Zn and Cd diffusion in InP were studied in the wide temperature range of 350-580°C to realize a guard ring in InP avalanche photodiodes (APD's). Hole-concentration profiles for Zn and Cd diffusions at various temperatures were found to be expressed by a unified empirical curve, which decreases exponentially with the distance from the surface, and abruptly decreases at the diffusion front. A graded junction can be formed by diffusion at temperatures lower than 500°C for the n-InP background carrier concentration of 10 16 cm -3 , while an abrupt junction can be formed by higher temperature diffusion. Breakdown voltages for the graded-junction diodes formed by low-temperature diffusion were confirmed to be higher than those for the abrupt-junction diodes formed by the higher temperature diffusion. A guard ring formed by the low-temperature Cd diffusion enabled planar-type InP and InGaAs/InP APD's to have uniform multiplication in the photosensitive area without any edge breakdown.