Bahram Nabet
Drexel University, Electrical and Computer Engineering, Faculty Member
The current-voltage (I-V) characteristics of metal-semiconductor-metal (MSM) photodetectors under various light intensities are examined. The current shows an initial increase followed by saturation and a subsequent sharp increase as bias... more
The current-voltage (I-V) characteristics of metal-semiconductor-metal (MSM) photodetectors under various light intensities are examined. The current shows an initial increase followed by saturation and a subsequent sharp increase as bias increases. We propose a theoretical model for bias dependence in all regions of operation except for breakdown, based on drift collection of carriers in the depleted regions under the contacts and diffusion and recombination in the undepleted region. This is based on the solution of the diffusion equation in the undepleted area between the two contacts of the MSM structure. The solution is subject to boundary conditions on excess minority carriers at the cathode end and continuity of current at the anode end. The latter is written in terms of a parameter, denoted as effective diffusion length, which describes the collection efficiency of carriers at the anode. The closed-form solution thus derived corroborates with physical expectations in several limiting cases. To compare theory with experiment, we propose methods to extract parameters that are used to normalize the I-V curves and calculate depletion widths under different light intensities, from current- and capacitance-voltage measurements. A close match between experimental and theoretical results is observed, and possible breakdown mechanisms are discussed.
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Enlargement of mammalian cells nuclei due to the cancerous inflammation can be detected early through noninvasive optical techniques. We report on the results of cellular experiments, aimed towards the development of a fiber optic... more
Enlargement of mammalian cells nuclei due to the cancerous inflammation can be detected early through noninvasive optical techniques. We report on the results of cellular experiments, aimed towards the development of a fiber optic endoscopic probe used for precancerous detection of Barrett's esophagus. We previously presented white light scattering results from tissue phantoms (polystyrene polybead microspheres). In this paper, we discuss light scattering properties of epithelial MDCK (Madine-Darby Canine Kidney) cells and cell nuclei suspensions. A bifurcated optical fiber is used for experimental illumination and signal detection. The resulting scattering spectra from the cells do not exhibit the predicted Mie theory oscillatory behavior inherent to ideally spherical scatterers, such as polystyrene microspheres. However, we are able to demonstrate that the Fourier transform spectra of the cell suspensions are well correlated with the Fourier transform spectra of cell nuclei, concluding that the dominate scatterer in the backscattering region is the nucleus. This correlation experimentally illustrates that in the backscattering region, the cell nuclei are the main scatterer in the cells of the incident light.
Research Interests:
ABSTRACT
Research Interests: Engineering, Materials Science, Modeling, Field effect transistors, Modulation, and 15 moreOptoelectronics, Numerical Simulation, Interfaces, Ion Implantation, Photodetector, Heterostructures, Fourier transform, Heterostructure Devices, Electrodes, Heterojunctions, Electric Field, Fourier Transforms, Dark Current, GaAs, and Heterojunction
Research Interests:
ABSTRACT The effects of grating geometry on a Surface Plasmon enhanced planar Metal-Semiconductor-Metal photodetector on GaAs are investigated via Finite-difference Time-domain simulations. Substrate absorption is increased by a factor... more
ABSTRACT The effects of grating geometry on a Surface Plasmon enhanced planar Metal-Semiconductor-Metal photodetector on GaAs are investigated via Finite-difference Time-domain simulations. Substrate absorption is increased by a factor greater than 10 without compromising time response.
Research Interests:
A framework for extremely compact, all analog, and massively parallel implementation of shunting recurrent and nonrecurrent neural networks that is applicable to a wide variety of FET-based integration technologies is proposed. A specific... more
A framework for extremely compact, all analog, and massively parallel implementation of shunting recurrent and nonrecurrent neural networks that is applicable to a wide variety of FET-based integration technologies is proposed. A specific circuit designed for implementation with gallium arsenide MESFETs is implemented. The implemented network's ability to perform contrast enhancement, data compression, and adaptation to mean input strength level, as well as its tunability of sensitivity, are well-suited for processing of sensory information in general and visual information in particular. Thus, if integrated with an array of photo-detectors, it can find widespread optoelectronics applications. These properties are also desirable for feature extraction for content addressable memory (CAM) systems. Furthermore, the network fits in self-organizing multilayer network architectures of Adaptive Resonance Theory. The asymptotic global stability of the network is shown by finding a global Liapunov energy function, which serves to show that the network itself is capable of CAM. The time trace of the local activity of the implemented network is found via a variational analysis which leads to the identification of higher order kernels in a Volterra series expansion. The same mathematical techniques are applied to find higher order spatial kernels which explains the extraordinary high order classification properties embedded in simple multiplicative nonlinearity.
Research Interests:
Along with breast and cervical cancer, esophageal adenocarcinoma is one of the most common types of cancers. The characteristic features of pre-cancerous tissues are the increase in cell proliferation rate and cell nuclei enlargement,... more
Along with breast and cervical cancer, esophageal adenocarcinoma is one of the most common types of cancers. The characteristic features of pre-cancerous tissues are the increase in cell proliferation rate and cell nuclei enlargement, which both take place in the epithelium of human body surfaces. However, in the early stages of cancer these changes are very small and difficult to
Research Interests:
Research Interests: Materials Science, Microelectronics, Nanoelectronics, Nanowire, Nanotechnology, and 15 moreMedicine, Multidisciplinary, Nanowires, Computer Simulation, Nano Optics, Nanotubes, Light, Nanostructures, Optical devices, Nanostructure, Cost Efficiency, Local Field Potential, Molecular Conformation, Materials Testing, and Near Field
Research Interests:
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
The GaNIAl,Ga,_,N material system is well suited as a photodetector material for operation in the ultraviolet (W).’-3 Such solid-state UV photodetectors are receiving much attention because of their applicability to a variety of military... more
The GaNIAl,Ga,_,N material system is well suited as a photodetector material for operation in the ultraviolet (W).’-3 Such solid-state UV photodetectors are receiving much attention because of their applicability to a variety of military as well as commercial needs. In this work we report on very low dark current metal-semiconductor-metal (MSM) UV photodetectors fabricated from single crystal GaN epitaxial layers, using Pt as the Schottky metal. The spectral response of these photodetectors is also good, showing the typical sharp solar-blind cutoff at the band-edge energy consistent with that seen from other g r o ~ p s . ~ , ~ The sample used in this study was grown by low-pressure metalorganic chemical vapor deposition (MOCVD) in an Emcore Model D125 rotating-disk vertical reactor. The MSM structure was fabricated on a 1.5 km GaN epitaxial layer using an interdigitated electrode mask set. The electrode fingers are 2-pm wide, 47-pm long, and on a pitch of 12 pm. A crosssectional view of the device structure is shown in Fig. 1. The dark current results for a typical device are shown (for one polarity) in Fig. 2. We obtained a very low dark current of less than <lo0 pA at a reverse bias of 10 V, which is better than an order of magnitude improvement over previously published data.6 Also shown is the photogenerated current under white light illumination. The I-Vcurve shows a steep increase for bias above 15 V indicative of a significant tunneling current as the width of the Schottky barrier thins under severe reverse voltages. We note that the devices exhibited no degradation of the Schottky barrier even when biased beyond 30 V for extended periods of time. The spectral response of these MSM photodiodes was measured with use of an Oriel white-light source and a Spex scanning monochromator. The output from the monochromator was chopped at 277 Hz, and a lock-in amplifier was used to insure low-noise measurement of the photogenerated current. The incident optical intensity is normalized as a function of X using a W-enhanced silicon photodetector of known responsivity. The responsivity of a typical GaN MSM is seen to be relatively flat below the bandedge, with a very steep dropoff at approximately 350 nm (Fig. 3). There is obvious photoconductive gain, although much lower than that found by other g r o ~ p s . ~ ” In summary, we have reported on the achievement of very low dark current MSM W photodetectors based on GaN epitaxial layers. The spectral response is typical of a solar-blind GaN-based UV photodetector, and shows evidence of a photoconductive gain mechanism. We believe these results demonstrate the high quality of the GaN crystals used to fabricate these devices, and the very low dark currents are encouraging for future device applications. This work was partially supported by the Office of Naval Research, the NSF under grant DMR-93-12947, and the NSF Science and % > 01 lo
Research Interests:
Research Interests:
Research Interests: Materials Science, Quantum Mechanics, Optoelectronics, Photodetectors, Current Density, and 15 morePhotodetector, Quantum Tunnelling, Thermionic Emission, Device Simulation, Effective mass, Quantum Well, Three Dimensional, Low Noise, Boundary Condition, Physical Model, Transient Response, Electrical And Electronic Engineering, Schottky barriers, Dark Current, and Optical Activity
Research Interests: Computer Science, Computer Vision, Neural Networks, Neural Network, Medicine, and 12 moreMultidisciplinary, Parameter estimation, Simulation, Volterra series, Motion Detection, Velocity Estimation, Nonlinear system, Image Motion Analysis, Input Output, Artificial Neural Network, Biological Circuits, and Application Software
Research Interests:
Research Interests:
Research Interests:
Quantum capacitors are demonstrated for gauging of quantum energy states, which provide an unprecedented electronic read-out for spintronic systems applied in quantum information processing. © 2019 The Author(s)
Research Interests:
Summary form only given. Molecular beam epitaxy (MBE) growth of the GaAs at temperature ranges of 200°C and 600 °C substantially affects its optical and electronic properties. Low-temperature (LT), around 200°C, and intermediate... more
Summary form only given. Molecular beam epitaxy (MBE) growth of the GaAs at temperature ranges of 200°C and 600 °C substantially affects its optical and electronic properties. Low-temperature (LT), around 200°C, and intermediate temperature (IT), around 400°C, growth by MBE has been employed in a variety of device applications including transistors, mixers, and photodetectors. In the device proposed here, a
Research Interests:
ABSTRACT
Research Interests:
Research Interests:
High speed photodetectors’ intrinsic response is limited by the transit time of the carriers that light generates to the contacts that collect these carriers generating an electric response in the external circuitry. By contrast, charge... more
High speed photodetectors’ intrinsic response is limited by the transit time of the carriers that light generates to the contacts that collect these carriers generating an electric response in the external circuitry. By contrast, charge plasma confined in a semiconductor can transfer energy, hence respond much faster, than the field-induced carrier drift current. The analogy is to a drop exciting a wave in a reservoir, which is detected more rapidly than the drop’s transport by current flow. Here we construct a photodetector device in which charge reservoirs of confined two dimensional electron and hole gasses (2DEG, 2DHG) mediate the photodetector response circumventing charge transport limitations in both expended energy and required velocity. In response to short optical pulses, this device produces electrical pulses which are almost two orders-of-magnitude shorter than the same device without the charge reservoirs. In addition to speed, the sensitivity of this process allows us to measure, at room temperature, as low as 11,000 photons. The device is shown to operate without applied bias, with high responsivity, at hundreds of gigahertz. These micro plasma devices can have a range of applications such as optical communication with fraction of a microwatt power compared to the present tens of milliwatts, ultrasensitive detection of light without need for cryogenic cooling, photovoltaic devices that are capable of harvesting dim light, detectors of THz radiation, and in detection of charged particles.
Research Interests:
ABSTRACT
Research Interests: Computer Science, Signal Processing, Neural Networks, Neural Network, Sensor Arrays, and 12 moreProtein Structure and Function, Artificial Neural Networks, Photodetectors, Transistor, Interconnection, Image Sensors, Dynamic Range, sense organs, Field effect transistor, Neural nets, Packing Density, and Edge enhancement
Research Interests: Materials Science, Doping, Optoelectronics, Semiconductor, Photodetectors, and 13 moreMetal, Two Dimensional Electron Gas, Optical physics, Photodetector, Photocurrent, Schottky diode, Schottky Barrier, Quantum Well, Rectification, Electrical And Electronic Engineering, Schottky barriers, Dark Current, and Heterojunction
2D transition metal carbides, known as MXenes, are transparent when the samples are thin enough. They are also excellent electrical conductors with metal‐like carrier concentrations. Herein, these characteristics are exploited to replace... more
2D transition metal carbides, known as MXenes, are transparent when the samples are thin enough. They are also excellent electrical conductors with metal‐like carrier concentrations. Herein, these characteristics are exploited to replace gold (Au) in GaAs photodetectors. By simply spin‐coating transparent Ti3C2‐based MXene electrodes from aqueous suspensions onto GaAs patterned with a photoresist and lifted off with acetone, photodetectors that outperform more standard Au electrodes are fabricated. Both the Au‐ and MXene‐based devices show rectifying contacts with comparable Schottky barrier heights and internal electric fields. The latter, however, exhibit significantly higher responsivities and quantum efficiencies, with similar dark currents, hence showing better dynamic range and detectivity, and similar sub‐nanosecond response speeds compared to the Au‐based devices. The simple fabrication process is readily integratable into microelectronic, photonic‐integrated circuits and silicon photonics processes, with a wide range of applications from optical sensing to light detection and ranging and telecommunications.
Research Interests:
ABSTRACT
Research Interests:
Spectral photoresponsivity measurements on AlGaAs/GaAs detectors with lateral heterodimensional metal contacts show a peak near the absorption edge of GaAs under low incident power and high biasing conditions. To model this behavior we... more
Spectral photoresponsivity measurements on AlGaAs/GaAs detectors with lateral heterodimensional metal contacts show a peak near the absorption edge of GaAs under low incident power and high biasing conditions. To model this behavior we expand on the photoreflection analysis of AlGaAs/GaAs heterostructures by spatially modulating the optical properties of the GaAs absorption layer as a result of the built-in electric field and the quantum confinement near the interface. Simulations which combine spatial variations in the absorption properties with an existing model of charge collection for metal-semiconductor-metal photodetectors have been able to reproduce the spectral results observed during experimentation.
Research Interests:
Research Interests:
ABSTRACT
Research Interests:
A quantum capacitor is described for low-power logic operations with ~9aJ/bit energy and fast photodetection based on “plasmorons,” unbounded by transport limitations. These are achieved by optoelectrical exploitation of quantum-exclusive... more
A quantum capacitor is described for low-power logic operations with ~9aJ/bit energy and fast photodetection based on “plasmorons,” unbounded by transport limitations. These are achieved by optoelectrical exploitation of quantum-exclusive energies of interacting 2D hole plasma.
Research Interests:
Research Interests:
The commenters refer to the above mentioned paper by Wolpert and Micheli-Tzanakou (ibid. p.955-61, 1993), and point out the oversight of previous activity in circuit design. In their opinion the paper in question does not seem to... more
The commenters refer to the above mentioned paper by Wolpert and Micheli-Tzanakou (ibid. p.955-61, 1993), and point out the oversight of previous activity in circuit design. In their opinion the paper in question does not seem to represent a novel approach either in circuitry or computationally. In reply, the authors acknowledge their oversight of the conference papers cited by the commenters, but point out that they never suggested that a simple shunting lateral inhibitory (SLI) pair was a novel circuit, and the overall objective of their effort was simply to carry analysis of SLI to a higher dimension.
Research Interests:
Asymmetrically connected inhibitory shunting networks are believed to occur in many areas of the brain. One such area is the visual system; these networks are found useful in explaining many peripheral visual phenomena. This paper... more
Asymmetrically connected inhibitory shunting networks are believed to occur in many areas of the brain. One such area is the visual system; these networks are found useful in explaining many peripheral visual phenomena. This paper examines a class of asymmetrically connected networks for stability and uniqueness of outputs; both time-invariant and moving inputs are considered.Under different sets of constaints upon
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
Research Interests:
ABSTRACT Incorporating InGaAs/GaAs/AlGaAs heterostructures creates a bi-layer electron and hole plasma producing a planar, top-illuminated photodetector with &lt;2.5-ps time response and sensitive to &lt;1-microWatt optical power,... more
ABSTRACT Incorporating InGaAs/GaAs/AlGaAs heterostructures creates a bi-layer electron and hole plasma producing a planar, top-illuminated photodetector with &lt;2.5-ps time response and sensitive to &lt;1-microWatt optical power, and can operate without applied bias.
Research Interests:
A plasmonic structure is introduced with a capacitance that is three orders of magnitude smaller than that of typical copper line. Furthermore, it can encode optical signal into binary states, making it suitable for on-chip optically... more
A plasmonic structure is introduced with a capacitance that is three orders of magnitude smaller than that of typical copper line. Furthermore, it can encode optical signal into binary states, making it suitable for on-chip optically enabled computation.
Research Interests:
We investigate mechanisms by which interaction of light and matter may be affected by electrons, and show how this can lead to optoelectronic devices with superior properties. In particular, confined cloud of electron gas allows sculpting... more
We investigate mechanisms by which interaction of light and matter may be affected by electrons, and show how this can lead to optoelectronic devices with superior properties. In particular, confined cloud of electron gas allows sculpting a wave function that affects both emission and absorption of radiation, while its collective, plasmonic, excitation may be used for optical wave guiding, coupling and radiation. Such processes require much less energy and are much faster than classical kinetic energy-based charge transport in traditional electronics. Here we present thin-film photodetectors in which 2D electron and hole charges allow operation in hundreds of GHz, without applied bias, requiring a fraction of microwatt of optical power. The 2D channel can also be structured to provide the momentum change that is required for coupling to excitation at THz range. The confined charge is then used as a plate of (an unconventional) capacitor which changes states by a factor of >1000, in tens of fs, requiring atto-joules of energy which is also switchable by light. This opto-plasmonic capacitor finds application in threshold logic based neuromorphic systems. These thin-film devices are produced in bottom-up core-shell nanowire (CSNW) technology, resulting in resonant optical cavities whose properties are controlled by 2D and 1D charge plasma, with orders of magnitude increase in absorption and emission of light that leads to lasing at room temperature even without vertical structure. Since CSNWs can be grown on Si, they can be good candidate platforms for Photonic Integrated Circuits (PIC) and Silicon Photonics.