N-on-p junction formation and drive-in in ion implanted Hg0.8Cd0.2Te photodiodes have been studied. A model of the junction formation and drive-in processes has been developed that accounts for the variations in injected Hg interstitial concentration, background point defect and extrinsic doping levels, sample geometry, and annealing conditions. The limiting mechanisms controlling junction drive-in were investigated using the model. Experimental data showed the junction drive-in rate was proportional to the square root of time, indicating a diffusion limited process. The diffusion limited process is the result of a solubility limit for the Hg interstitial concentration. This limit is approximately the same value as that obtained for Hg interstitials in Hg saturated Hg0.8Cd0.2Te in type conversion and self-diffusion experiments (DICI = 1.43 × 1013exp(−.457 eV/ kT)*PHg).