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We compare the experimentally measured and theoretically calculated quantum efficiency (QE), where an analytical drift-diffusion photocurrent model is used, of n+ -on-p InAs-GaSb superlattice (SL) photodiodes. With inputs of the transport parameters obtained by the electron-beam-induced current technique and absorption coefficient spectra calculated by the eight-band kldrp method for the p-SL, n+ -SL, and depletion region, taking into account the band filling effect, we show that the drift-diffusion photocurrent model is a good approximation for the InAs-GaSb type-II SL photodiodes, which implies that the SL depletion region in InAs-GaSb SL photodiodes is as effective as that in bulk semiconductor photodiodes in terms of collecting the photo-excited electron-hole pairs. Using this theoretical model, we also find that the high doping density in n-type SL degrades the QE by reducing the absorption coefficient. As a result, the n-type doping density is suggested to be below 1times1017cm-3 in order to optimize the QE for the studied InAs-GaSb SL photodiodes.