Low bandgap Si(1-x)Ge(x) solar cells are designed, fabricated, characterized and analyzed for the purpose of acting as the bottom cell in a GaAsP-SiGe tandem solar cell. The development of the SiGe cell under a GaAsP cell can lead to a 34% relative increase in efficiency over that of a silicon solar cell. This work focuses on making a SiGe bottom cell that can generate 21 mA/cm(2) of short circuit current (J(sc)) at 1 Sun from photons beyond 780 nm and with a 450 mV band gap-voltage offset (W-oc) under 20 Suns illumination. Numerical and analytical methods are introduced to demonstrate the SiGe solar cells' performance limits and to examine the trade-offs among I-V performance, cell structure and material composition. Material compositions are confirmed with energy-dispersive X-ray spectroscopy (EDS) and electrochemical capacitance voltage profiling (ECV). First principles analysis shows that a Si.15Ge.85 cell with a 5 mu m base can produce the expected current and an efficiency of 8.8% under 20 Suns. By increasing base doping, we achieve a W-oc of 435 mV under 20 Suns with a Si.18Ge.82 cell. The best 1 Sun short circuit current measured from a Si.12Ge.88 cell at wavelengths beyond 780 nm without antireflection coating, back surface reflector, or back texturing is 12.9 mA/cm(2). Adding a back surface field to the structure will lead to a higher V-oc and lower W-oc. Implementing light trapping with a higher base angle of pyramids can lead to the target J(sc) of 21 mA/cm(2). (C) 2014 Elsevier B.V. All rights reserved.