Obtaining higher quantum efficiency and more stable GaAs photocathodes has been an important developmental direction in the investigation of GaAs photocathodes. One significant approach to this problem is to improve the electron diffusion length. We put forward and investigate an exponential doping mode GaAs photocathode. It was proved by theoretical and experimental results that, because the exponential doping structure is in favor of forming a directional constant built-in electric field, the electron diffusion and drift length of the cathode material can accordingly be enhanced. The mathematical expression of the electron diffusion and drift length $L_{DE}$ of an exponential doping photocathode was deduced, and the relationship between the doping coefficient and the electron diffusion and drift length is made certain. This investigation contributes to the understanding of varied doping GaAs photocathodes and provides guidance to optimize the doping structure of GaAs photocathodes for higher quantum efficiency.