The fundamental parameters of infrared (IR) detection are discussed to compare a wide range of materials. In comparative studies both photon and thermal detectors are considered. More attention is paid to photon detectors such as : HgCdTe photodiodes, InSb photodiodes, quantum well IR photoconductors (QWIPs) and doped silicon photoconductors. Different competitive technologies in long wavelength IR(LWIR) and very LWIR(VLWIR) spectral ranges with emphasis on the material properties, device structure, and their impact on FPA performance are considered. The potential performance of materials as infrared detectors is examined utilizing the (alpha) /G ratio, where (alpha) is the absorption coefficient and G is the thermal generation. It results that LWIR QWIP cannot compete with HgCdTe photodiode as the single device, especially at higher temperature operation (>70K), due to fundamental limitations associated with intersubband transitions. However, the advantage of HgCdTe is less distinct in temperature range below 50K due to problems involved in an HgCdTe material (p- type doping, Shockley-Read recombination, trap-assisted tunneling, surface and interface instabilities). Even though QWIP is a photoconductor, several its properties such as high impedance, fast response time, long integration time, and low power consumption, well comply requirements of fabrication of large FPAs. Due to the high material quality at low temperature, QWIP has potential advantages over HgCdTe for VLWIR FPA applications in terms of the array size, uniformity, yield and cost of the systems. State of the art of QWIP and HgCdTe FPAs provides similar performance figure of merit, because they are predominantly limited by the readout circuits. Decision of the best technology implementation is therefore driven by the specific needs of a system.