This paper compares the technical merits of two IR detector arrays technologies; photovoltaic HgCdTe and quantum well IR photoconductors (QWIPs). It is clearly shown that long wavelength IR (LWIR) QWIP can not complete with HgCdTe photodiode as the single device especially at higher temperature operation due to fundamental limitations associated with intersubband transitions. However, the advantage of HgCdTe is less distinct in temperature range below 50 K due to problems involved in a HgCdTe material. Even though that 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 large focal plane arrays. Due to the high material quality at low temperature, QWIP has potential advantages over HgCdTe for very LWIR FPA applications in terms of the array size, uniformity, yield and cost of the systems.