F. Levent Degertekin

Georgia Institute of Technology, Atlanta, Georgia, United States

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Publications (252)288.47 Total impact

  • Sarp Satir · F Levent Degertekin
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    ABSTRACT: We present a nonlinear lumped model that predicts the electrical input-output behavior of an ultrasonic system using CMUTs with arbitrary array/membrane/electrode geometry in different transmit-receive configurations and drive signals. The receive-only operation, where the electrical output signal of the CMUT array in response to incident pressure field is calculated, is included by modifying the boundary elementbased vibroacoustic formulation for a CMUT array in rigid baffle. Along with the accurate large signal transmit model, this formulation covers pitch-catch and pulse-echo operation when transmit and receive signals can be separated in time. In cases when this separation is not valid, such as CMUTs used in continuous wave transmit-receive mode, pulse-echo mode with a nearby hard or soft wall or in a bounded space such as in a microfluidic channel, an efficient formulation based on the method of images is used. Some of these particular applications and the overall modeling approach have been validated through comparison with finite element analysis on specific examples including CMUTs with multiple electrodes. To further demonstrate the capability of the model for imaging applications, the two-way response of a partial dual-ring intravascular ultrasound array is simulated using a parallel computing cluster, where the output currents of individual array elements are calculated for given input pulse and compared with experimental results. With its versatility, the presented model can be a useful tool for rapid iterative CMUT-based system design and simulation for a broad range of ultrasonic applications.
    No preview · Article · Oct 2015 · IEEE transactions on ultrasonics, ferroelectrics, and frequency control
  • Source
    S. Tol · F.L. Degertekin · A. Erturk
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    ABSTRACT: Existing research in vibration-based energy harvesting has focused mostly on the harvesting of deterministic or stochastic vibrational energy available at a fixed position in space. Such an approach is convenient for designing and employing linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates. This work presents a mathematical framework for the harvesting of one-dimensional bending waves propagating in infinite and semi-infinite beams as an alternative. For this purpose, the fully coupled electroelastic problem with piezoelectric patches bonded to a long slender beam is solved and conversion of incident wave energy into usable electricity while minimizing the traveling waves reflected and transmitted from the harvester domain is analyzed. The analysis shows that the efficiency of power transfer from elastic waves can be significantly improved beyond the typical wavelength matching in terms of both efficiency and bandwidth by resistive–inductive loading. It is also shown that enhancements to efficiency can be obtained by localized obstacles in mechanical domain, and fully anechoic boundary conditions can be obtained on finite beams by resistive–inductive impedance matching. These enhancement methods are most effective and practical when piezoelectric patch lengths and obstacle to patch distances are /4, where evanescent fields become insignificant, while the model can readily accommodate the presence of evanescent waves for arbitrary patch lengths. The validity and application of the proposed methods are demonstrated with experimental case studies using a long slender beam.
    Full-text · Article · Sep 2015 · Wave Motion
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    Wenchao Zhou · Drew Loney · Andrei G. Fedorov · F. Levent Degertekin · David W. Rosen
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    ABSTRACT: Purpose – The aim of this paper is to advance the understanding of the droplet deposition process to better predict and control the manufacturing results for ink-jet deposition. Design/methodology/approach – As material interface has both geometric and physical significance to manufacturing, the approach the authors take is to study the interface evolution during the material joining process in ink-jet deposition using a novel shape metric and a previously developed powerful simulation tool. This tool is an experimentally validated numerical solver based on the combination of the lattice Boltzmann method and the phase-field model that enabled efficient simulation of multiple-droplet interactions in three dimensions. Findings – The underlying physics of two-droplet interaction is carefully examined, which provides deep insights into the effects of the printing conditions on the interface evolution of multiple-droplet interaction. By studying line printing, it is found that increasing impact velocity or decreasing fluid viscosity can reduce manufacturing time. For array printing, the authors have found the issue of air bubble entrapment that can lead to voids in the manufactured parts. Research limitations/implications – The array of droplets impinges simultaneously, in contrast to most ink-jet printers. Sequential impingement of lines of droplet needs to be studied. Also, impingement on non-planar surfaces has not been investigated yet, but is important for additive manufacturing. Finally, it is recognized that the droplet hardening mechanisms need to be incorporated in the simulation tool to predict and control the final shape and size of the arbitrary features and manufacturing time for ink-jet deposition. Practical implications – The research findings in this paper imply opportunities for optimization of printing conditions and print head design. Furthermore, if precise droplet control can be achieved, it may be possible to eliminate the need for leveling roller in the current commercial printers to save machine and manufacturing cost. Originality/value – This work represents one of the first attempts for a systematic study of the interface dynamics of multiple-droplet interaction in ink-jet deposition enabled by the novel shape metric proposed in the paper and a previously developed numerical solver. The findings in this paper advanced the understanding of the droplet deposition process. The physics-based approach of analyzing the simulation results of the interface dynamics provides deep insights into how to predict and control the manufacturing relevant outcomes, and optimization of the deposition parameters is made possible under the same framework.
    Full-text · Article · Jun 2015 · Rapid Prototyping Journal
  • Mostafa Fatemi · Yook-Kong Yong · F Levent Degertekin · Paul Wilcox
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    ABSTRACT: The 2014 IEEE Ultrasonics Symposium brought together researchers and scientists from academia and industry to present cutting-edge advances in ultrasonic research and inspire new ideas and collaborations between scholars. This symposium took place in the magnificent and worldclass city of Chicago from September 3-6, 2014. The symposium consisted of 938 contributed presentations, 24 invited talks including the plenary speaker, 12 short courses, and 18 student paper competition finalists. A total of 642 manuscripts were accepted for publication in the conference proceedings in the following major topics: • Medical ultrasonics; • Ultrasonic nondestructive testing and imaging; • Industrial ultrasound; • Physical acoustics; • SAW, FBAR, and MEMS; and • Ultrasonic sensors, transducers, and actuators.
    No preview · Article · Jun 2015 · IEEE transactions on ultrasonics, ferroelectrics, and frequency control
  • Shane W. Lani · Karim Sabra · F. Levent Degertekin
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    ABSTRACT: Surface acoustic waves can propagate above immersed membrane arrays, such as of capacitive micromachined ultrasonic transducers (CMUTs). Similar waves on metamaterials and metasurfaces with rigid structures (typically in the kHz range) have been studied and used for tunable band gaps, negative refraction, and subwavelength focusing and imaging. This work demonstrates through simulation and experiments that a 2D membrane array can be used for subwavelength focusing utilizing a time reversal method. The studied structure consisted of the focusing region, which is a dense grid of 7x7 membranes (6.6 MHz resonance) that support the slow surface acoustic waves. Eight additional membranes are located on the same surface outside the focusing region. Subwavelength focusing was performed by using a time reversal method in which the external eight membranes were used as excitation transducers. Modeling results were verified with experiments that were performed with the membranes being actuated electrostatically and the membrane displacements were measured with a laser Doppler vibrometer. Subwavelength focusing (lambda/5) was achieved on the metasurface while a modal decomposition of the spatial focus from an iterative time reversal method was done to illustrate that optimal focusing resolution requires efficient excitation of the mode shapes containing subwavelength features.
    No preview · Article · Apr 2015 · The Journal of the Acoustical Society of America
  • Shane Lani · Karim G. Sabra · F. Levent Degertekin
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    ABSTRACT: Dispersive surface waves on an acoustic 2D metamaterial, a metasurface consisting of membranes on a rigid surface, have certain propagation characteristics with potential applications for resonance based sensing and subwavelength imaging. The trapped modes of the system that is responsible for the dispersive properties of these acoustic waves are analyzed through modal analysis for a small linear membrane array to obtain the mode shapes, resonant frequencies, quality factors, and wavenumbers. Transient analysis is used for larger arrays to obtain the dispersive properties of the traveling waves and is compared to the modal analysis. Equifrequency contours of the 2D metasurface illustrate interesting features of the metasurface at different frequency regimes around the membrane resonance. These features include anisotropic wave propagation, directional band gap, negative refraction, and complete band gap. Effects of membrane pitch, randomness of resonance, and aperiodic membrane spacing on dispersion, band gaps, and quality factor of the trapped modes on the metasurface are investigated as they relate to realistic implementations for different applications.
    No preview · Article · Jan 2015 · Journal of Applied Physics
  • Toby Xu · Coskun Tekes · F. Levent Degertekin
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    ABSTRACT: Use of high-κ dielectric, atomic layer deposition (ALD) materials as an insulation layer material for capacitive micromachined ultrasonic transducers (CMUTs) is investigated. The effect of insulation layer material and thickness on CMUT performance is evaluated using a simple parallel plate model. The model shows that both high dielectric constant and the electrical breakdown strength are important for the dielectric material, and significant performance improvement can be achieved, especially as the vacuum gap thickness is reduced. In particular, ALD hafnium oxide (HfO2) is evaluated and used as an improvement over plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (Six)Ny)) for CMUTs fabricated by a low-temperature, complementary metal oxide semiconductor transistor-compatible, sacrificial release method. Relevant properties of ALD HfO2) such as dielectric constant and breakdown strength are characterized to further guide CMUT design. Experiments are performed on parallel fabricated test CMUTs with 50-nm gap and 16.5-MHz center frequency to measure and compare pressure output and receive sensitivity for 200-nm PECVD Six)Ny) and 100-nm HfO2) insulation layers. Results for this particular design show a 6-dB improvement in receiver output with the collapse voltage reduced by one-half; while in transmit mode, half the input voltage is needed to achieve the same maximum output pressure.
    No preview · Article · Dec 2014 · IEEE Transactions on Ultrasonics Ferroelectrics and Frequency Control
  • Source
    M Balantekin · S Satır · D Torello · F L Değertekin
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    ABSTRACT: We present the proof-of-principle experiments of a high-speed actuation method to be used in tapping-mode atomic force microscopes (AFM). In this method, we do not employ a piezotube actuator to move the tip or the sample as in conventional AFM systems, but, we utilize a Q-controlled eigenmode of a cantilever to perform the fast actuation. We show that the actuation speed can be increased even with a regular cantilever.
    Full-text · Article · Dec 2014 · Review of Scientific Instruments
  • T. Xu · C. Tekes · F.L. Degertekin
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    ABSTRACT: Higher transmit sensitivity and receive sensitivity at lower bias voltages would be achieved by CMUTs when their structure converges to an ideal parallel plate structure without an insulation layer. Using an insulation layer with high relative dielectric constant (high-K) helps achieving this goal with increased capacitance and large electric field in the vacuum gap. The effect of the insulation layer is more pronounced for high frequency, small gap CMUTs, where large pressures can be generated with smaller displacements. In this paper, we present a simple model for optimal insulation layer properties and significant improvement in both transmit and receive sensitivity by replacing the silicon nitride (SixNy) insulation layer with high-K hafnium oxide (HfO2) for a low temperature CMUT-on-CMOS process. Experiments are performed on parallel fabricated test CMUTs with 50-nm gap and 16.5-MHz center frequency to measure and compare pressure output and receive sensitivity for PECVD SixNy and HfO2 insulation layers. Results for this particular design show 6-dB improvement in receive sensitivity (V/Pa) with the collapse voltage reduced by one half. Successful CMUT-on-CMOS integration with high-k dielectric is also demonstrated.
    No preview · Article · Oct 2014
  • M. Wasequr Rashid · Coskun Tekes · Maysam Ghovanloo · F. Levent Degertekin
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    ABSTRACT: Designing Intracardiac Echocardiography (ICE) imaging catheters using 2D arrays with large number of elements is extremely challenging with standard interconnect and transducer technology. Using microbeamformers inside the catheter reduces the number of cables; however it requires complex front-end electronics which consumes considerable amount of real estate. By using the CMUT-on-CMOS technology, one can incorporate on-chip transmit electronics and efficiently optimize the 2D transducer array to use less number of active elements to reduce the number of cables by combining multiple channels on a single output cable. Reducing the number of cables with integrated electronics may provide a flexible catheter which can be used for 3D ICE imaging under MRI as well as X-Ray. In this study, we explore the design of the CMOS receiver circuit which implements a Frequency Division Multiplexing (FDM) scheme to achieve reduced number of output channels. We discuss design details of an 8×1 multiplexing chip designed in 0.35μm CMOS process. We also present post layout simulation results from the chip showing the viability of the approach.
    No preview · Conference Paper · Sep 2014
  • Bernard Shieh · F. Levent Degertekin · Karim Sabra
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    ABSTRACT: We present a method for the realistic simulation of tissue backscatter in the ultrasound simulation program Field II. The method, based on an application of the Rayleigh speckle model, is implemented as an additional convolution step which appropriately scales and filters RF data to match an arbitrary backscattering coefficient spectrum. We verify that the method accurately captures the statistics and frequency-dependent behavior of diffuse tissue by simulating a standard backscattering coefficient measurement for focused transducers. We provide an example of how this method can be used to create simulation phantoms for assessing tissue detectibility in low SNR applications.
    No preview · Conference Paper · Sep 2014
  • Sarp Satir · F. Levent Degertekin
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    ABSTRACT: We present a fast lumped model that predicts the electrical input-output behavior of an ultrasound system using CMUT arrays with arbitrary membrane/electrode geometries. Model capabilities include selection of arbitrary drive signals and transmit-receive circuit impedances for individual array elements, enabling accurate modeling of arrays of single and multiple electrode CMUTs for different ultrasound modalities in a computationally efficient manner. Given the CMUT system description, the pressure at an arbitrary point in the immersion medium and individual membrane displacements are calculated in the time domain for transmit mode. In receive mode, the output electrical signals are calculated as if another ultrasound source is present in the immersion medium. Pulse-echo operation is considered as the transmit-receive mode where the incident field is generated by the image of the CMUT array itself. An example CMUT system consisting of a single circular membrane with multiple electrodes is used to validate all these different modes through comparisons with COMSOL. The results show the ability to capture the nonlinear CMUT behavior accurately and the versatility of the model for different scenarios.
    No preview · Conference Paper · Sep 2014
  • Coskun Tekes · Toby Xu · F. Levent Degertekin
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    ABSTRACT: Synthetic phased array based high frequency imaging devices on catheter probes suffer from low SNR and limited penetration due to small active transducer area and frequency-dependent tissue attenuation. Although using signal averaging improves the SNR, it increases data collection time which degrades real time imaging performance. Temporally coded pulses emit high energy to the imaging medium and enhance SNR, using a suitable decoding procedure, without compromising frame rate. We previously demonstrated the volumetric imaging performance of a single-chip forward looking CMUT-on-CMOS system. In this study, we describe the implementation of temporal coding excitation in our single-chip system with reduced DC voltage levels and present the imaging results. A 2-cycle 13-bit Barker code was generated to encode a rectangular unipolar pulse waveform. 24-ns pulse width was chosen for maximum energy transfer to the system. Due to the limitation of internal pulsers, a 25-V pulse amplitude was used to produce the encoded excitation signal. The received signals were decoded by using spectrum inversion filtering. Imaging experiments were performed using a pig artery phantom immersed in water. The reconstructed 2D cross-sectional images using coded excitation produces 9 dB improvement in image SNR compared to single pulse excitation without any frame rate degradation.
    No preview · Conference Paper · Sep 2014
  • Jaemyung Lim · Evren F Arkan · F Levent Degertekin · Maysam Ghovanloo
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    ABSTRACT: We present a system-on-a-chip (SoC) for use in high-frequency capacitive micromachined ultrasonic transducer (CMUT) imaging systems. This SoC consists of trans-impedance amplifiers (TIA), delay locked loop (DLL) based clock multiplier, quadrature sampler, and pulse width modulator (PWM). The SoC down converts RF echo signal to baseband by quadrature sampling which facilitates modulation. To send data through a 1.6 m wire in the catheter which has limited bandwidth and is vulnerable to noise, the SoC creates a pseudo-digital PWM signal which can be used for back telemetry or wireless readout of the RF data. In this implementation, using a 0.35-μm std. CMOS process, the TIA and single-to-differential (STD) converter had 45 MHz bandwidth, the quadrature sampler had 10.1 dB conversion gain, and the PWM had 5-bit ENoB. Preliminary results verified front-end functionality, and the power consumption of a TIA, STD, quadrature sampler, PWM, and clock multiplier was 26 mW from a 3 V supply.
    No preview · Article · Aug 2014
  • Karim G Sabra · Justin Romberg · Shane Lani · F. Levent Degertekin
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    ABSTRACT: Monolithic integration of capacitive micromachined ultrasonic transducer arrays with low noise complementary metal oxide semiconductor electronics minimizes interconnect parasitics thus allowing the measurement of thermal-mechanical (TM) noise. This enables passive ultrasonics based on cross-correlations of diffuse TM noise to extract coherent ultrasonic waves propagating between receivers. However, synchronous recording of high-frequency TM noise puts stringent requirements on the analog to digital converter's sampling rate. To alleviate this restriction, high-frequency TM noise cross-correlations (12-25 MHz) were estimated instead using compressed measurements of TM noise which could be digitized at a sampling frequency lower than the Nyquist frequency.
    No preview · Article · Jun 2014 · The Journal of the Acoustical Society of America
  • F. L. Degertekin · C. Tekes · T. Xu · S. Satir · G. Gurun · J. Zahorian
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    ABSTRACT: High frequency ultrasonic imaging devices are widely used in minimally invasive catheter-based interventions especially in intravascular imaging, but there is still a need for more flexible devices with true 3-D volumetric imaging capability in front of the catheter tip. Single chip capacitive micromachined ultrasonic transducer (CMUT) arrays with on-chip CMOS electronics implementation provides great flexibility in designing array geometry and integrated front-end electronics. We have successfully implemented dual ring CMUT arrays for forwardlooking intravascular ultrasound (IVUS) and intracardiac echography (ICE) imaging which provide 3-D volumetric imaging functionality. In this paper, we report on the details of the singlechip microscale imaging system and experimental results. We also discuss system improvements with initial experimental results.
    No preview · Conference Paper · May 2014
  • S. Tol · F. L. Degertekin · A Erturk
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    ABSTRACT: Vibration-based energy harvesting has been heavily researched over the last decade to enable self-powered small electronic components for wireless applications in various disciplines ranging from biomedical to civil engineering. The existing research efforts in this interdisciplinary field have mostly focused on the harvesting of deterministic or stochastic vibrational energy available at a fixed position in space. Such an approach is convenient to design and employ linear and nonlinear vibration-based energy harvesters, such as base-excited cantilevers with piezoelectric laminates. However, persistent vibrations at a fixed frequency and spatial point, or standing wave patterns, are rather simplified representations of ambient vibrational energy. As an alternative to energy harvesting from spatially localized vibrations and standing wave patterns, this work presents an investigation into the harvesting of one-dimensional bending waves in infinite beams. The focus is placed on the use of piezoelectric patches bonded to a thin and long beam and employed to transform the incoming wave energy into usable electricity while minimizing the traveling waves reflected and transmitted from the harvester domain. To this end, performance enhancement by wavelength matching, resistiveinductive circuits, and a localized obstacle are explored. Electroelastic model predictions and performance enhancement efforts are validated experimentally for various case studies.
    No preview · Conference Paper · Apr 2014
  • Shane Lani · Karim G Sabra · F Levent Degertekin
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    ABSTRACT: Subwavelength focusing and imaging has been a long sought after goal and one that metamaterials can possibly achieve. In 2011, Lemoult et al. used time reversal techniques to focus sound to as small as λ/25 in air by using the evanescent wave field above a gird of soda cans acting as Helmholtz resonators [Lemoult et al. Phys. Rev. Lett. 107, 064301, (2011)]. This paper will demonstrate subwavelength focusing in immersion in the 11-0 MHz frequency range with capacitive micromachined ultrasonic transducer (CMUT) arrays. CMUTs are microscale (10-100 μm wide) membrane arrays, which support evanescent surface waves that derive their dispersive properties not only from the periodic structure of the array, but also from the membrane resonance. Furthermore, CMUTs have embedded electrodes for electrostatic excitation and detection of acoustic waves which allow implementation of time reversal techniques to focus the dispersive evanescent surface waves using only the CMUTs on the same substrate as sources and receivers. Using a finite boundary element method simulation, we demonstrate subwavelength focusing at points in the near-field above a 2D CMUT array in immersion.
    No preview · Article · Apr 2014 · The Journal of the Acoustical Society of America
  • Source
    Wenchao Zhou · Drew Loney · Andrei G Fedorov · F Levent Degertekin · David W Rosen
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    ABSTRACT: A lattice Boltzmann (LB) formulation, which is consistent with the phase-field model for two-phase incompressible fluid, is proposed to model the interface dynamics of droplet impingement. The interparticle force is derived by comparing the macroscopic transport equations recovered from LB equations with the governing equations of the continuous phase-field model. The inconsistency between the existing LB implementations and the phase-field model in calculating the relaxation time at the phase interface is identified and an approximation is proposed to ensure the consistency with the phase-field model. It is also shown that the commonly used equilibrium velocity boundary for the binary fluid LB scheme does not conserve momentum at the wall boundary and a modified scheme is developed to ensure the momentum conservation at the boundary. In addition, a geometric formulation of the wetting boundary condition is proposed to replace the popular surface energy formulation and results show that the geometric approach enforces the prescribed contact angle better than the surface energy formulation in both static and dynamic wetting. The proposed LB formulation is applied to simulating droplet impingement dynamics in three dimensions and results are compared to those obtained with the continuous phase-field model, the LB simulations reported in the literature, and experimental data from the literature. The results show that the proposed LB simulation approach yields not only a significant speed improvement over the phase-field model in simulating droplet impingement dynamics on a submillimeter length scale, but also better accuracy than both the phase-field model and the previously reported LB techniques when compared to experimental data. Upon validation, the proposed LB modeling methodology is applied to the study of multiple-droplet impingement and interactions in three dimensions, which demonstrates its powerful capability of simulating extremely complex interface phenomena.
    Full-text · Article · Mar 2014 · Physical Review E
  • Shane W Lani · M. Wasequr Rashid · Jennifer Hasler · Karim G Sabra · F. Levent Degertekin
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    ABSTRACT: Capacitive Micromachined Ultrasonic Transducers (CMUTs) operating in immersion support dispersive evanescent waves due to the subwavelength periodic structure of electrostatically actuated membranes in the array. Evanescent wave characteristics also depend on the membrane resonance which is modified by the externally applied bias voltage, offering a mechanism to tune the CMUT array as an acoustic metamaterial. The dispersion and tunability characteristics are examined using a computationally efficient, mutual radiation impedance based approach to model a finite-size array and realistic parameters of variation. The simulations are verified, and tunability is demonstrated by experiments on a linear CMUT array operating in 2-12 MHz range.
    No preview · Article · Feb 2014 · Applied Physics Letters

Publication Stats

3k Citations
288.47 Total Impact Points

Institutions

  • 2000-2015
    • Georgia Institute of Technology
      • • School of Electrical & Computer Engineering
      • • School of Mechanical Engineering
      Atlanta, Georgia, United States
  • 1994-2000
    • Stanford University
      • • E. L. Ginzton Laboratory
      • • Department of Electrical Engineering
      • • Center for Integrated Systems
      Stanford, California, United States
  • 1992-1998
    • Bilkent University
      • Department of Electrical & Electronic Engineering
      Ankara, Ankara, Turkey
    • Ankara University
      Engüri, Ankara, Turkey