channel. Accounting for these factors permits setting more realistic performance limits for the transistor, especially the magnetoresistance, which is found to be lower compared to earlier predictions. The interplay between tunneling and spin scattering is elucidated by numerical simulation. Insertion of the tunneling barrier leads to an increased magnetoresistance. Simulations are used to explore the tunneling barrier design issues.

Universality Of Non Ohmic Shunt Leakage In Thin Film Solar Cells, Sourabh Dongaonkar, J. D. Sevaites, G. M. Ford, S. Loser, J. Moore, R. M. Gelfand, H. Mohseni, H. W. Hillhouse, R. Agrawal, M. A. Ratner, T. J. Marks, Mark S. Lundstrom, Muhammad A. Alam

Department of Electrical and Computer Engineering Faculty Publications

http://dx.doi.org/10.1063/1.3518509

Abstract can be found at the above link

Influence Of Dimensionality On Thermoelectric Device Performance, Raseong Kim, Supriyo Datta, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

The role of dimensionality on the electronic performance of thermoelectric devices is clarified using the Landauer formalism, which shows that the thermoelectric coefficients are related to the transmission, T(E)T(E), and how the conducting channels, M(E)M(E), are distributed in energy. The Landauer formalism applies from the ballistic to diffusive limits and provides a clear way to compare performance in different dimensions. It also provides a physical interpretation of the “transport distribution,” a quantity that arises in the Boltzmann transport equation approach. Quantitative comparison of thermoelectric coefficients in one, two, and three dimensions shows that the channels are utilized more effectively in …

Modeling Of Spin Metal-Oxide-Semiconductor Field-Effect Transistor: A Nonequilibrium Green’S Function Approach With Spin Relaxation, Tony Low, Mark S. Lundstrom, Dmitri E. Nikonov

Department of Electrical and Computer Engineering Faculty Publications

A spin metal-oxide-semiconductor field-effect transistor spin MOFSET, which combines a Schottky-barrier MOFSET with ferromagnetic source and drain contacts, is a promising device for spintronic logic. Previous simulation studies predict that this device should display a very high magnetoresistance MR ratio between the cases of parallel and anitparallel magnetizations for the case of half-metal ferromagnets HMF. We use the nonequilibrium Green’s function formalism to describe tunneling and carrier transport in this device and to incorporate spin relaxation at the HMF-semiconductor interfaces. Spin relaxation at interfaces results in nonideal spin injection. Minority spin currents arise and dominate the leakage current for antparallel …

Influence Of Phonon Scattering On The Performance Of P-I-Np-I-N Band-To-Band Tunneling Transistors, Siyuranga O. Koswatta, Mark S. Lundstrom, Dmitri E. Nikonov

Department of Electrical and Computer Engineering Faculty Publications

Power dissipation has become a major obstacle in performance scaling of modern integrated circuits and has spurred the search for devices operating at lower voltage swing. In this letter, we study p-i-n band-to-band tunneling field effect transistors taking semiconducting carbon nanotubes as the channel material. The on current of these devices is mainly limited by the tunneling barrier properties, and phonon-scattering has only a moderate effect.We show, however, that the off current is limited by phonon absorption assisted tunneling, and thus is strongly temperature dependent. Subthreshold swings below the 60 mV/decade conventional limit can be readily achieved even at room …

Inversion Capacitance-Voltage Studies On Gaas Metal-Oxide-Semiconductor Structure Using Transparent Conducting Oxide As Metal Gate, T. Yang, Y. Liu, P. D. Ye, Y. Xuan, H. Pal, Mark S. Lundstrom

Department of Electrical and Computer Engineering Faculty Publications

A systematic capacitance-voltage C-V study has been performed on GaAs metaloxide- semiconductor MOS structures with atomic-layer-deposited Al2O3 as gate dielectrics and indium tin oxide ITO as the metal gate. The transparent conducting ITO gate allows homogeneous photoillumination on the whole MOS capacitance area, such that one can easily observe the low-frequency LF C-V and quasistatic C-V of GaAs at room temperature. The semiconductor capacitance effect on GaAs MOS devices has also been identified and insightfully discussed based on the obtained LF C-V curves. The semiconductor capacitance effect becomes more important for devices with high-mobility channel materials and aggressively scaled high-k …

Ballistic Graphene Nanoribbon Metal-Oxide-Semiconductor Field-Effect Transistors: A Full Real-Space Quantum Transport Simulation, Gengchiau Liang, Neophytos Neophytou, Mark S. Lundstrom, Dmitri Nikonov

Department of Electrical and Computer Engineering Faculty Publications

A real-space quantum transport simulator for graphenenanoribbon (GNR) metal-oxide-semiconductor field-effect transistors (MOSFETs) has been developed and used to examine the ballistic performance of GNR MOSFETs. This study focuses on the impact of quantum effects on these devices and on the effect of different type of contacts. We found that two-dimensional (2D) semi-infinite graphene contacts produce metal-induced-gap states (MIGS) in the GNR channel. These states enhance quantum tunneling, particularly in short channel devices, they cause Fermi level pinning and degrade the device performance in both the ON-state and OFF-state. Devices with infinitely long contacts having the same width as the channel …