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Two gallium arsenide solar cell configurations have evolved over the last decade-the heteroface solar cell  which uses a (GaAl) As surface passivating layer over a p-n solar cell; and a n+-p-p+shallow homojunction solar cell . Energy conversion efficiencies of 18.8 percent have been reported with the heteroface structure . In this paper, we report an improved design, shallow junction heteroface, n-p, gallium arsenide solar cell for space applications, with a predicted AMO efficiency in the range of 22.7 to 23.9 percent. The analysis was based on the technique of loss minimization . A similar analysis for p-n heteroface solar cells leads to a projected AMO efficiency in the 21.9 to 23.0 percent range. The optimized n-p structure, while slightly more efficient, has the added advantage of being less susceptible to radiation-induced degradation by virtue of its thin top junction layer . Detailed spectral response curves and an analysis of the loss mechanisms are reported. The details of the design are readily measurable. The optimized designs were reached by quantifying the dominant loss mechanisms and then minimizing them using computer simulations. This methodology is also reported.