The carrier emission efficiency of light emitting diodes is of fundamental importance for many technological applications, including the performance of GaN and other semiconductor photocathodes. We have measured the evolution of the emitted carriers and the associated transient electric field after femtosecond laser excitation of n-type GaN single crystals. These processes were studied using subpicosecond, ultrashort, electron pulses and explained by means of a three-layer analytical model. We find that for pump laser intensities on the order of 10^11 W/cm^2, the electrons that escaped from the crystal surface have a charge of about 2.7 pc and a velocity of about 1.8 um/ps. The associated transient electrical field evolves at intervals ranging from picoseconds to nanoseconds. These results provide a dynamic perspective on the photoemission properties of semiconductor photocathodes.