Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “junctions”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,477 records · Page 82
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

The spectral response of a front surface field solar cell

A calculation of the short-circuit current as a function of wavelength of the incident light, induced in a front surface field solar cell, is presented. The cell consists of a P(+)-P-N(+) junction. The electric field present in the P(+) layer situated at the front surface (the surface exposed to the sun) boosts the minority carrier collection at the P-N(+) junction sufficiently to make this structure a viable alternative to ordinary solar cells. A distinct advantage derives from the placement of all ohmic contacts on the back-surface facilitating array assemblage.

Von Roos, O.↗
NASA NTRS1980

The effects of titanium impurities in N/+//P silicon solar cells

Microscopic and electrical measurements were performed to explain the degradation mechanisms associated with the presence of titanium impurities in silicon. The measurements included X-ray topography, transmission electron microscopy, and deep level transient spectroscopy, before and after processing. The results indicated the presence of TiO2 precipitates, the density of which increased after phosphorus diffusion. A majority carrier trapping level was observed in the wafers before processing. It was concluded that 10% of the Ti in the N(+)/P silicon solar cells formed electrically active centers which caused degradation of the cell junction. 14% of the remaining Ti precipitated out as TiO2, forming electrically active defects, which also caused junction degradation.

Salama, A. M.↗
NASA NTRS1980

Effects of grain boundaries in polycrystalline solar cells

In polycrystalline solar cells, the short-circuit current is reduced (compared to single-crystal cells) due to recombination of minority carriers at the grain boundaries. The magnitude of this reduction is calculated for the case of a monochromatic beam of light, assuming that both the light beam and the grain boundaries are perpendicular to a p-n junction. The photoresponse of the junction is calculated in terms of the absorption coefficient of the light, the minority-carrier diffusion length, the surface recombination velocity at the grain boundary and the distance of the light beam from the grain boundary. The analysis gives an expression for an effective grain boundary width which depends on wavelength. This width determines the reduction in short-circuit current of solar cells made from material with columnar-type grains.

Zook, J. D.↗
NASA NTRS1980

Incorporation of superlattice crystal layers in multijunction solar cells

Superlattice layers are effective in decreasing the density of dislocations in lattice mismatched heterostructures at least four orders of magnitude. Hence it was proposed to utilize this feature of superlattices to alleviate the problems due to misfit dislocations generated in the regions between two or more photovoltaic collecting junctions. A further advantage is that the possibility is presented for using silicon as a low cost substrate as well as for the low band gap junction. In the test case, a silicon low gap cell was connected to a GaAs.7P.3 high gap cell through a connecting region containing a GaAs/GaP superlattice.

Blackslee, A. E.↗
NASA NTRS1981

Buck/boost regulator

A voltage regulated DC to DC converter uses an inductor and a capacitor as storage elements. The inductor is composed of two windings having a common junction. A transformer with a center tap connected to the common junction of the two windings is connected at either end of its winding to ground through controlled switches. One winding of the inductor and either end of the transformer winding are connected by power diodes to the capacitor which supplies the output voltage to a load. The other winding of the inductor is connected to a fourth power diode as a clamping diode. Input voltage is supplied to the inductor through a third controlled switch. A pulse width modulator connected to the output of the converter alternately closes and opens the switches connected to either end of the transformer winding and also closes the switch supplying input voltage to the inductor each time either of the switches connected to the ends of the transformer winding are closed. The duty cycle of the closing and opening of the several switches is adjusted by the pulse modulator to regulate the output voltage.

Paulkovich, J.↗
NASA NTRS1980

Silicon solar cells with high open-circuit voltage

Open-circuit voltages as high as 0.645 V (AM0-25 C) have been obtained by a new process developed for low-resistivity silicon. The method utilizes high-dose phosphorus implantation, followed by furnace annealing and simultaneous oxide growth to form high-efficiency, shallow junctions. The effect of the thermally grown oxide is a reduction of surface recombination velocity; the oxide also acts as a moderately efficient AR coating. Boron doped silicon with resistivities from 0.1 to 0.3 ohm-cm has been processed according to this sequence; results show highest open-circuit voltage is attained with 0.1-ohm-cm starting material. The effects of bandgap narrowing, caused by high doping concentrations in the junction, were also investigated by implanting phosphorus over a wide range of dose levels.

Minnucci, J. A.↗
NASA NTRS1980

Low cost processes for fabricating silicon solar cells

Solar cell fabrication processes, in particular junction formation and metallization, are evaluated in terms of cell efficiencies, process yields, module packing factors, and energy cost effectiveness. It is shown that for junction formation, the diffusion processes provide a relatively low-cost approach. The costs per unit cell area can be further reduced by increased wafer area and mechanized wafer handling. The costs for a large number of metallization processes, excluding the costs of the metal, are roughly comparable. However, their varying influence on cell performance leads to a significant spread in the allowable process costs.

Goldman, H.↗
NASA NTRS1980

Experimental and theoretical investigations of the quality factor for N/+/P silicon solar cells

Many N(+)P silicon solar cells made with silicon from different growth techniques have current-voltage relations of the form: I = I(0) (exp(qV/AkT) - 1), where the quality factor A is nonintegral, is greater than 1, and shows a temperature dependence. The dark forward characteristics of such cells have been measured over a range of temperatures and the behavior of the factor A derived from them. Lack of agreement with previous models has led to the development of a new model, in which N(+) conduction electrons tunnel to deep levels near that side, these levels being due to junction contamination by impurities. Electron recombination then occurs with holes thermally assisted into the junction from the P side. This mechanism involves increased I(0) values over those for diffusion diode processes and thus reduces the cell power conversion efficiency.

Garlick, G. F. J.↗
NASA NTRS1982

Solar Cells From Metallurgical-Grade Silicon

Epitaxial deposition produces acceptable solar cells from metallurgicalgrade silicon. Instead of diffusing dopants into silicon to form pn junction, junction is formed by growing epitaxial layer--one having crystal structure continuous with that of substrate--on metallurgical-grade silicon. Less pure forms of silicon, such as metallurgical-grade, are less expensive than semiconductor-grade material and help to lower cost of solar cells.

Daiello, R. V.↗
NASA NTRS1981

Design of throat section of conical corrugated horns

A simple graphical method is presented for designing the junction between a conical corrugated horn and a smooth-wall circular waveguide. The method gives both an insight into the field behavior along the horn and an indication of the reflection coefficient at the horn-waveguide junction. It has been applied to the design of the DSN X-S band feedhorn and an improved corrugated throat section.

Olver, A. D.↗
NASA NTRS1981

Processing of silicon solar cells by ion implantation and laser annealing

Methods to improve the radiation tolerance of silicon cells for spacecraft use are described. The major emphasis of the program was to reduce the process-induced carbon and oxygen impurities in the junction and base regions of the solar cell, and to measure the effect of reduced impurity levels on the radiation tolerance of cells. Substrates of 0.1, 1.0 and 10.0 ohm-cm float-zone material were used as starting material in the process sequence. High-dose, low-energy ion implantation was used to form the junction in n+p structures. Implant annealing was performed by conventional furnace techniques and by pulsed laser and pulsed electron beam annealing. Cells were tested for radiation tolerance at Spire and NASA-LeRC. After irradiation by 1 MeV electrons to a fluence of 10 to the 16th power per sq cm, the cells tested at Spire showed no significant process induced variations in radiation tolerance. However, for cells tested at Lewis to a fluence of 10 to the 15th power per sq cm, ion-implanted cells annealed in vacuum by pulsed electron beam consistently showed the best radiation tolerance for all cell resistivities.

Minnucci, J. A.↗
NASA NTRS1980

The SPS interference problem-electronic system effects and mitigation techniques

The potential for interference between solar power satellites (SPS) and other Earth satellite operations was examined along with interference problems involving specific electronic devices. Conclusions indicate that interference is likely in the 2500 MHz to 2690 MHz direct broadcast satellite band adjacent to SPS. Estimates of the adjacent channel noise from SPS in this band are as high as -124 dBc/4 kHz and -100 dBc/MHz, where dBc represents decibels relative to the total power in the fundamental. A second potential problem is the 7350 MHz, 3d harmonic from SPS that falls within the 7300 MHz to 7450 MHz space to Earth, government, satellite assignment. Catastrophic failures can be produced in integrated circuits when the microwave power levels coupled into inputs and power leads reach 1 to 100 watts. The failures are typically due to bonding wire melting, metallization failures, and junction shorting. Nondestructive interaction or interference, however, generally occurs with coupled power levels of the order of 10 milliwatts. This integration is due to the rectification of microwave energy by the numerous pn junctions within these circuits.

Juroshek, J. R.↗
NASA NTRS1982

Mathematical analysis of the Photovoltage Decay (PVD) method for minority carrier lifetime measurements

When the diffusion length of minority carriers becomes comparable with or larger than the thickness of a p-n junction solar cell, the characteristic decay of the photon-generated voltage results from a mixture of contributions with different time constants. The minority carrier recombination lifetime tau and the time constant l(2)/D, where l is essentially the thickness of the cell and D the minority carrier diffusion length, determine the signal as a function of time. It is shown that for ordinary solar cells (n(+)-p junctions), particularly when the diffusion length L of the minority carriers is larger than the cell thickness l, the excess carrier density decays according to exp (-t/tau-pi(2)Dt/4l(2)), tau being the lifetime. Therefore, tau can be readily determined by the photovoltage decay method once D and L are known.

Vonroos, O. H.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the "flat-spot" (FS) power loss phenomenon observed in silicon solar cells operating deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallurization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗
NASA NTRS1981

Long term behavior of silicon germanium thermoelectric generators

Results of tests of the long term performance of SiGe radioisotope thermoelectric generators (RTG) are presented. Three modules were monitored for 17,000-32,300 hr at hot junction temperatures of 1,085, 1,055, and 1,000 C; coating the unicouples with a 12,000 A thick layer of Si3N4 protected the modules from Si sublimation. Output degraded less than 0.3-0.4%/1,000 hr over the testing period. Life tests on a multihundredwatt (MHW) SiGe generator with 312 couples at a hot shoe temperature of 1,040 C dealt with power of 150 W at 30 V, with 0.5%/1,000 hr performance degradation. Si deposition on the insulation was found to enhance electrical conductance until a saturation point was reached. Disassembly of a test module after 16,750 hr revealed a Mo build-up, SiN4 coating deterioration, and Ti diffustion from the hot shoe to cooler regions of the junction. The presence of an Al2O3 insulator was recognized as preventing coating loss. Performance records from the Voyager and Les-8 satellites' RTG's are compared and show similar, 0.25%/1,000 hr degradation rates; RTG storage is judged to be feasible and the tests lead to projections of a 600,000 hr lifetime for a SiGe RTG.

Shields, V.↗
NASA NTRS1981

Advanced high temperature thermoelectrics for space power

Preliminary results from a spacecraft system study show that an optimum hot junction temperature is in the range of 1500 K for advanced nuclear reactor technology combined with thermoelectric conversion. Advanced silicon germanium thermoelectric conversion is feasible if hot junction temperatures can be raised roughly 100 C or if gallium phosphide can be used to improve the figure of merit, but the performance is marginal. Two new classes of refractory materials, rare earth sulfides and boron-carbon alloys, are being investigated to improve the specific weight of the generator system. Preliminary data on the sulfides have shown very high figures of merit over short temperature ranges. Both n- and p-type doping have been obtained. Pure boron-carbide may extrapolate to high figure of merit at temperatures well above 1500 K but not lower temperature; n-type conduction has been reported by others, but not yet observed in the JPL program. Inadvertant impurity doping may explain the divergence of results reported.

Lockwood, A.↗
NASA NTRS1982

Computer-aided prediction of high-frequency performance limits in silicon bipolar integrated circuits

A circuit model for an existing silicon integrated bipolar junction transistor (IBJT) is used to evaluate presently achievable high frequency circuit performance. The relationship between circuit model and processing parameters are semi-quantitatively explored to make predictions on the frequency response, which can be achieved through realistic device fabrication modifications. A new figure of merit is introduced, which is defined as the signal frequency at which an integrated bipolar junction transistor can deliver a power gain of G. The most sensitive parameter influencing attainable high frequency IBJT performance is base resistance.

Burns, J. L.↗
NASA NTRS1982

On the cause of the flat-spot phenomenon observed in silicon solar cells at low temperatures and low intensities

A model is presented that explains the 'flat-spot' power-loss phenomenon observed in silicon solar cells operating under deep space (low temperature, low intensity) conditions. Evidence is presented suggesting that the effect is due to localized metallurgical interactions between the silicon substrate and the contact metallization. These reactions are shown to result in localized regions in which the PN junction is destroyed and replaced with a metal-semiconductor-like interface. The effects of thermal treatment, crystallographic orientation, junction depth, and metallization are presented along with a method of preventing the effect through the suppression of vacancy formation at the free surface of the contact metallization. Preliminary data indicating the effectiveness of a TiN diffusion barrier in preventing the effect are also given.

Weizer, V. G.↗