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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.

Transport properties of many semiconductor devices are dominated by the scattering of unscreened ionized impurities. Unscreened ions can be found either in depletion regions, low-doped bulk material, or in barriers of quantum well infrared photodetectors (QWIP). In QWIPs the electrons are concentrated in the well regions, leaving the ions in the barriers unscreened. Scattering due to unscreened ions is usually investigated using the Conwell‐Weisskopf (CW) model . . This model, originally introduced for interpretation of nuclear processes, uses classical physics by applying the Rutherford formula for particle scattering along with the Born approximation. The divergence of the relaxation time and momentum relaxation time in this model is avoided by neglecting scattering at small angles. This assumption is not valid for quantum particles which are scattered at all angles. In this work we present a different approach to prevent the relaxation divergence without avoiding smallangle scatterings. We show that this approach may result in scattering times and mobilities which are different by an order of magnitude than those derived from the CW model for electrons with kinetic energy below 0.1 eV, resulting in a more realistic simulation of transport properties. This model was applied to transport of optically excited electrons in QWIP structures. The concentration of dopants derived from the model, agree well with known figures.

The transport properties of channel delta-doped quantum well structures were characterized by conventional Hall effect and light-modulated Shubnikov-de Haas (SdH) effect measurements. The large number of carriers that become available due to the delta-doping of the channel, leads to an apparent degeneracy in the well. As a result of this degeneracy, the carrier mobility remains constant as a function of temperature from 300 K down to 1.4 K. The large amount of impurity scattering, associated with the overlap of the charge carriers and the dopants, resulted in low carrier mobilities and restricted the observation of the oscillatory magneto-resistance used to characterize the two-dimensional electron gas (2DEG) by conventional SdH measurements. By light-modulating the carriers, we were able to observe the SdH oscillation at low magnetic fields, below 1.4 tesla, and derive a value for the quantum scattering time. Our results for the ratio of the transport and quantum scattering times are lower than those previously measured for similar structures using much higher magnetic fields.

In this study we investigate both the transport and quantum scattering times as a function of the carrier concentration for a modulation doped Al0.3Ga0.7As/GaAs structure. Carriers in the well are generated as a result of the persistent photoconductivity effect. When more than one subband becomes populated, digital filtering is used to separate the components for each of the excited subbands. We find that the quantum scattering time for the ground subband increases initially as the carrier concentration is increased. However, once the second subband becomes populated, the ground subband scattering time begins to decrease. The quantum scattering time for the excited subband is also observed to decrease as the concentration is increased. Our results are consistent with the theoretical results by A. Isihara and L. Smrcka [J. Phys. C 19, 6777 (1986)]. Finally, from the ratio of the transport and quantum scattering times, it is seen that the transport in the well becomes more isotropic as the concentration is increased.

A simultaneous fit of Hall and conductivity data gives quantitative results on the carrier concentration and mobility in both the quantum well and the parallel conduction channel. In this study this method was applied to reveal several new findings on the effect of persistent photoconductivity (PPC) on free-carrier concentrations and mobilities. The increase in the two-dimensional electron-gas (2DEG) concentration is significantly smaller than the apparent one derived from single carrier analysis of the Hall coefficient. In the two types of structures investigated, delta doped and continuously doped barrier, the apparent concentration almost doubles following illumination, while analysis reveals an increase of about 20% in the 2DEG. The effect of PPC on mobility depends on the structure. For the sample with a continuously doped barrier the mobility in the quantum well more than doubles. This increase is attributed to the effective screening of the ionized donors by the large electron concentration in the barrier. In the delta doped barrier sample the mobility is reduced by almost a factor of 2. This decrease is probably caused by strong coupling between the two wells, as is demonstrated by self-consistent analysis.

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.

Excess carrier distribution and saturation current generated by a graded p‐n junction are investigated, approximating the dopant profile by an exponential function. Analytical solutions of the steady‐state continuity equation are presented for lifetime dominated by Auger, radiative, or Shockley–Read recombination mechanisms, for both ohmic and electrically reflecting boundary conditions. The saturation current generated by the graded region is derived for each of the dominant mechanisms and boundary conditions. Unlike previous published works the result of this analysis is a set of concise analytical expressions that make them a useful tool for simulation and investigation of graded structures such as silicon solar cells. The results are calculated for HgCdTe diodes, demonstrating the effect of the profile slope as determined by surface and bulk concentrations and junction depth. It is shown that the contribution of the graded region to the saturation current, as compared to that of the substrate, is sign...

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. >

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 $S\text{\ensuremath{-}}{P}_{x,y}$ $(S\text{\ensuremath{-}}{P}_{z})$ transitions. The transition energies and the oscillator strengths were analyzed by numerical solution of the Schr\"odinger 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 $\ensuremath{\sim}1\text{ }\text{eV}$ 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.

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.

The contribution of the graded region of implanted p-n junctions is analyzed using an exponential profile. Though previously neglected, it was recently shown that this contribution to the saturation current of HgCdTe diodes is significant. Assuming a dominant Auger recombination, an analytical solution to the continuity equation is obtained. An expression for the current generation by the graded region is presented for both ohmic and reflecting boundary conditions. A revised condition for a wide region is derived. When the region is narrow, the current differs drastically from that of the zero-gradient case. The effects of the junction depth and the substrate and surface concentrations on the current are investigated. It is shown that the reverse current does not saturate.

Comparative analysis of photoconductive (PC) spectra obtained from SiGe quantum dots grown on Si shows large dependence on the location of doping. The sample with doping in the silicon barrier has much larger response than the sample in which the doping was placed in the wetting layer, i.e. in the self assembled dots. This is due to the penetration of the wave-function into the barrier in the first structure, as shown by the 1-D simulation. Several peaks in the infrared were observed, ranging from 70 to 220 meV, associated with inter-level transitions of holes in the valance band of the dot or the wetting layer. The overall spectra and the relative intensity of the various peaks change drastically with bias magnitude and polarity. The PC signals increase super-linearly with bias, indicating that the carriers are released into the quasi-continuum through tunneling.

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.

Intersubband transitions indirect in both real and momentum spaces were observed in GaAs/AlAs type II short period superlattices. Significant absorption of normal incident radiation, with ‘‘forbidden’’ polarization was measured, in addition to absorption in the ‘‘allowed’’ configuration. The transition energy shows a strong temperature dependence. This absorption is attributed to X–Γ transition. Both doped and undoped samples were investigated. Normal incidence absorption is stronger for the doped superlattices. Simulations using a two band model show good agreement to experimental data.

The contribution of more than one carrier to the conductivity in modulation‐doped field effect transistors (MODFET) affects the resultant mobility and complicates the characterization of these devices. Mixed conduction arises from the population of several subbands in the two‐dimensional electron gas (2DEG), as well as the presence of a parallel path outside the 2DEG. We characterized GaAs/AlGaAs MODFET structures with both delta and continuous doping in the barrier. Based on simultaneous Hall and conductivity analysis we conclude that the parallel conduction is taking place in the AlGaAs barrier, as indicated by the carrier freezeout and activation energy. Thus, simple Hall analysis of these structures may lead to erroneous conclusions, particularly for real‐life device structures. The distribution of the 2D electrons between the various confined subbands depends on the doping profile. While for a continuously doped barrier the Shubnikov–de Haas analysis shows superposition of two frequencies for concent...

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.

Deep traps in semi‐insulators (SI) are characterized using a junction composed of an epitaxial p‐type layer grown on SI n‐type layer. At a reverse bias electrons are released from the traps resulting in a current transient through the substrate. Simultaneously the depletion region in the epilayer expands until the entire layer is depleted leading to a decaying epitaxial current. The analysis of these transients renders the electron emission and capture coefficients and lifetime, and the energy location of the traps. The long current decay are accelerated by illuminating the sample with photons of energy below the band gap, as long as their energy is larger than the difference between trap energy and the bottom of the conduction band. Thus we determined directly this energy difference.

A technique for determination of room-temperature two-dimensional electron gas (2DEG) concentration and mobility in heterostructures is presented. Using simultaneous fits of the longitudinal and transverse voltages as a function of applied magnetic field, we were able to separate the parameters associated with the 2DEG from those of the parallel layer. Comparison with the Shubnikov-de Haas data derived from measurements at liquid helium temperatures proves that the analysis of the room-temperature data provides an excellent estimate of the 2DEG concentration. In addition we were able to obtain for the first time the room-temperature mobility of the 2DEG, an important parameter to device application. Both results are significantly different from those derived from conventional Hall analysis.

A drastic enhancement of the Shubnikov–de Haas (SdH) pattern is obtained by recording the changes in the quantum oscillations of magnetoresistance due to modulation of the carrier concentration. The technique enables measurement of the SdH waveform at relatively high temperatures and in samples with moderate mobilities. The modulated waveform shows selective enhancement of the low‐frequency SdH oscillations associated with the upper subband. Thus, we were able to record very clear oscillations generated by a carrier concentration well below 5×1010 cm−2. The theory for this selective enhancement is provided.

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.

Heat treatment at 70 °C of low carrier concentration p‐type HgCdTe samples (p0=8×1014 cm−3) generates an inverted surface layer. A two day anneal process below 95 °C did not affect the Hall coefficient, whereas an almost complete recovery was obtained by annealing at 120 °C. While bulk electron mobility, obtained from PEM data, remained high (about 9×104 cm2/V s at 77 K), surface mobility is lower by more than an order of magnitude. Surface recombination velocity indicates a continuous improvement with increased temperature, and the activation energy remains equal to the vacancies energy level. The proposed mechanism is that of positive charges in the sulfide migrating towards the interface and generating an image inversion layer.

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.

The oscillatory magnetoresistance wave form of a 2-D electron gas shows multiple structures when two subbands are populated. In addition to high-field oscillations at a frequency equal to the sum of the two frequencies corresponding to the concentrations of the subbands, and to a superposition at intermediate fields, we observed oscillations at the difference frequency at low fields and higher temperatures. The field range at which the frequency difference is observed increases with increasing temperature. The crossover from superposition to frequency difference is accompanied by a beat. Similar beats, whose field location shows identical temperature dependence, can be observed in data obtained by other groups on different structures. The various components of the wave form can be attributed to different phase relations between the diagonal and off-diagonal elements of the conductivity tensor. It is shown how the intermodulation term, when inserted into the extended oscillatory equation, can give rise to all three structures.

Transport properties of a pseudomorphic In .20Ga.80As/Al.23Ga.77As HEMT structure have been measured by Hall and SdH techniques. Two samples of identical structures but different doping levels were compared. Low temperature mobility measurements as a function of concentration shows a sharp peak at a Hall concentration of 1.9.1012/cm2. This concentration coincides with the onset of second subband occupancy, indicating that the decrease in mobility is due to intersubband scattering. In spite of the low Al content (23%) large PPC was observed in the highly doped sample only, showing a direct correlation between the PPC and doping concentration of the barrier layer.

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...

A temperature independent device is presented which uses a quantum well structure and delta doping within the channel. The device requires a high delta doping concentration within the channel to achieve a constant Hall mobility and carrier concentration across the temperature range 300-1.4 K. Transistors were RF tested using on-wafer probing and a constant G sub max and F sub max were measured over the temperature range 300-70 K.

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.

A new technique for deriving the electron mobility as minority carriers in semiconductor materials is introduced. The technique is based on the modulation of the Hall voltage through the introduction of a low‐level optical injection. The photogenerated excess carriers produce a signal which is a function of the magnetic field, the carrier mobilities and concentrations, and the concentration of excess carriers and thus to their lifetime. The measurement is easy to implement and it can be performed along with the measurement of the Hall coefficient and the conductivity. The only modification needed in a conventional galvanomagnetic setup is the introduction of a chopped low‐intensity laser. Using these three simultaneous measurements, we determined the equilibrium carrier concentrations, the two mobilities, and the effective excess carrier lifetime of p‐type HgCdTe narrow gap semiconductor. This novel technique allows us to present the mobility of electrons as minority‐carrier throughout the temperature range of 13.6–300.2 K in this material.

The lifetime and carrier‐concentration profile of Hg1−xCdxTe photodiodes (x=0.2–0.3) formed by implantation of B+ ions (100 keV, 1013–1014 cm−2) into a p‐type substrate was investigated. Using graded C‐V measurements, concentration of carriers around the n+p junction was obtained. It was found that the junction is located at a depth of 0.7–1.5 μm, depending on the bulk hole concentration. The technique enables profiling of the same sample after annealing. The annealing process was shown to make the electron concentration shallower and thus to bring the junction closer to the surface. Various techniques were used to measure excess carrier lifetimes. All indicate that the extent of degradation of the excess electron lifetime in the p region is very limited. For all tested samples, an etch of 2 μm was enough to recover the bulk lifetime. These results contradict the general assumption of deep damages in the p‐type substrate. Thus, the contribution of the n+ side to the saturation current and to the R0A produ...

The optical absorption edges of Hg0.71Cd0.29Te and CdTe were measured over the temperature range 80≤T≤300 K. The refractive indices of these materials are determined over a large energy range at T=300 K and their dispersion relation is given. The absorption band tail of Hg1−xCdxTe has an exponential shape. Its slope increases with decreasing temperature. It is suggested that this tail is not caused by permanent lattice disorder but is a material property. An empirical expression for the absorption coefficient as a function of temperature and composition is derived. Using this expression, the zero‐intercept cut‐on energy of the infrared transmission spectrum at room temperature is used to define the composition for any desired thickness of Hg1−xCdxTe sample.

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.