The electron emission properties of planar-type electron emission devices based on a graphene-oxide-semiconductor (GOS) structure before and after vacuum annealing were investigated. The fluctuation of the electron emission current was around 0.07%, which is excellent stability compared to the conventional field emitter array. The GOS devices were operable in very low vacuum of 10 Pa without any deterioration of their electron emission properties. Improvement of the electron emission properties of the GOS devices was achieved by vacuum annealing at 300 degrees C. The electron emission efficiency of the GOS type electron emission devices reached 2.7% from 0.2% after vacuum annealing. The work function of the graphene electrode was found to decrease 0.26 eV after vacuum annealing by Kelvin force probe microscopy analysis. These results indicated that the improvement of the electron emission efficiency of the GOS devices by vacuum annealing is due to the decrease in the work function of the graphene electrode. Published by the AVS.