The cost and performance of hybrid HgCdTe infrared focal plane arrays are constrained by the necessity of fabricating the detector arrays on a CdZnTe substrate. These substrates are expensive, fragile, are available only in small rectangular formats, and are not a good thermal expansion match to the silicon readout integrated circuit. We discuss in this paper an infrared sensor technology based on monolithically integrated infrared focal plane arrays that could replace the conventional hybrid focal plane array technology. We have investigated the critical issues related to the growth of HgCdTe on Si read-out integrated circuits and the fabrication of monolithic focal plane arrays: (1) the design of Si read-out integrated circuits and focal plane array layouts, (2) the low temperature cleaning of Si(001) wafers, (3) growth of CdTe and HgCdTe layers on read-out integrated circuits, (4) array fabrication, interconnection between focal plane array and read-out integrated circuit input nodes and demonstration of the photovoltaic operation, and (5) maintenance of the read-out integrated circuit characteristics after substrate cleaning, molecular beam epitaxy growth and device fabrication. Crystallographic, optical and electrical properties of the grown layers are presented. Electrical properties for diodes fabricated on misoriented Si and read-out integrated circuit substrates are discussed. The fabrication of arrays with demonstrated I-V properties show that monolithic integration of HgCdTe-based infrared focal plane arrays on Si read-out integrated circuits is feasible and could be implemented in the 3rd generation of infrared systems.