To meet the requirement of photon-starved detection, separate absorption grading charge multiplication avalanche photodiodes (SAGCM APDs) have drawn more and more attentions. In this work, we presents a detailed analysis of dark currents for planar-type SAGCM InGaAs–InP APDs with different sub-micron thicknesses of multiplication layer, considering the effect of the diffusion process, generation-recombination process and tunneling process. According to our knowledge, it is the first time clearly demonstrating that the dominant component of dark current for a SAGCM APD changes with multiplication gain from surface to volume contribution. Our finding could be beneficial to the optimization of an APD with a SAGCM structure. An optimal InGaAs–InP APD with low-voltage operation (<40 V) in combination with ultra-low dark current (0.10 [email protected]M = 8) has been demonstrated.