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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1984

Performance characteristics of multi-anode microchannel array detector systems

The multi-anode microchannel arrays (MAMAs) are state-of-the-art, pulse-counting, photoelectric array detectors designed specifically for use in space astrophysics instruments. The present paper provides a description of recent progress related to the development of ultraviolet and visible-light versions of the MAMA detectors, taking into account a comparison of the operating characteristics of these devices with those of photoconductive array detectors, such as the CCDs. Attention is given to MAMA detector system design parameters, the operating characteristics of MAMAs and CCDs, MAMA performance characteristics, and future developments.

Timothy, J. G.↗
NASA NTRS1985

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/A10.15Ga0.85As modulation-doped field effect transistors (MODFET's) exhibiting extremely good dc characteristics have been successfully fabricated. dc transconductance in these strained-layer structures of 270 mS/mm was measured for 1-micron gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.

Ketterson, A.↗
NASA NTRS1986

Dark current measurement in photoconductors

The Space Infrared Telescope Facility (SIRTF) is envisioned as a next generation space observatory for observations between 2 and 700 microns. In order to address many of the important scientific questions in areas such as cosmology, star formation, and galaxy evolution, infrared detectors of unparalleled sensitivity will be required. Dark current measurements are described for a number of different discrete photoconductive detectors that may be of importance at the very low backgrounds expected with SIRTF.

Young, E. T.↗
NASA NTRS1986

Transient response of Ge:Be and Ge:Zn FIR photoconductors under low background photon flux conditions

An experimental study of the transient behavior of Ge:Be and Ge:Zn photoconductors to changes in photon flux rates has been performed under the low background flux conditions (10 to the 8th photon/s) typical of astronomy and astrophysics applications. A characteristic transient behavior with time constants ranging from 0.1 to greater than 5 s has been observed in both materials when the shallow levels are very closely compensated. The detector response consists of both a fast and a slow component. The amplitude of the slow component can be up to 10 times greater than the initial fast component. It has been established that this phenomenon cannot be explained by current models of carrier sweep-out or dielectric relaxation. The transient behavior has been characterized as a function of temperature, electric field, photoconductive gain and material parameters.

Haegel, N. M.↗
NASA NTRS1986

MIPS - The Multiband Imaging Photometer for SIRTF

The Multiband Imaging Photometer for SIRTF (MIPS) is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700 micron spectral region. It will use high performance photoconductive detectors from 3 to 200 micron with integrating JFET amplifiers. From 200 to 700 microns, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution.

Rieke, G. H.↗
NASA NTRS1986

MIPS - The Multiband Imaging Photometer for SIRTF

The Multiband Imaging Photometer System (MIPS) for SIRTF is to be designed to reach as closely as possible the fundamental sensitivity and angular resolution limits for SIRTF over the 3 to 700 microns spectral region. It will use high performance photoconductive detectors from 3 to 200 microns with integrating JFET amplifiers. From 200 to 700 microns, the MIPS will use a bolometer cooled by an adiabatic demagnetization refrigerator. Over much of its operating range, the MIPS will make possible observations at and beyond the conventional Rayleigh diffraction limit of angular resolution.

Rieke, G. H.↗
NASA NTRS1987

Charge distribution and response time for a modulation-doped extrinsic infrared detector

The electric charge distribution and response time of a modulation-doped extrinsic infrared detector are determined. First, it is demonstrated theoretically that the photoconductive layer is effectively depleted of ionized majority-impurity charges so that scattering is small and mobility is high for photogenerated carriers. Then, using parameters appropriate to an actual detector, the predicted response time is 10 to the -8th to about 10 to the -9th s, which is much faster than comparable conventional detectors. Thus, the modulation-doped detector design would be valuable for heterodyne applications.

Hadek, Victor↗
NASA NTRS1987

II-VI and I-III-VI2 alloy crystals for photovoltaic applications

The results of the photoreflectance, photoconductivity, and photoluminescence studies on Mn(x)Cd(1-x)Te single crystals are discussed from the viewpoint of the utility of these compounds as gap components in multijunction solar cells. Attention is also given to recent results on CuInSe2 and on quaternary I-III-VI2 alloys based on CuInSe2. Recent Kelvin probe and X-ray photoemission spectroscopy studies of the surface properties of CuInSe2 and Mn(x)Cd(1-x)Te, respectively, are described. In addition, recent results on the radiation damage and annealing kinetics of Mn(x)Cd(1-x)Te are considered in the context of future work on the sputtering and ion beam deposition of thin films of this material.

Bachmann, K. J.↗
NASA NTRS1988

High frequency GaAlAs modulator and photodetector for phased array antenna applications

A waveguide Mach-Zehnder electro-optic modulator and an interdigitated photoconductive detector designed to operate at 820 nm, fabricated on different GaAlAs/GaAs heterostructure materials, are being investigated for use in optical interconnects in phased array antenna systems. Measured optical attenuation effects in the modulator are discussed and the observed modulation performance up to 1 GHz is presented. Measurements of detector frequency response are described and results presented.

Claspy, P. C.↗
NASA NTRS1987

Characterization of graded interface In(x)Ga(1-x)As/In(0.52)Al(0.48)As (x between 0.53 and 0.70) structures grown by molecular-beam epitaxy

Modulation-doped In(x)Ga(1-x)As/In(0.52)Al(0.48)As/InP structures were grown by molecular-beam epitaxy with x values between 53 and 70 percent. For pseudomorphic cases, graded instead of abrupt interfaces were used. Hall mobility and persistent photoconductivity measurements as a function of temperature were used to characterize samples with different structural parameters. Consistent trends in the variation of mobilities and two-dimensional carrier concentration, n(2D), under light and dark conditions have been observed and discussed in terms of applicable scattering mechanisms. The Hall mobilities are comparable to the best results obtained to date but with significantly higher n(2D) concentration.

Peng, C. K.↗
NASA NTRS1989

Microwave characteristics of GaAs MMIC integratable optical detectors

Interdigitated photoconductive detectors were fabricated on microwave device structures, making them easily integratable with Monolithic Microwave Integrated Circuits (MMIC). Detector responsivity as high as 2.5 A/W and an external quantum efficiency of 3.81 were measured. Response speed was nearly independent of electrode geometry, and all detectors had usable response at frequencies to 6 GHz. A small signal model of the detectors based on microwave measurements was also developed.

Claspy, Paul C.↗
NASA NTRS1989

Blocked impurity band hybrid infrared focal plane arrays for astronomy

High-performance infrared hybrid focal plane arrays using 10- x 50-element Si:As blocked-impurity-band (BIB) detectors (cutoff wavelength = 28 microns) and matching switched MOSFET multiplexers have been developed and characterized for space astronomy. Use of impurity-band-conduction technology provides detectors which are nuclear-radiation-hard and free of the many anomalies associated with conventional silicon photoconductive detectors. Emphasis in the present work is on recent advances in detector material quality which have led to significantly improved detector and hybrid characteristics. Results demonstrating increased quantum efficiency (particularly at short-wavelength infrared), obtained by varying the BIB detector properties (infrared active layer thickness and arsenic doping profile), are summarized. Measured read noise and dark current for different temperatures are reported. The hybrid array performance achieved demonstrates that BIB detectors are well suited for use in astronomical instrumentation.

Reynolds, D. B.↗
NASA NTRS1990

Optically-Tuned Far-Infrared Device

Spacing of visible interference pattern determines frequency of infrared radiation. Variable photoconductivity grating formed by interference of two visible laser beams intersecting on surface of proposed AlGaAs device substituted for fixed metal grating. Proposed device provides higher output-power density and greater tunability.

Katz, Joseph↗
NASA NTRS1990

Detection Of Gas-Phase Polymerization in SiH4 And GeH4

Inelastic scattering of laser light found to indicate onset of gas-phase polymerization in plasma-enhanced chemical-vapor deposition (PECVD) of photoconductive amorphous hydrogenated silicon/germanium alloy (a-SiGe:H) film. In PECVD process, film deposited from radio-frequency glow-discharge plasma of silane (SiH4) and germane (GeH4) diluted with hydrogen. Gas-phase polymerization undesirable because it causes formation of particulates and defective films.

Shing, Yuh-Han↗
NASA NTRS1990

Binary Operation Of A Liquid-Crystal Light Valve

Conditions for operation of commercially available liquid-crystal light valve as binary spatial light modulator discovered. In mode, modulator turns on sharply and then saturates as intensity of writing beam increases. Valve comprises photoconductive layer and liquid-crystal layer separated by dielectric mirror and sandwiched between two transparent electrodes. Potential applications include enhancement of images, optical recording, and holography.

Davis, Jeffrey A.↗
NASA NTRS1990

Fast Feature-Recognizing Optoelectronic System

Proposed optoelectronic system recognizes features or classifies images by processing outputs of photosensors rapidly, in parallel, through circuits developed in research on neural networks. Array of photoconductive elements serve as photomodulated connections in electronic neural network, which provides high speed data compression to generate feature vector. System able to "learn" new patterns for subsequent recognition. Potential applications in robotic vision systems and pattern recognition.

Thakoor, S.↗
NASA NTRS1989

Germanium Blocked Impurity Band (BIB) detectors

Information is given in viewgraph form. The advantages of the Si blocked impurity band (BIB) detector invented by M. D. Petroff and M. G. Stabelbroek are noted: smaller detection volume leading to a reduction of cosmic ray interference, extended wavelength response because of dopant wavefunction overlap, and photoconductive gain of unity. It is argued that the stated advantages of Si BIB detectors should be realizable for Ge BIB detectors. Information is given on detector development, subtrate choice and preparation, wafer polising, epitaxy, characterization of epi layers, and preliminary Ge BIB detector test results.

Haller, E. E.↗