Researchers have studied the use of double layer heterojunction HgCdTe devices for application in focal plane arrays (FPA's). Such devices are built with a wide bandgap semiconductor on top of a narrow bandgap semiconductor. With a highly doped p-type material at the surface, these devices enhance the ability to make contact with the anode side of the diode with interconnect metal. Optimizing FPA performance with heterojunction detectors has posed serious problems because many of the HgCdTe's material parameters vary as a function of composition, temperature and doping concentration. AET, with funding from the US Army Night Vision Labs, has developed a new system for design of focal plane arrays using heterojunction HgCdTe detectors. By using this new software modeling technique, a double layer heterojunction detector device has been designed with consideration for many of the material and environmental variations. This paper develops the models employed in the simulation program and will compare the simulation results with experimental data.