The 2-2.8 μm vertically stacked multiple-junction PIN GalnAsSb/GaSb photodetectors are analyzed, optimized, and designed based on an improved general mode to predict the optimal performance of PIN infrared photodetectors arising from nonequilibrium carrier generation, diffusion, and recombination theory. Optimal thickness of intrinsic absorption region (Wopt), response quantum efficiency (RQE), detectivity (D*), and -3-dB cutoff frequency (f_3dB) are calculated and optimized for a 2-2.8 μm room-temperature high-frequency operation. Ways to achieve optimal performance in practice, material, and device structures are proposed. The optimized vertically stacked multiple-junction PIN GalnAsSb/GaSb photodetector structure shows a D* of (1.6-1.9) ?? 1012 cm?? Hz1/2/W,an RQE of 52%-69%, and a f- 3 dB ≫ 20 GHz with Wopt = 3 μm and junction number K = 5, effective illumination area Ad = 1000 μm2, and reverse bias voltage VRB = 0.5 V. The proposed general model is validated by simulation and measurement data of fabricated single-junction detectors.