We show that photorefractive beam coupling gain as a function of grating period measured in semi‐insulating GaAs with an external ac electric field or with a dc field and moving fringes can be explained theoretically by a mobility‐lifetime product about four orders of magnitude smaller than that obtained in low‐field conditions. This reduction is caused by enhanced occupation of theLband at high fields coupled with the lowLband mobility and by the increase in recombination due to the cascade capture process that occurs for electric fields above a few kV/cm.