Two possibilities for observing the infrared absorption of residual phosphorus inp‐type silicon are discussed. Experimental results show that inp‐type silicon the pairing of Mg, presumably with boron and/or oxygen, produces donor complexes with ionization energy less than that of phosphorus. The integrated intensity of the phosphorus lines of the sample after Mg diffusion indicates a phosphorus concentration of ∼ 1.5 × 1013atoms/cm3.