Electrical and Computer Engineering Commons^™

Design Concepts Of Terahertz Quantum Cascade Lasers: Proposal For Terahertz Laser Efficiency Improvements, Tillmann Kubis, Saumitra Raj Mehrotra, Gerhard Klimeck

Birck and NCN Publications

Conceptual disadvantages of typical resonant phonon terahertz quantum cascade lasers 􏰎THz-QCLs􏰍 are analyzed. Alternative designs and their combination within a concrete device proposal are discussed to improve the QCL performance. The improvements are 􏰎1􏰍 indirect pumping of the upper laser level, 􏰎2􏰍 diagonal optical transitions, 􏰎3􏰍 complete electron thermalization, and 􏰎4􏰍 materials with low effective electron masses. The nonequilibrium Green’s function method is applied to predict stationary electron transport and optical gain. The proposed THz-QCL shows a higher optical gain, a lower threshold current, and a higher operation temperature. Alloy disorder scattering can worsen the QCL performance.

Theory And Applications Of Compressive Sensing, Atul Divekar, Okan Ersoy

Department of Electrical and Computer Engineering Technical Reports

This thesis develops algorithms and applications for compressive sensing, a topic in signal processing that allows reconstruction of a signal from a limited number of linear combinations of the signal. New algorithms are described for common remote sensing problems including superresolution and fusion of images. The algorithms show superior results in comparison with conventional methods. We describe a method that uses compressive sensing to reduce the size of image databases used for content based image retrieval. The thesis also describes an improved estimator that enhances the performance of Matching Pursuit type algorithms, several variants of which have been developed for …

Universality Of Non-Ohmic Shunt Leakage In Thin-Film Solar Cells, Sourabh Dongaonkar, Jonathan D. Servaites, Grayson M. Ford, Stephen Loser, James E. Moore, Ryan M. Gelfand, Hooman Mohseni, Hugh W. Hillhouse, Rakesh Agrawal, Mark A. Ratner, Tobin J. Marks, Mark Lundstrom, Muhammad A. Alam

Birck and NCN Publications

We compare the dark current-voltage (IV) characteristics of three different thin-film solar cell types: hydrogenated amorphous silicon (a-Si:H) p-i-n cells, organic bulk heterojunction (BHJ) cells, and Cu(In,Ga)Se₂ (CIGS) cells. All three device types exhibit a significant shunt leakage current at low forward bias (V< ∼ 0.4) and reverse bias, which cannot be explained by the classical solar cell diode model. This parasitic shunt current exhibits non-Ohmic behavior, as opposed to the traditional constant shunt resistance model for photovoltaics. We show here that this shunt leakage (I_sh), across all three solar cell types considered, is characterized by the following common phenomenological features: (a) voltage symmetry about V = 0, (b) nonlinear (power law) voltage dependence, and (c) extremely weak temperature dependence. Based on this analysis, we provide a simple method of subtracting this shunt current component from …

Throughput And Delay Analysis On Uncoded And Coded Wireless Broadcast With Hard Deadline Constraints, Xiaohang Li, Chih-Chun Wang, Xiaojun Lin

Department of Electrical and Computer Engineering Technical Reports

Multimedia streaming applications have stringent QoS requirements. Typically each packet is associated with a packet delivery deadline. This work models and considers realtime streaming broadcast over the downlink of a single cell. The broadcast capacity of the system subject to deadline constraints are studied for both uncoded and coded wireless broadcast schemes. For the uncoded scenario, an optimal transmission policy is devised based on finite-horizon dynamic programming, and a closed-form expression of the optimal throughput is developed in the asymptotic regime, as the size of the file approaches infinity. For the coded scenario, the optimal capacity in the asymptotic regime …

Probabilistic Matching Pursuit For Compressive Sensing, Atul Divekar, Okan K. Ersoy

Department of Electrical and Computer Engineering Technical Reports

Compressive sensing investigates the recovery of a signal that can be sparsely represented in an orthonormal basis or overcomplete dictionary given a small number of linear combinations of the signal. We present a novel matching pursuit algorithm that uses the measurements to probabilistically select a subset of bases that is likely to contain the true bases constituting the signal. The algorithm is successful in recovering the original signal in cases where deterministic matching pursuit algorithms fail. We also show that exact recovery is possible when the number of nonzero coefficients is upto one less than the number of measurements. This …

H-Matrix-Based Fast Direct Finite Element Solver For Large-Scale Electromagnetic Analysis, Haixin Liu, Dan Jiao

Department of Electrical and Computer Engineering Technical Reports

In this work, we prove that the sparse matrix resulting from a finite-element-based analysis of electrodynamic problems can be represented by an H-matrix without any approximation, and the inverse of this sparse matrix has a data-sparse H-matrix approximation with error well controlled. Based on this proof, we develop an H-matrix-based direct finite-element solver of O(kNlogN) memory complexity and O(k2Nlog2N) time complexity for solving electromagnetic problems, where k is a small variable that is adaptively determined based on accuracy requirements, and N is the number of unknowns. Both inversebased and LU-based direct solutions are developed. The LU-based solution is further accelerated …

A Theoretically Rigorous Full-Wave Finite-Element-Based Solution Of Maxwell's Equations From Dc To High Frequencies, Jianfang Zhu, Dan Jiao

Department of Electrical and Computer Engineering Technical Reports

It has been observed that finite element based solutions of full-wave Maxwell's equations break down at low frequencies. In this paper, we present a theoretically rigorous method to fundamentally eliminate the low-frequency breakdown problem. The key idea of this method is that the original frequency-dependent deterministic problem can be rigorously solved from a generalized eigenvalue problem that is frequency independent. In addition, we found that the zero eigenvalues of the generalized eigenvalue problem cannot be obtained as zeros because of finite machine precision. We hence correct the inexact zero eigenvalues to be exact zeros. The validity and accuracy of the …

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

Theory Of ‘Selectivity’ Of Label-Free Nanobiosensors – A Geometro-Physical Perspective, Pradeep R. Nair, Muhammad A. Alam

Birck and NCN Publications

Modern label-free biosensors are generally far more sensitive and require orders of magnitude less incubation time compared to their classical counterparts. However, a more important characteristic regarding the viability of this technology for applications in Genomics/Proteomics is defined by the ‘Selectivity’, i.e., the ability to concurrently and uniquely detect multiple target biomolecules in the presence of interfering species. Currently, there is no theory of Selectivity that allows optimization of competing factors and there are few experiments to probe this problem systematically. In this article, we use the elementary considerations of surface exclusion, diffusion limited transport, and void distribution function to …