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B. Popescu

Bio: B. Popescu is an academic researcher from Technische Universität München. The author has contributed to research in topics: Charge carrier & Dynamic random-access memory. The author has an hindex of 4, co-authored 6 publications receiving 52 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, the authors investigate the dynamic response of two different bulk heterojunction organic photodetectors over a large illumination and frequency range, which they attribute to interface-related phenomena.
Abstract: In this paper, we investigate the dynamic response of two different bulk heterojunction organic photodetectors over a large illumination and frequency range. To our knowledge, there is no similar study that includes the nW/cm2 regime. Photocurrent transient measurements reveal that the interlayer at the hole-extracting electrode is critical for the device performance under ultralow illumination. Furthermore, we observe a nonlinear cutoff frequency behavior over the illumination range, which we attribute to interface-related phenomena. We perform a detailed simulation study of the transient response for the measured samples. Making use of a drift diffusion model that also takes into account charge trapping and detrapping effects, both in bulk and at material interfaces, we are able to successfully reproduce the measured transients. Based on our simulations, we propose an explanation for this effect: it can be attributed to the interplay between the potential landscape seen by the charge carriers and to the presence of a large concentration of interface trap states, as well as of fixed interface charges. The importance of smart interface engineering as a key factor for device optimization is also highlighted.

23 citations

Journal ArticleDOI
TL;DR: In this paper, the leakage current versus voltage characteristic of high- $k$ thin film capacitors over a large temperature range was investigated, and a detailed simulation study for the measured samples, making use of a modified drift diffusion model, which also took into account charge trapping/detrapping effects and nonlocal tunneling.
Abstract: In this paper, we have investigated the leakage current versus voltage characteristic of high- $k$ thin film capacitors over a large temperature range. Fabricated samples, consisting of a 10-nm thin ${\rm SrTiO}_{3}$ (STO) layer as a dielectric material and ${\rm SrRuO}_{3}$ as electrodes, have been examined. Electrical measurements performed at different temperatures reveal leakage currents that exceed $10^{-7}~{\rm A}/{\rm cm}^{2}$ at 1 V, a requirement needed for dynamic random access memory (DRAM) applications. We perform a detailed simulation study for the measured samples, making use of a modified drift diffusion model, which also takes into account charge trapping/detrapping effects and nonlocal tunneling. Based on our simulations, we propose an explanation for the large leakage currents observed experimentally. They can be attributed to a trap-assisted tunneling process that is enhanced by oxygen vacancies in the STO dielectric layer. We are thus able to reproduce the temperature and voltage dependence of the measured currents and can use our model to examine the impact of different physical parameters on the behavior of the capacitor structure—a first step toward device optimization. A feasibility analysis is performed for a 1T1C DRAM cell using an optimized deep trench STO capacitor with a reduced oxygen defect density. The simulation results underline the advantages of our modeling procedure using a commercial technology computer aided design (TCAD) framework: once the complex leakage mechanism is implemented, it can be activated on arbitrary 3-D structures, taking advantage of all the postprocessing or visualization capabilities.

19 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a modeling framework suited for the theoretical study of electrolyte-gated organic thin-film transistors and analyze the response of biosensors for varying bias conditions and ion concentrations in the electrolyte.
Abstract: In this paper, we present a modeling framework suited for the theoretical study of electrolyte-gated organic thin-film transistors. Employing a novel, fully self-consistent, coupled Poisson–Boltzmann/drift-diffusion simulator, we analyze the response of biosensors for varying bias conditions and ion concentrations in the electrolyte. Our model considers the diffusive nature of ions in the electrolyte region, the formation of a Helmholtz layer at the electrolyte/organic semiconductor interface and the particular charge transport mechanisms of organic semiconductors, such as field-dependent mobility and the presence of defect states. We calibrate our model on a set of current–voltage measurements for a fabricated device. Once validated, our simulation model offers useful insights in the underlying physics and helps us quantify the impact of the electrolyte solution on the surface potential at the electrolyte/semiconductor interface. A sensitivity analysis is performed to determine the inaccuracy of simpler models, such as the Helmholtz approximation, on the response of our biosensor. Improving our understanding of the working principle and charge transport in such novel electrolyte-gated Organic Thin Film Transistors is an indispensable step toward performance optimization and can pave the way for the design of new, more sensitive biosensor devices.

9 citations

Proceedings ArticleDOI
01 Mar 2012
TL;DR: This paper describes a feasibility study of a fully organic, low cost, frequency doubler for harmonic RFID applications and proofs for the first time the feasibility of a full organic non linear electronic circuit for RFID Applications.
Abstract: This paper describes a feasibility study of a fully organic, low cost, frequency doubler for harmonic RFID applications. The proposed structure is formed by two antennas (RX and TX) printed on paper and an organic Shottky diode able to double the frequency of the received signal. To the author knowledge this contribution proofs for the first time the feasibility of a fully organic non linear electronic circuit for RFID applications.

5 citations

01 Jan 2013
TL;DR: In this article, the authors investigated the transport in InAs nanowire-based wrap gate field effect transistors and their high-frequency performance and deduced that the intrinsic highfrequency performance of these devices is at least one order of magnitude higher than currently reached today, in terms of maximum oscillation frequency and cutoff frequency, and that the parasitic network is the major limiting factor.
Abstract: In this paper, we have investigated the transport in InAs nanowire-based wrap gate field-effect transistors and their high-frequency performance. State-of-the-art InAs devices reveal excellent dc performance in terms of transconductance, subthreshold slope, and saturation behavior. Only, very recently high-frequency measurements have been performed on these devices, demonstrating that they can operate well in the gigahertz range. However, their intrinsic high-frequency performance and the limiting mechanism in reaching the optimal limit have not been fully understood yet. One of the main reasons lies in the technological difficulties in contacting the nanometer devices and in the parasitic elements that arise from this imperfect measurement setup. Making use of a sophisticated hydrodynamic simulator, we are able to successfully reproduce the experimental output characteristics over the entire measurement range. Next, we also perform a detailed simulation study of the frequency response for the measured samples, with and without an extrinsic parasitic network. Based on our simulations, we deduce that the intrinsic high-frequency performance of these devices is at least one order of magnitude higher than currently reached today, in terms of maximum oscillation frequency and cutoff frequency, and that the parasitic network is the major limiting factor. Furthermore, we identify the parasitic elements that have the greatest impact on the device performance and we explain their working principles.

4 citations


Cited by
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Book ChapterDOI
01 Jan 2003
TL;DR: In this paper, an expanded and thoroughly revised edition of Thomas H. Lee's acclaimed guide to the design of gigahertz RF integrated circuits features a completely new chapter on the principles of wireless systems.
Abstract: This expanded and thoroughly revised edition of Thomas H. Lee's acclaimed guide to the design of gigahertz RF integrated circuits features a completely new chapter on the principles of wireless systems. The chapters on low-noise amplifiers, oscillators and phase noise have been significantly expanded as well. The chapter on architectures now contains several examples of complete chip designs that bring together all the various theoretical and practical elements involved in producing a prototype chip. First Edition Hb (1998): 0-521-63061-4 First Edition Pb (1998); 0-521-63922-0

207 citations

Journal ArticleDOI
TL;DR: Bulk-heterojunction based organic photodetectors are fabricated by means of drop-on-demand inkjet printing with vertical topology, inverted structure, and small footprint due to optimization of the deposition technique.
Abstract: Bulk-heterojunction based organic photodetectors are fabricated by means of drop-on-demand inkjet printing with vertical topology, inverted structure, and small footprint (about 100 μm x 100 μm). Due to optimization of the deposition technique, an external quantum efficiency in excess of 80% at 525 nm and a -3dB bandwidth of a few tens of kHz is achieved.

145 citations

Journal ArticleDOI
TL;DR: ESM data indicate that nanoscale variations in ion uptake are associated with local changes in polymer packing that may impede ion transport to different extents within the same macroscopic film and can inform future materials optimization.
Abstract: Ionic transport phenomena in organic semiconductor materials underpin emerging technologies ranging from bioelectronics to energy storage. The performance of these systems is affected by an interplay of film morphology, ionic transport and electronic transport that is unique to organic semiconductors yet poorly understood. Using in situ electrochemical strain microscopy (ESM), we demonstrate that we can directly probe local variations in ion transport in polymer devices by measuring subnanometre volumetric expansion due to ion uptake following electrochemical oxidation of the semiconductor. The ESM data show that poly(3-hexylthiophene) electrochemical devices exhibit voltage-dependent heterogeneous swelling consistent with device operation and electrochromism. Our data show that polymer semiconductors can simultaneously exhibit field-effect and electrochemical operation regimes, with the operation modality and its distribution varying locally as a function of nanoscale film morphology, ion concentration and potential. Importantly, we provide a direct test of structure-function relationships by correlating strain heterogeneity with local stiffness maps. These data indicate that nanoscale variations in ion uptake are associated with local changes in polymer packing that may impede ion transport to different extents within the same macroscopic film and can inform future materials optimization.

129 citations

Journal ArticleDOI
13 Oct 2014
TL;DR: This paper focuses on a particular, yet broad, class of systems that falls in the IoT category of large area electronics (LAE), represented by “smart surfaces,” and deals with technologies and architectures involved, namely, materials, antennas, RFID systems, and chipless structures.
Abstract: Energy harvesting is well established as one of the prominent enabling technologies [along with radio-frequency identification (RFID), wireless power transfer, and green electronics] for the pervasive development of Internet of Things (IoT). This paper focuses on a particular, yet broad, class of systems that falls in the IoT category of large area electronics (LAE). This class is represented by “smart surfaces.” The paper, after an introductory overview about how smart surfaces are collocated in the IoT and LAE scenario, first deals with technologies and architectures involved, namely, materials, antennas, RFID systems, and chipless structures; then, some exemplifying solutions are illustrated to show the present development of these concurrent technologies in this area and to stimulate further solutions. Conclusions and future trends are then drawn.

87 citations

Journal ArticleDOI
TL;DR: The current issues, recent progress in and the future of DRAM materials, and fabrication technologies are discussed.
Abstract: Dynamic random-access memory (DRAM) is the main memory in most current computers. The excellent scalability of DRAM has significantly contributed to the development of modern computers. However, DRAM technology now faces critical challenges associated with further scaling toward the ∼10-nm technology node. This scaling will likely end soon because of the inherent limitations of charge-based memory. Much effort has been dedicated to delaying this. Novel cell architectures have been designed to reduce the cell area, and new materials and process technologies have been extensively investigated, especially for dielectrics and electrodes related to charge storage. In this article, the current issues, recent progress in and the future of DRAM materials, and fabrication technologies are discussed.

82 citations