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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 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 × 17 μm 2 size provide a frequency response above 10 GHz at 1.5 μm wavelength.

Various structures of self-assembled Ge/Si quantum dot infrared photodetectors were implemented and investigated. The electronic structure of the QDIPs was studied by electrical and optical techniques including I–V characteristics, dark current, photoconductivity, photoluminescence, and photo-induced infrared absorption. The photoconductive spectra consist of a broad multi-peak, composed of peaks ranging from 70 to 220 meV. Their relative intensity changes with bias. Comparative dark current measurements were performed. Dark current limits the performance of this first generation of Ge/Si QDIPs. It is plausible that direct doping in the dot layer is a viable way of reducing the dark current. � 2003 Elsevier B.V. All rights reserved.

A junction between an epitaxial semiconductor and a semi‐insulating substrate serves as a tool for analyzing transient behavior in semi‐insulators (SI). At equilibrium a narrow region which is fully depleted from electrons exists in the SI. The application of a reverse bias results in an additional partially depleted region, initially depleted just from free carriers, adjacent to the fully depleted one. As the transient progresses electrons are released from the deep traps in the SI. The partially depleted region collapses into a wider fully depleted region. This process is manifested by a substantial current transient through the SI. The charges emitted recombine in the epitaxial layer leading to a decrease in its conductance due to a narrowing conducting path. Thus it is possible to characterize this process by measuring the (large) current through the epilayer rather than the (small) current through the SI. These transients are a function of the emission coefficient. Their analysis provides data also on the energy gap of the traps, their capture coefficient, and the electron lifetime. The long decays can be accelerated by illuminating with photons of energy below the band gap but larger than the energy spacing between the deep traps and the conduction band.

A statistical approach is used to characterize image transmission through misaligned multifibers, i.e., differently arranged at the input and output faces, leading to a spatially invariant line spread function. The resulting MTF is the product of the MTF for the aligned system and a function characterizing stochastic departures from alignment. The case of circular fibers with Gaussian misalignment is treated theoretically and is found to account satisfactorily for experimentally observed results.

The conditions necessary to obtain simultaneous oscillation of two laser transitions sharing a common energy level are examined. The general form of the equations is the same for all three possible cases, and it is shown that the operating point should lie within the area limited by the axes of coordinates and two straight lines in the transmittance plane. Parameters determining those lines can either be calculated or measured experimentally; their slopes depend only upon the transition parameters and are thus the same for all lasers operating on given transitions.

A detailed comparison between optical and electrical interconnections is presented, with the emphasis on advantages and drawbacks of optical link utilization. The impact of attenuation, dispersive degradation, and fan out on signal integrity is discussed. The implementation of a polyimide optical waveguide in the MCNC package is discussed. >

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.

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.

A high detectivity multiquantum well midinfrared photodetector is reported. It is based on a strain compensated InGaAs/InGaP on InP structure, using bound-to-continuum intersubband absorption, with λP=4.9 μm and ∼0.5 μm full width at half maximum. This design is unique by enabling a large critical thickness, thus increasing the quantum efficiency. Photodetectors with background-limited performance (BLIP) with detectivity of Dλ*(BLIP)=3.2×1010 cmHz/W up to 110 K, with only ten quantum well periods were implemented. Arguments are given to predict an optimized background-limited performance for this design up to 135 K.

The observation of intersubband (e1–e2) absorption induced by current injection in GaAs/GaAlAs multiquantum wells is reported. The new technique enables sensitive measurement of intersubband transitions in undoped quantum wells. The carrier concentration in the wells is easily controlled by varying the current density. The technique is demonstrated on undoped multiquantum well layers sandwiched in a p‐i‐n laser‐diodelike structure. The lowest confined level was populated by biasing the junction and hence injecting carriers into the quantum wells. The spectra were taken by a Fourier transform infrared (FTIR) spectrometer, modulating the current and using the double modulation method. The dependence of the absorption on current intensity was examined and excess carrier concentration and lifetime were calculated. The induced absorption saturates at current densities higher than 100 mA/cm2.

InAs/InAlAs-on-InP quantum dots were implemented for infrared photodetection. The photoconductive spectra were measured as a function of polarization angle and bias. Normal incidence configuration was employed, to emphasize the contrast to the response in quantum well infrared photodetectors (QWIPs). Several peaks were observed, ranging from 90 to 420 meV. The strongest peak, at around 95 meV, is highly polarized. The superlinear increase of the intensity with bias, due to bound-to-bound transition followed by tunneling, was modeled successfully by WKB approximation. I–V characteristics show no improvement over QWIP performance.

Wave functions and oscillator strengths for interlevel transitions in InAs dots embedded in an InAlAs matrix, calculated using an eight-band strain-dependent k.p Hamiltonian are compared to measured photoconductive spectra. Several polarized peaks are present in the 80-400-meV range. A large peak is present around 160 meV for side illumination, in spite of small oscillator strength, while it is absent for normal incidence, and as a result of absorption. The high carrier concentrations of the contact layers produce a unique cavity. As the plasma frequency is approached, the real part of the refractive index drops. Radiation entering from the side is totally internally reflected, while front illumination is completely reflected. This effect of the plasma-confined micro-cavity was not reported previously, to the best of our knowledge.

Infrared detectors were implemented on InAs self-assembled quantum dots fabricated using Stranski–Krastanov growth mode on InAlAs matrix, lattice matched to InP (0 0 1) substrates. These dots grow with a shape of small elongated boxes, with their long axis along the [ 1 1 0] direction, and with a high concentration of 7×10 10 cm −2 . Photoconductive measurements were performed in all three polarizations. Rich spectra in the range of 50–500 meV, with different polarization selection rules were observed. The bias dependence of peak intensity of the intraband transitions serves as an additional tool to identify their origin. Some of the peaks, which increase linearly with bias, are attributed to bound-to-continuum transitions. Others, which appear only at larger biases, and increase superlinearly, are due to bound-to-bound transitions. The magnitude of detector responsivity at normal-incidence is similar to that obtained for polarization normal to the layers, and is comparable to that achieved in QWIPs. BLIP conditions prevail at 77 K for integral photocurrent response at F#1. The effect of unintentional doping is discussed. It is shown that this doping can be destructive for detector operation unless the density of dots is large.

In a graded junction, the formalism for handling reflecting boundary conditions must be modified. Since a significant drift term is present, zero recombination velocity at the surface does not imply a zero excess carrier gradient but rather zero overall flux. A model for analyzing p‐n junctions fabricated by implantation or diffusion is presented, assuming the dominant recombination mechanism in the graded region is Auger. The model enables optimization of diode design. By proper selection of parameters, mainly by reducing surface concentration or by increasing the steepness of the dopant profile, it is possible to drastically reduce the saturation current generated by the graded region.

Theoretical predictions show that thermal generation rate in quantum dots can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. Quantum Dots Infrared Photodetectors (QDIP) were implemented in a GaAs/InAs material system. The device is composed of 10 layers of self assembled InAs dots grown on GaAs substrate. The illumination was introduced through a wedge in order to investigate polarization effects. A clear spectral response was obtained at 13K, with two peaks. The first, at 6.5 μm, is polarized parallel to the growth axis, and is attributed to excitation from the first confined level to the continuum. The second, centered around 15 μm, is unpolarized, and is due to excitation from the second confined level to the continuum. The fact that a photoconductive signal was recorded even though no absorbance was observed, indicates a very large gain due to a very long lifetime, consistent with theoretical predictions. It is estimated that once a device with quantum efficiency comparable to that of Quantum Well Infrared Photodetectors (QWIPs) is realized, it will be possible to operate QDIPs at temperatures higher than QWIPs.

3 páginas, 3 figuras.-- PACS: 85.60.Gz, 85.35.Be, Thermal generation rate in quantum dots (QD) can be significantly smaller than in quantum wells, rendering a much improved signal to noise ratio. QDs infrared photodetectors were implemented, composed of ten layers of self-assembled InAs dots grown on GaAs substrate. Low temperature spectral response shows two peaks at low bias, and three at a high one, polarized differently. The electronic level structure is determined, based on polarization, bias, and temperature dependence of the transitions. Although absorbance was not observed, a photoconductive signal was recorded. This may be attributed to a large photoconductive gain due to a relatively long lifetime, which indicates, in turn, a reduced generation rate., This research was partially supported by the Israeli Ministry of Science and Technology. Two of the authors (P.P. and J.G.) received financial support from QUEST, an NSF Science and Technology Center (DMR 11-20007)

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 Schrodinger-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.

InAs nanostructures on InAlAs / InP (0 0 1) have been fabricated using Stranski–Krastanov growth mode. Depending on growth conditions, a full coverage of the InAlAs surface by either InAs strings of elongated dots or isolated dots can be achieved. Giant intraband absorptions are observed at mid-infrared wavelengths in both nanostructures. The intraband resonances are strongly polarized in the layer plane as a consequence of the quantum confinement along either [1 1 0] and [1 −1 0] directions.

Photoconductive spectra of InAs quantum dots embedded within InAlAs barriers show several peaks, ranging from 4 to 12 μm . The nature of these peaks differs, as indicated by their dependence on bias and on temperature. The long wavelength peak drops rapidly with temperature, while the first peak hardly changes up to 60 K . While the wide peak around 4 μm depends linearly on bias, indicating a direct excitation into the quasi-continuum, the peak at 12 μm increases super-linearly since the photocarriers are generated by a two-step process, excitation into a bound level followed by tunneling to the continuum. This explains the drop with temperature, since as the temperature increases more carriers relax from the excited state back to the ground level rather than tunneling into the continuum.

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.

A monolithic infrared focal plane array where photo-diodes and field effect transistors are integrated is presented. The photodiode is connected directly to the integrating capacitor while the transistor controls the integrated signal-charge transfer to the video line. The operating modes of such array are discussed and especially the pseudo-staring mode. Such arrays were produced and their system performance were investigated. The detectors average specific detectivity D * (lambda )(f/no equals 1, (lambda) equals 3.83 micrometers ) was equal to 1.3 X 10 11 [cm-Hz 1/2 /W] and was used to calculate the Noise Equivalent Temperature Difference (NETD) as a function of the number of detectors and vectors. The NETD was estimated to be of 0.039 K for an optimal 5 vectors array.

A new approach to the fabrication of monolithic infrared focal plane arrays is presented and examined in this paper. The array is based on photovoltaic diodes, parallel integration capacitors, and MIS field effect transistors (FET). The photodiode is connected directly to the integrating capacitor while the MISFET serves as a pass gate to the video line. This configuration is operated in the pseudo-staring mode. The array was implemented in InSb, in a process based on a new passivation in which a photo chemical oxidation of InSb is followed by a conventional photo chemical SiO2 growth. A two-level metallization process was developed serving both for electrical connection and optical coverage. Two configurations were tested for the layout of the two metal layers. In addition, the lower metallization was implemented in Cr, Ti, and Al. The optimal structure is a planar array with Cr as the first metal layer which forms the source and drain contacts.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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.

The issue of thermal imaging systems exposed to large photon fluxes is of great practical importance, as it is encountered while observing a scenery which includes ‘‘hot’’ sources. High illumination level phenomena appear when excess carrier density is comparable or larger than majority carrier density in the bulk. This state can be reached quite easily in n‐type HgCdTe detectors due to the moderate doping levels (typically ≤1015 cm−3). A modified expression for the current responsivity is derived, suitable for use at both high and low illumination levels. The quasineutrality assumption is verified; thus the ambipolar equation can be applied to describe high illumination behavior. Photoconductive detectors with and without covered electrodes are measured and analyzed. It is shown that under high illumination levels (1) the dominating recombination mechanism is the phonon assisted Auger process, and (2) the relative photoresponse for a covered electrode structure decreases monotonously with increasing cove...

Various surface passivations of p-type Hg1-xCdxTe were studied to understand their interface properties and and their potential for device technology. Anodic oxide forms an inverted layer near the interface. This n-type skin layer exhibits extremely good n-type properties, which equal, and even surpass, bulk properties. The high electron mobility may be explained by quantization of the electron levels in the space-charge region, and the formation of a two dimensional electron gas near the interface. Thick (approx 500 A) anodic sulfide generates a similar inversion layer. The charge density is proportional to the sulfide thickness. Carefully prepared thin (approx 100 A) anodic sulfide films as well as ZnS coating on freshly etched sur-faces, form nearly Hatband conditions which are suitable for n+ on p diode technology. The surface recombination velocity, determined for these two passivations using the photoelectromagnetic effect, is shown to be similar at low temperatures, increasing with decreasing temperatures. The dominant trapping mechanism at the surface is similar to that in the bulk, and is probably mostly due to vacancies.

Experimental results show that the contribution of the graded region to the current of Hg(1-x)Cd(x)Te diodes is not negligible, as compared to that of the p type bulk. The theoretical analysis reveals the influence of the electric field present outside the depletion region on the current generated by the graded region. The analysis shows the importance of the lifetime profile in the graded region, which is a function of the specific recombination mechanism and its dependence on the local dopant concentration. The effect of parameters such as substrate concentration, surface concentration, and junction depth on this current is discussed.

The distribution of excess carriers in the three dimensions of a semiconductor in closely packed sensing arrays is analytically derived using a novel three‐dimensional simulation. In addition to the introduction of lateral transport, the results show significant deviation from the distribution obtained out of the one‐dimensional model, both in magnitude and gradients. Thus, the net flow of carriers is drastically different than previously predicted. The technical implications of the exact three‐dimensional distribution on quantum efficiency and crosstalk is visualized. The effects of physical parameters such as absorption coefficient,diffusion length, array geometry, and detector structure are thoroughly investigated.

A new method for measuring the ambipolar mobility is described. It is based on the determination of the travel time of excess carriers between two probes. Maintaining the device under a constant voltage, the travel time is depicted as the separation between two zeros on the output trace. Using this technique the ambipolar mobility of electrons in p‐type Hg 0.71Cd0.29Te was measured for the temperature range of 78–256 K.

Decreasing dimensions, along with an increasing number of elements in imaging photodiode arrays, result in the degradation of spatial resolution and sensitivity due to lateral transport. This effect is modeled using a novel 3-D analytical solution of the continuity equation. The model enables the full 3-D analysis of lateral transport as manifested in excess carrier distribution, photocurrent, and self- and cross-responsivities. Three detector structures are investigated: the semi-infinite substrate, the perfectly collecting, and the perfectly reflecting backside. The front and rear illuminations are treated. The calculated results for the 3-D case deviate fundamentally from those predicted by the 1-D model. The 3-D model succeeds in explaining the reduced quantum efficiency of small-area detectors. It also predicts the limited effect diffusion length has on self-responsivity and cut-off wavelength. The calculated spectral responses fit extremely well data measured on InSb and HgCdTe test arrays. As a powerful design tool, the model enables optimizing responsivity and crosstalk by varying element geometry and spacing, optical aperture, lifetime, and spectral range. The model can be applied to any semiconductor photodiode array provided the relevant physical and geometrical parameters are known.

For heterodyning two optical beams it is necessary to obtain two almost coherent waves. Rather than using an extremely stable laser source, it is possible to split the beam from a single source and use a modulator to shift the frequency of one of the waves. The heterodyne signal generated is at the frequency of modulation, thus, it is susceptible to rf interference by the high modulation voltage. Using two electro‐optical modulators, one can utilize the signal at the sum or difference frequency at which no high voltage is present. It is shown that using a single source and two modulators one obtains a signal without any interference leakage. Thus it is possible to measure easily the frequency response of photodetectors up to high frequencies. This configuration also doubles the frequency range obtainable from a single modulator. The theoretical expressions for the intensities at the various frequencies are presented. Noise equivalent power (NEP) measurements of heterodyne signals performed using this setu...

The time and space distribution of excess carriers in semiconductor devices is derived through the solution of the ambipolar continuity equation. Both build-up of excess carriers due to external excitation and their decay following its termination are presented. A detailed solution for a 1-D photoconductive device with various boundary conditions is obtained. The solution is expanded to the 3-D distribution, where an analytical solution is given for the diffusion region of photodiodes. The procedure for solving the 3-D problem in photoconductor devices is outlined. The derived equations are used to demonstrate the advantages of the “covered electrodes” structure and in order to obtain an accurate value for the ambipolar mobility as measured in a four-probe device.

A novel interface to InSb based on photochemical native oxide (PNOX), which improves conventional ultraviolet (UV) enhanced deposition, is reported. Prior to the deposition of SiO2, an additional stage of growing a native oxide by exposing the semiconductor to N2O and Hg vapors at low pressure under UV illumination is introduced. Composition and electrical properties of the interface are discussed. Compositional analysis was performed by Auger electron spectroscopy. Electrical properties were characterized using metal‐insulator semiconductor (MIS) capacitors and photodiodes implemented with UV enhanced chemical vapor deposition (CVD) oxide/PNOX/InSb. Interface state densities of 2–4×1011 cm−2 eV−1 are obtained with good uniformity and stability. The hysteresis of the MIS structure with 1000 A photoinduced CVD oxide on PNOX is very small, about 0.32 V for a ±20 V span measured at 77 K.

Decreasing dimensions along with increasing number of elements in imaging photodiode arrays result in degradation of spatial resolution and sensitivity due to lateral transport(1-5). This effect is modelled using a novel 3-D analytical solution of the continuity equation. The model enables the full 3-D analysis of lateral transport as manifested in excess carrier distribution photocurrent, self and cross responsivities. Three detector structures are investigated: semi-infinite substrate, perfectly collecting, and perfectly reflecting backside. Front and rear illuminations are treated. Calculated results for the 3-D case deviate fundamentally from those predicted by the 1-D model. The 3-D model succeeds in explaining measured reduced quantum efficiency of small area detectors. It also predicts the observed limited effect of diffusion length on self responsivity and cutoff wavelength. Calculated spectral responses fit extremely well data measured on InSb and HgCdTe test arrays.

A combination of light emitting diodes, as light sources, and optical fibers, as channels for the emitted light to form patterns, has resulted in a pattern reversal stimulator that is miniature in size, has pattern color options, is low in power requirements, and hence in possible stimulus artifacts. In addition, the use of optical fibers enables better separation of pattern elements and easier focusing on the pattern. The stimulator incorporates all the assets of previous versions of miniature pattern stimulators, is comparable or even less in price, and can interface with commercial averagers.

The validity of the assumption that the contribution of the n region to the current of Hg1-xCdxTe diodes is negligible was examined. By placing such diodes in a magnetic field we were able to separate between the current component originating in the p -type substrate, and that from the graded n region. Experimental results show that the ratio between these currents is 0.5 - 3, depending on temperature. The theoretical analysis reveals the influence of the electric field present outside the depletion region on the current generated by the graded region. This field not only produces a drift component, which drives the minority carriers into the junction, it also greatly modifies the excess carrier distribution, thereby changing diffusion part of the current. The analysis shows the importance of the lifetime profile in the graded region, which is a function of the specific recombination mechanism and its dependence on the local dopant concentration. The effect of parameters such as substrate concentration, surface concentration, and junction depth on this current is discussed.

Efficient generation of synthesized multicolor laser light, which ordinarily is limited by the competition between lasing lines, is obtained by means of time multiplexing of oscillations at various wavelengths. Experimental results are described wherein a rotating mirror with sectors of varying reflectivity is used in a mixed-ion argon-krypton laser.

Multiwavelength lasers are used to compensate for color distortion in transmission of light through plastic optical fibers. Efficient methods to eliminate the speckle effect and to generate multicolor laser light are described.

By magnifying the high-quality image produced by a rigid miniature endoscope onto a large flexible multifiber, an instrument has been developed which provides both high resolution and flexibility. It is shown theoretically, and verified experimentally, that the MTF of a system of coupled multifibers is the product of the individual MTFs. Measurements show that the coupled instruments outperform state-of-the-art all-flexible systems. The technique should greatly facilitate observation, display, and photography in percutaneous endoscopy and in a variety of industrial applications.