This Letter proposes a simple realisation of a circuit (referred to as electronic capacitor) capable of emulating large capacitance of controllable value while utilising a much smaller capacitor and a DC-DC converter. The circuit may be used in a plug-and-play fashion instead of any real large capacitor within the control bandwidth. The proposed control algorithm forces the terminal voltage of the DC-DC converter to follow the well-known capacitance equation by sensing the terminal current and simultaneously compensates conversion losses by an auxiliary control loop. The proposed algorithm is successfully verified by emulating bulk DC-link capacitance of a commercial power factor correction front end.

A high sensitivity rounded bow-tie antenna was optimized based on electromagnetic simulations. The bicrystal Josephson junction detector with a thin layer of YBCO was located exactly at the bicrystal misorientation point at the center of the antenna. The Au antenna was fabricated on an (100) MgO bicrystal substrate. In the presence of 200 GHz RF radiation, a noise equivalent power (NEP) of around 6x10(-12) W Hz(-1/2) was measured, with a voltage sensitivity, eta(V), of 30 dB with the detector at 60 K. Even though no coupling lens was used, these results are as good as those reported in systems in which the impinging radiation was focused with lenses over the detectors. Thus, important simplification of the system was obtained without loss of sensitivity.

A bow-tie antenna with a step-edge Josephson junction as the detection element, located at the center of the antenna, was investigated. The performance of these detection systems is highly sensitive to impedance mismatch between the antenna and the Josephson junction. Impedance matching optimization was performed using advance microwave simulations. Based on simulations results a bow-tie antenna system was implemented and analyzed. Following our experimental results, in the presence of a 200 GHz RF signal, and further simulations, we developed an improved structure where the junction is placed between the ends of two matching strips rather than in center of the antenna. This configuration was analyzed for two antenna structures: bow-tie and planar wire dipole. We found that the dimensions of the matching strips affect both detection magnitude and frequency of return loss dip, and that reflection is extremely sensitive to the junction length. The reflectivity obtained for an antenna without a matching unit was around −14 dB while for an antenna with a matching unit return loss up to −35 dB was derived. Simulation results also showed a high directivity of the antenna, with high radiation gain and low return loss. By varying antenna and junction dimensions, we derived an optimal design in the range of 200–280 GHz.

Active capacitance reduction circuits (ACRCs) are dc-dc converters, terminated by a small auxiliary capacitor, typically utilized to replace bulk electrolytic capacitors in dual-stage grid-connected power conversion systems. Electronic capacitors are ACRCs supporting "plug-and-play" operation in addition to the ability to emulate virtually any finite capacitance at dc-link connected terminals. Despite excellent steady-state functionality, all ACRCs possess the poor response to steplike no-load-to-full-load (and vice versa) transients due to the fact that the auxiliary capacitor utilized (and thus corresponding energy stored) is much smaller than the bulk dc-link capacitor being replaced. Nevertheless, in grid-feeding applications where fuel cells of photovoltaic generators act as a power source, the rate of generated power change is limited. This is also true regarding some off-grid systems such as LED lighting with dimming, where sudden load changes are unnecessary and electronic capacitors seem to be directly applicable in such systems. Therefore, this paper reveals the limits of steplike and ramplike load variations, tolerable by a system equipped with an electronic capacitor, based on its power rating and auxiliary capacitance value utilized. Examples are given for typical systems with 400-V dc link, connected to 50-Hz mains. The validity of presented findings is well-supported by simulations and experiments.

The paper suggests a way of operating a DC-DC converter to directly emulate a large capacitance of a controllable value when a much smaller capacitor is actually utilized. Converters that are operated in this way are referred to as Electronic Capacitors (EC). The proposed system has the potential of replacing in a plug-and-play fashion any real large capacitor (within the control bandwidth). The control algorithm forces the converter terminal voltage to follow capacitance equation as a function of sensed (or estimated) terminal current while simultaneously compensating for conversion losses. The proposed method is verified by emulating bulk DC link capacitance of a commercial power factor correction (PFC) front end. It is shown that steady state performance of the system with electrolytic and electronic capacitors is nearly the same at both DC link and grid sides. On the other hand, transients are handled slightly differently due to dissimilar hold-up abilities of real and electronic capacitors and are therefore subject of future research.

The growing demand for communication systems requires multi-channel solutions. However, the number of antennas in communication systems is dictated by the number of channels in each system which is limited by the available space on a broadcasting platform and by the limited available resources. We have developed a Multi-Channel Phase Control coupler in the VHF frequency range enabling a reduction in the number of antennas to a third of the original setup. The system is based on a phase shifter in a meander stripline geometry connected to each channel individually. The channels are then phase matched simultaneously to a single antenna through a computer-controlled capacitor bank connected to each phase shifter. The system performance shows a low insertion loss of 0.5 dB and a low return loss of -15 dB for the multi-channel setup.

An enhancement of a conventional PI controller used on grid connected power factor corrector (PFC) systems is suggested in this paper. The transient length is reduced by applying two different methods. The first one consists of an auxiliary unit which increases the error signal, while the second one is a hybrid controller which completely replaces the basic PI controller when DC-link voltage drop is detected. The applied control method is based on hybrid controller proposed in the literature for dc-dc power converters. In order to apply this method to the PCF case the controller was properly analyzed for variable input voltage and simulation results demonstrate the theoretical findings.

We have investigated optical properties of high-density InAs self-assembled quantum dots ~ QDs ! in an In x Al 1 2 x As matrix, lattice matched to an InP ~ 001 ! substrate. The weak lattice mismatch ( ; 3%) results in a 90% coverage of the In x Al 1 2 x As surface with InAs QDs. By means of interband and intraband spectroscopies crossed with atomic force microscopy ~ AFM ! measurements, we have determined that the InAs QDs optical properties depend on the deposited amount of InAs. Photoinduced absorption spectroscopy has been used to investigate midinfrared intraband absorptions. For three monolayers ~ ML ! InAs deposit thickness, just above two-dimensional ~ 2D ! /3D growth mode transition ~ 2.5 ML ! , the islands form as isolated elliptical dots elongated along the @ 11 ¯ 0 # direction and exhibit intraband resonances polarized either along the @ 110 # or the @ 11 ¯ 0 # direction. For thicker deposition ( . 3 ML), InAs islands form chains of elliptical dots along the @ 11 ¯ 0 # direction where the quantum confinement is lost, resulting in a quantum-wire-like behavior. In this paper, we also report on photoluminescence and photocurrent spectroscopies, in order to get insight into the InAs/In x Al 1 2 x As island band structure. These experimental results are in good agreement with that of a multiband k • p model.

In this paper, a robust control method based on a disturbance observer is proposed to regulate the terminal voltage of a photovoltaic generator, interfaced by a current mode-controlled boost dc-dc converter. The combined generator-converter-load system possesses a nonlinear behavior, highly dependent on operation point and environmental variables, thus burdening the control task. It is shown that employing a typical linear controller, designed according to a single nominal operating point, results in a closed-loop performance, varying from a highly overdamped near open-circuit condition to greatly underdamped around short-circuit conditions. On the other hand, when the proposed robust controller is utilized, the closed-loop performance remains nearly nominal throughout the whole operation range. In addition, it is shown that, by sacrificing the performance in the vicinity of the open-circuit point, it is possible to implement the controller using a single op-amp with a reduced part count. Simulation and experimental results are presented to verify the proposed method.

We demonstrate intersuband InGaN/(Al)GaN quantum well infrared photodetectors grown on a free standing non-polar m-plane GaN substrate. The devices are grown by metal organic chemical vapor deposition and exhibit TM-polarized photocurrent at peak wavelengths of 7.5 and 9.3 μm at temperature of 14 K. Based on the experimental data of intersubband and interband transition energies and 8-band k · p Schrödinger-Poisson solver calculations, we were able to estimate the conduction band offset to valence band offset discontinuity ratio (ΔEc:ΔEv) of 57:43 for In0.1Ga0.9N/GaN and 55:45, for In0.095GA0.905N/Al0.07Ga0.93N non-polar m-plane multi-quantum well structures.

The ultra fast carrier dynamic in GaN/AlGaN quantum cascade detector was investigated using a time-resolved bias-lead monitoring technique. It is demonstrated that the intrinsic speed limitation, governed by the carrier transit time, is smaller than 1 ps, corresponding to a frequency cut off above 200 GHz.

We have investigated intraband absorptions in GaN/AlN QDs for ultrafast all-optical switching applications at telecommunication wavelengths. Using time-resolved pump-probe experiments the electron lifetime in the excited state of the QDs is measured to be 165 fs. Intraband absorption saturation experiments reveal a record low switching energy. We then investigate quantum cascade ISB photodetectors based on GaN/AlGaN QWs and show that mesa devices with 17 x 17 mu m(2) size provide a frequency response above 10 GHz at 1.5 mu m wavelength. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In-plane photoconductivity (PC) measurements in a GaN/AlN quantum dots (QDs) layer show a TM-polarized infrared (IR) peak, at 0.9 eV, and a visible-UV (vis-UV) peak, at 2.8 eV. Based on the energy and polarization dependence, the 0.9 eV is associated with the polarized S to Pz intraband transition within the QDs. The IR PC turns from positive PC (PPC) to negative PC (NPC) as temperature is raised, increasing exponentially from 50 to 300 K. Vis-UV radiation renders PPC at all temperatures. Combined with vis-UV radiation, the IR PC is negative even at low temperatures. Based on these observations, we suggest a model in which IR excited carriers in the QD layer are coupled to deep levels (DLs) in the AlN barrier and turn immobile, i.e., NPC is observed. Vis-UV radiation re-excites them into the QDs, resulting in PPC. At lower temperatures coupling into the DL becomes inefficient, thus, IR excitation results in PPC. This model was translated into rate equations. Simulations based on these rate equations reproduce well the experimental results.

Photoconductivity (PC) in GaN/AlN quantum dot (QD) matrix due to in-plane transport is characterized. When exposed to near infrared (NIR) radiation a peak is observed at 1.4 |am. Based on its energy and the polarization dependence, it is associated with the polarized S to Pz intra-band transition in the QDs. This PC signal turns from positive to negative as temperature is raised, increasing exponentially from 50K to 300K. UV excitation at 2.8 eV renders a positive PC at all temperatures, and when combined with NIR radiation, negative PC is observed even at 12 K. We claim that following NIR excitation the electrons get trapped in deep levels in the AlN barrier, from which the UV radiation re-excite them into the QDs.

Using photocurrent infrared spectroscopy, we characterized bound-to-bound transitions in the conduction band of samples with high density of GaN/AlN quantum dots (QDs). TE (TM) polarized peaks at 0.15-0.23 eV (0.7-0.9 eV) depending on the dot size, are associated with S-P-x,P-y (S-P-z) transitions. The transition energies and the oscillator strengths were analyzed by numerical solution of the Schrodinger equation in three dimensions for truncated hexagonal pyramid dots. Assuming peak broadening due to dot size fluctuations, we derived the dot dimensions for which all observed photocurrent peak energies can be accounted for. It is shown that for these dimensions to agree with the quantum well limit both nonparabolicity and anisotropy of the conduction band must be taken into account. The appearance of photocurrent due to bound-to-bound intraband transitions within the QDs is attributed to lateral hopping conductivity. Analysis of the photoresponse magnitude due to optical excitation of electrons to different states in the QD yields a two-dimensional mobility edge at similar to 1 eV above the GaN conduction-band edge. The hopping model is further supported by the temperature dependence of the dark conductivity and its sensitivity to the size and the density of the QDs.

This paper reviews recent progress toward intersubband (ISB) devices based on III-nitride quantum wells (QWs). First, we discuss the specific features of ISB active region design using GaN/AlGaN materials, and show that the ISB wavelength can be tailored in a wide spectral range from near-to long infrared wavelengths by engineering the internal electric field and layer thicknesses. We then describe recent results for electro-optical waveguide modulator devices exhibiting a modulation depth as large as 14 dB at telecommunication wavelengths. Finally, we address a new concept of III-nitride QW detectors based on the quantum cascade scheme, and show that these photodetectors offer the prospect of high-speed devices at telecommunication wavelengths.

A quantum cascade detector in the GaN/AlGaN/AlN material system was implemented. The design takes advantage of the large internal field existing in the nitrides in order to generate the essential saw tooth energy level structure. The device operates in the near IR spectral range with a room temperature responsivity at λ=1.7μm of 10mA∕W (1000V∕W) at zero bias. The spectroscopic measurements are in good agreement with simulations.

We demonstrate room-temperature, high-speed operation of GaN/AlGaN quantum cascade detectors. The devices are processed as square mesas with 50 Ω coplanar access lines. Frequency response measurements were performed under illumination by a modulated laser diode emitting at λ=1.55 μm. The electrical response exhibits a first-order filter frequency response. For 17×17 μm2 (25×25 μm2) detectors the −3 dB cutoff frequency is 11.4 GHz (6.5 GHz). S-parameter analysis confirms that the cutoff frequency is extrinsically limited and that the speed of the device can be further increased by reducing the device size.

Photocurrent spectra due to interlevel transitions of holes in Ge/Si quantum dots show several peaks in the range of 60–300 meV, which superlinearly increase with bias, indicating release of carriers by tunneling. The relative peak intensity drastically changes with applied voltage, its polarity, and the measurement system. Lower energy peaks, at 69 and 86 meV, are observed only with a Fourier transform IR (FTIR) spectrometer. The 69 and 86 meV transitions excite holes into intermediate levels from which they are re-excited to shallow levels in a two-photon process. This is observed with FTIR only where the sample is simultaneously exposed to a wide range of energies. Direct band-to-band excitation at 1.25 eV increases the midinfrared signals by orders of magnitude by pumping the intermediate levels. Placing dopants in the barrier greatly increases photocurrent intensity and reduces noise. One-dimensional and three-dimensional numerical analyses confirm our findings.