criticality. It should be realized, however, that much more than a comparable amount of funding would be required for gallium arsenide than for silicon because, by comparison, the former is not nearly as advanced in technology. Though similar as far as its operation in a solar cell is concerned, gallium arsenide has entirely different properties and requires unique manufacturing techniques. At present, the only significant manufacturing capability in gallium arsenide exists in one U.S. company. Expanding this effort into a dedicated large-scale production would put industry on a new learning curve that would invariably cause major problems. Furthermore, the time in which to accomplish this mammoth feat, for the quantity of cells that would be required, is extremely short. For silicon, however, it would mean only that the companies who are presently manufacturing integrated circuits would expand their already familiar operations to include solar cells. The cost of gallium arsenide solar cells will never be as low as silicon because of the reasons already stated. However, the cost would not have to be the same to be competitive from a system performance point of view. A reduction in system weight could be an important factor in offsetting the higher initial cost of the cells. The same can also be said concerning minimizing radiation degradation and increasing life in orbit. Too much uncertainty exists at this time to predict what the break-point in cell cost would have to be to make the use of gallium arsenide cells feasible in the SPS arrays. (d) CADMIUM SULFIDE A 5 GW SPS solar array fabricated with 0.025 mm cadmium sulfide solar cells which have a 7.11 percent efficiency at 373 K would require an active area of 51.1 x 10° m . Since the specific gravity of cadmium sulfide is 4.84, the array would contain 6286 tons of cadmium sulfide. This corresponds to 4903 tons of cadmium and 1383 tons of sulfur. During 1972, 3640 tons of cadmium and over 7 x 10® tons of sulfur were produced in the U.S. While there is an ample supply of sulfur, the production of cadmium would have to be increased. The estimated U.S. reserves of cadmium are 2 x 10$ tons. (6) SOLAR CELL FABRICATION In principle, a silicon solar cell is merely a p-n junction diode. The junction gives rise to a potential barrier, and the associated electric field separates the hole-electron pairs, which are generated by incident photon radiation, into a unidirectional electrical current, For silicon, the semiconductor bandgap is 1.1 electron volts. However, the effective potential difference across the junction barrier is 0.55-0.60 volts. P-N junctions are formed by diffusing a n-type dopant into a silicon wafer that has been doped uniformly with p-type dopants. The conventional silicon solar cell is fabricated by taking a p-type solar cell blank and diffusing phosphorous
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