We present a detailed study of double barrier strained Al0.35Ga0.65As/AlAs/GaAs/In0.2Ga0.8As quantum well infrared photodetectors on GaAs substrate. Measurements were made on four different well widths active layers and on several mesa pixels with different optical coupling structures. We obtained responses peaked in the spectral range 3.6–4.6 μm. Based on the experimental results, we show that in the background limited regime, the impact ionization is the restrictive transport process for midwave detectors. We also demonstrate a 4 μm structure with the high background limited detectivity of 2×1011 Jones at 77 K and 9×1010 Jones at 110 K (2π field of view, 300 K background).