A p-type graphene–oxide–semiconductor (GOS) planar-type electron emission device holds promise as a photocathode with a high quantum efficiency (QE) for generating a high-speed modulated electron beam. We report the electron emission properties of a planar-type electron emission device based on a GOS structure using lightly doped p-type silicon as a substrate under laser light irradiation. The electron emission efficiency is 12% with and without laser light irradiation. The emission current shows a four-order of magnitude enhancement in the high field region compared to that in the dark, and a 0.3% QE is obtained without a negative electron affinity surface. This GOS device exhibits advanced photoassisted electron emission characteristics with high photosensitivity on the order of mA/W. The photoemission from the p-type GOS device shows a fast photoresponse with a rise time of less than 0.2 µs, which is the same as that of the current amplifier. By contrast, it shows a slow response of about 2.4 µs at the fall time, which is determined by the diffusion process of the photoexcited electrons in the bulk. The use of a highly doped p-type silicon substrate can be a practical route for further improvement.