Summary form only given. Satisfying the fast turn-on time and high sensitivity requirements, photorefractive multiple quantum well (MQW) devices have become the candidate for real-time holographic applications. The diffraction efficiency of the present devices decreases with the grating spacing much faster than the theoretically calculated. In order to understand the charge transport process that reduces the resolution, we have measured the picosecond response time of diffraction gratings. The grating build-up processes and the diffraction efficiency as a function of grating period have been presented in an earlier conference. In this work, we investigate the picosecond effects of applied field on diffraction efficiency in GaAs MQWs.