Gordon C. Taylor

Quaternary InGaAsSb Thermophotovoltaic Diodes

InxGa1-xAsySb1-y thermophotovoltaic (TPV) diodes were grown lattice matched to GaSb substrates by metal-organic vapor phase epitaxy in the bandgap range of EG = 0.5 to 0.6 eV. InGaAsSb TPV diodes, utilizing front-surface spectral control filters, are measured with thermal-to-electric conversion efficiency and power density (PD) of nTPV = 19.7% and PD = 0.58 W/cm2, respectively, for a radiator temperature of Tradiator = 950 degC, diode temperature of Tdiode = 27 degC, and diode bandgap of EG = 0.53 eV. Practical limits to TPV energy conversion efficiency are established using measured recombination coefficients and optical properties of front surface spectral control filters which for 0.53-eV InGaAsSb TPV energy conversion are nTPV = 28% and PD = 0.85 W/cm2 at the above operating temperatures. The most severe performance limits are imposed by 1) diode open-circuit voltage (VOC) limits due to intrinsic Auger recombination and 2) parasitic photon absorption in the inactive regions of the module. Experimentally, the diode VOC is 15% below the practical limit imposed by intrinsic Auger recombination processes. Analysis of InGaAsSb diode electrical performance versus diode architecture indicates that VOC and thus efficiency are limited by extrinsic recombination processes such as through bulk defects

Silicon-based reconfigurable antennas-concepts, analysis, implementation, and feasibility

We report on an innovative reconfigurable antenna concept with significant practical relevance based on the dynamic definition of metal-like conductive plasma channels in high-resistivity silicon that are activated by the injection of DC current. The plasma channels are precisely formed and addressed using current high-resolution silicon fabrication technology. These dynamically defined plasma-reconfigurable antennas enable frequency hopping, beam shaping, and steering without the complexity of RF feed structures. This concept shows promise for delivering the performance and capabilities of a phased array, but at a reduced cost. However, challenges such as p-i-n biasing circuit complexity and their nonlinearities, as well as antenna efficiency, would still require further investigations.

Silicon based reconfigurable antennas

Summary form only given. Selects solid-state plasmas generated by PIN junctions to effectively implement various reconfigurable antennas. Other alternative technologies include MEMs switches to reconfigure antennas. PIN junctions can also be utilized as switches, and can replace MEMs to reconfigure antennas where speed of response is an issue. We have utilized these PIN junctions to dynamically define plasma regions with sufficient conductivity. Injecting high dc currents into these junctions in high resistivity silicon creates plasma regions (domains) with relatively high conductivity. The locations and shapes of these defined islands can be precisely controlled over the whole processed silicon wafer, as high resolution is a routine matter of todays silicon technology. The defined metallic-like patterns can include the radiating elements and the feed structures as well. The dipole antenna described is segmented into three different sections to allow wide frequency coverage from 1 to 20 GHz frequency range.

Curved CCDs and their application with astronomical telescopes and stereo panoramic cameras

The creation of curved CCD’s and the mosaicing of contoured CCD’s mounted within the curved focal planes of telescopes and stereo panoramic imaging cameras introduces a revolution in optical design that greatly enhances the scientific potential of such instruments. In the alteration of the primary detection surface within the instrument’s optical system from flat to curved, and precisely matching the applied CCD’s shape to the contour of the curved focal plane, a major increase in the amount of transmittable light at various wavelengths through the system is achieved, thereby enabling multi-spectral ultra-sensitive imaging for a variety of experiments simultaneously, including autostereoscopic image acquisition. For earth-based and space-borne optical telescopes, the advent of curved CCD’s as the principle detectors provides a simplification of the telescope’s adjoining optics, reducing the number of optical elements and the occurrence of optical aberrations associated with large corrective optics used to conform to flat detectors. New astronomical experiments may be devised in the presence of curved CCD applications, including 3 dimensional imaging spectroscopy conducted over multiple wavelengths simultaneously, wide field real-time stereoscopic tracking of remote objects within the solar system at high resolution, and deep field mapping of distant objects such as galaxies with much greater precision and over larger sky regions. Stereo panoramic cameras equipped with arrays of curved CCD’s will require less optical glass and no mechanically moving parts to maintain proper stereo convergence over wider perspective viewing fields than their flat CCD counterparts, making the cameras lighter and faster in their ability to scan and record 3 dimensional objects moving within an industrial or terrain environment. Preliminary experiments conducted at the Sarnoff Corporation indicate the feasibility of curved CCD imagers with acceptable electro-optic integrity. Currently, we are in the process of evaluatingthe electro-optic performance of a curved wafer scale CCD imager. Detailed ray trace modeling and experimental electro-optical data performance obtained from the curved imager will be presented at the conference.

High Performance InGaAsSb TPV Cells via Multi‐Wafer OMVPE Growth

The fabrication and performance of InGaAsSb thermophotovoltaic cells are described. The InGaAsSb layers, grown by organometallic vapor‐phase epitaxy in a multi‐wafer reactor, with a 0.53 eV bandgap are lattice‐matched to a GaSb substrate. Growth series with up to thirty 50 mm wafers have been done with good control of material composition and carrier transport properties. With improved materials and metallization and with a modification to the cell edges, fill factors near 70% and a greater than 60% peak external quantum efficiency are obtained. A two order‐of‐magnitude increase in shunt resistance with a consequent 15% improvement in fill factor was achieved with the improved edge structure. Series resistance, about 20 mΩ, is the remaining limitation to cell performance and is closely correlated with fill factor.

Ion-Implanted K-Band GaAs Power FET

This paper reports the performance up to 26 GHz from a GaAs power FET produced by ion implantation. At 15 GHz, an output power of 250 mW at 3 dB gain with 27.4% power-added efficiency was obtained. At 26 GHz, 55 mW at 3 dB gain with 5% power-added efficiency was demonstrated. Capless annealing and a novel lift-off gate fabrication scheme was employed.

Novel selective-plated heatsink, key to compact 2-watt MMIC amplifier

We describe an improved heatsink technology fully compatible with a standard MMIC processing that significantly decreases the thermal limitation of the MMIC format. By etching to reduce the substrate thickness by 75 micron only immediately under the active areas of the MESFETs and selectively plating solid gold heatsinks to replanarize the back of the wafer, a 45 percent reduction in the thermal resistance is obtained. Despite a very compact design, 2.4 mm MESFETs fabricated on 100 micron thick substrates demonstrated a thermal resistance of only 18 C/W. Using these devices, a 2 stage 2 watt power MMIC was designed to fit a compact 1.4 mm x 3.25 mm chip footprint. The nominal 2 watt, 7-11 GHz power MMIC amplifier was designed for phased array applications where small size, high power and high efficiency are primary concerns. With fixed off-chip tuning, the MMIC delivers 1.7-2.3 watts with 18-24 percent power added efficiency across the full 7-11 GHz band.

High‐Efficiency Multi‐Cell TPV Module Fabrication and Performance

Sarnoff Corporation has developed thermophotovoltaic (TPV) modules using either Sarnoff grown and processed InGaAsSb/GaSb TPV cells or cells fabricated by our co‐workers. The TPV module fabrication includes substrate design, fabrication and module assembly. The substrate comprises an AlN base plate and Al2O3 septa. The septa separate the cells and support metal layers to electrically connect the bottom of a cell to the top of the adjacent septum. A welded gold ribbon connects the septum to the topside of the adjacent cell. The detailed structure of the substrate, as well as the module assembly process, is discussed. TPV modules of 1 cm × 1 cm area with two 1 cm × 0.5 cm cells and 2 cm × 2 cm area with eight cells are fabricated routinely. Alternative approaches to the Al2O3/AlN substrate and ribbon connection have been explored for low‐cost, large‐scale production. Silicon based substrates show promising results. KOH wet etching of silicon produces septa with straight walls having perfect 90‐degree angles...

Minority-carrier transport in InGaAsSb thermophotovoltaic diodes

Uncoated InGaAsSb/GaSb thermophotovoltaic (TPV) diodes with 0.56 eV (2.2 {micro}m) bandgaps exhibit external quantum efficiencies of 59% at 2 {micro}m. The devices have electron diffusion lengths as long as 29 {micro}m in 8-{micro}m-wide p-InGaAsSb layers and hole diffusion lengths of 3 {micro}m in 6-{micro}m-wide n-InGaAsSb layers. The electron and hole diffusion lengths appear to increase with increasing p- and n-layer widths. At 632.8 nm the internal quantum efficiencies of diodes with 1- to 8-{micro}m-wide p-layers are above 89% and are independent of the p-layer width, indicating long electron diffusion lengths. InGaAsSb has, therefore, excellent minority carrier transport properties that are well suited to efficient TPV diode operation. The structures were grown by molecular-beam epitaxy.

K- and Ka-Band High Efficiency Amplifier Modules Using GaAs Power FETs

Sub-half-micrometer GaAs power FETs fabricated with a side-etched-gate technology (1-3) have been developed that exhibit low junction temperatures suitable for space application (4). With these devices high efficiency amplifier modules have been developed and will be reported on. At 20 GHz, using these devices, a high gain module was assembled that had an output power of 775 +- 75 mW from 18- to 20-GHz with 6.9 +- 0.2 dB gain, and a maximum efficiency of 20.3%. At 35 GHz, an output power of 210 mW with 3 dB gain and 22% efficiency has been obtained with one 0.6 mm width cell, and two cells were combined to obtain 300 mW with 3 dB gain and 18.8% efficiency. The power and efficiency results obtained at 35 GHz are some of the highest reported to date and indicate that Ka-band solid state power amplifiers are feasible.