The standard diffusion length (Lp) test procedure is apt to damage the p-n junction, so we often use characteristic decay length (L) obtained by laser beam induced current (LBIC) instead of Lp. Two dimensional modeling is used to get the relations of L and Lp. Calculated L/Lp ratio is about 1.1, and it is not affected by doping concentration, Shockley-Read-Hall (SRH) lifetime and mobility. For HgCdTe photodiodes, we get characteristic decay length from the LBIC experiment, and get the true diffusion length by division by the ratio 1.1.