The asymmetry in the characteristic of GaAs/AlGaAs quantum well infrared photodetector (QWIP) due to different Ga desorption rate upon opening /closing the Al shutter and different diffusion of Si doping in the quantum well during MBE growth was analyzed,and that in the parameters of GaAs/AlGaAs QWIP versus bias was discussed and compared with the materials and devices grown by MOVCD method. It was found that the asymmetry of QWIP grown by MBE is higher than that of QWIP grown by MOCVD.