We report on the design, fabrication, testing, and modeling of single-heterojunction Al0.27Ga0.73As/GaAs p-i-n photodiodes for use as components in optical receivers. The photodiodes are grown by molecular beam epitaxy and fabricated as 1100-mu-m2 mesa structures. At 5-V reverse bias and 850 nm, we typically measure 100 fF of capacitance, 90 pA of leakage current, 73% external quantum efficiency, < 2% reflectivity, and 22-GHz bandwidths.