Light atom species, such as Li, Mg, B, Be, Cl, F and Al implanted in bulk and epitaxial HgCdTe of compositions from 3 to 12 µm cut-off yielded n/p junctions. The nature of these junctions has not previously been understood. Implantation of light atoms has been observed to induce n-type electrically active defects which propagate deep into the material during the implantation process. The experiments, performed on an epitaxial wafer of ∼5 µm cut-off wavelength, showed an electron profile of the implanted layer 3 to 4 times deeper than the mean projected range determined by SIMS measurements. The results are in good agreement with the graded junction profile determined from C-V measurement, and have junction depths of 2–3 µm determined from EBIC measurements. It has been concluded that the junction formation mechanism in HgCdTe is determined primarily by radiation induced mobile defects.