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This paper analyzes the performance improvements obtainable from extending the traditionally thin back‐surface‐field (BSF) layer deep into the base of silicon solar cells under terrestrial solar illumination (AM1). This extended BSF cell is also known as the back‐drift‐field cell. About 100 silicon cells have been analyzed, each with a different emitter or base dopant impurity distribution whose selection was based on physically anticipated improvements. The four principal performance parameters (the open‐circuit voltage, the short‐circuit current, the fill factor, and the maximum efficiency) are computed using a program which numerically solves the six Shockley equations under AM1 solar illumination at 88.92 mW/cm2, at an optimum cell thickness of 50 μm. The results show that very significant performance improvements can be realized by extending the BSF layer thickness from 2 μm (18% efficiency) to 40 μm (20% efficiency). The immunity of cell performance to recombination defect or impurity center is also improved by a factor of 2 to 3 in the recombination center density. For 20% p+/n/n+ cells with about 20‐μs base lifetime, a 20‐μm BSF penetration is sufficient. At this or deeper penetrations, interband Auger recombination in the emitter layer becomes the limiting factor and affects mainly the short‐circuit current but not the open‐circuit voltage which has saturated to about 710 mV.