By solving the one-dimensional diffusion equation for equilibrium minority carriers of transmission-mode GaAs phtotocathodes, the equations for surface photovoltage spectroscopy of exponential and uniform doping transmission-mode GaAs photocathodes are deduced. Through metal organic chemistry vaporation deposition (MOCVD) technique, two GaAs photocathodes of different doping structures with the same active layer depth are designed. By measuring and theoretically emulating the surface photovoltage spectroscopy curves for two materials, the exponential doping structure can well increase the electric diffusion length for transmission-mode GaAs photocathodes. The reason is that the internal electric field can drive the photo-excited electrons to move toward surface barriers through both diffusing and electric field drifting, thus can finally elevate the emission efficiency and the surface photovoltage spectroscopy. The difference of two surface photovoltage spectroscopy waves can be well explained by the internal energy band equations and electron scattering theory.