A study was made of the diffusion of Cd into nominally undoped (4×1016 cm−3) n‐type InP using elemental sources consisting of (Cd and In) or (Cd and P). Two diffusion fronts whose depths are controlled by the amount of P or In in the diffusion source were delineated by etching. Electron beam induced current profiles and photoluminescence measurements revealed the formation of a p+‐ p−‐ n− junction, with the p− region bounded by the two diffusion fronts. Mesa p+‐ p−‐ n− diodes, 150 μm in diameter, fabricated from a wafer with the p+‐ p− and p−‐ n− junctions at 3.1 and 20.0 μm, respectively, have reverse breakdown voltages between 90 and 220 V and reverse leakage currents from 5 to 10 pA at half breakdown. The I‐V characteristics of these p+‐p−‐n− diodes exceed those of state of the art, abrupt p+‐n− InP avalanche photodiodes because of the formation of the p− region.