Quantum Well Infrared Photodetectors (QWIPs) have the advantages of excellent uniformity and mature material properties. Thanks to these properties, large format and low cost QWIP focal plane arrays (FPAs) can be fabricated. The standard material system used for QWIP FPAs is AlGaAs/GaAs in the long wavelength infrared (LWIR) band. AlGaAs/GaAs material system has some disadvantages such as low quantum and conversion efficiencies under high frame rate and/or low background conditions. These limitations of the standard material system give rise to research on alternative material systems for QWIPs. InP/InGaAs material system is an alternative to AlGaAs/GaAs for LWIR QWIPs. This thesis focuses on the development of InP/InGaAs QWIP FPAs. A large format (640x512) LWIR QWIP FPA constructed with strained InP/InGaAs system is demonstrated with high quantum and conversion efficiencies. The FPA fabricated with the 40-well epilayer structure yielded a peak quantum efficiency as high as 20% with a broad spectral response (15%). The responsivity peak and the cut-off wavelengths of the FPA are 8.5 and ~9 um, respectively. The peak responsivity of the FPA pixels is larger than 1 A/W with a conversion efficiency as high as ~17 % in the bias region where the detectivity is reasonably high. The FPA provides a background limited performance (BLIP) temperature higher than 65 K (f/1.5) and satisfies the requirements of most low integration time/low background applications. Noise equivalent temperature difference (NETD) of the FPA is as low as 25 mK with integration times as short as 2 ms (f/1.5, 68 K).