We report the design, characterization and fabrication of GaAs/AlGaAs quantum well infrared photodetectors (QWIPs) to achieve intersubband transitions at expected long wavelengths. With eight-band k·p model, we calculated the E2-E1 transition energies of GaAs/Al0.25Ga0.75As QWs with the different well widths. According to the calculation, we designed a QWIP with the estimated detection wavelength around 9 μm. The actual device structure parameters, such as well width and Al composition, were confirmed by the XRD measurements. The absorption peak at 9.46 μm and the peak responsivity at 8.7 μm are obtained, which are very close to the simulation results.