This paper describes a new multi-heterojunction (n) under bar(+)p (p) under bar photovoltaic infrared photodetector. The device has been developed specifically for operation at temperatures of 200-300K in the long wavelength (8-14 mu m) range of the infrared spectrum. The new structure solves the perennial problems of poor quantum efficiency and low dynamic resistance found in conventional long wavelength infrared photovoltaic detectors when operated near room temperature. Computer simulations show that devices with properly optimized multiple heterojunctions are capable of achieving the performance limits imposed by the statistical nature of thermal generation-recombination processes. In order to demonstrate the technology, multiple heterojunction devices have been fabricated on epilayers grown by isothermal vapor phase epitaxy of HgCdTe and in situ As p-type doping. The detector structures were formed using a combination of conventional dry etching, angled ion milling, and angled thermal evaporation for contact metal deposition. These multi-junction (n) under bar(+)p (p) under bar HgCdTe heterostructure devices exhibit performances which make them useful for many applications. D* of optically immersed multiple heterostructure photovoltaic detectors exceeding 10(8)cmHz(1/2)/W were measured at lambda, = 10.6 mu m and T = 300K.