Diffraction and background-limited operation at room temperature is the ultimate goal of the infrared physicist. The achievement of this goal is strongly dependent on the hybridization technology utilized in bonding the IR focal plane array to the Si ROIC. To this end, this book describes a disruptive architecture in the field of hybridization known as the vertically integrated photodiode. This technology, as applied specifically to HgCdTe, is described in detail by the author. However, it can in principle be applied to any IR semiconducting material system, provided suitable dry etch and passivation technologies exist. The limitations of the many current and varied IR architectures are discussed with regard to diffraction and background-limited performance as a function of operating temperature. Shockley-Read-Hall lifetime is found to be a major factor in these considerations, for which HgCdTe has no IR equal. However, a number of scientific and engineering challenges remain within the HgCdTe materials system before ultimate performance can be achieved. A discussion of those challenges, together with possible outcomes, constitutes the basis of this book.