Diamond has unique physical and chemical properties such as the highest thermal conductivity. Especially, the negative electron affinity (NEA), where the conduction band minimum is higher than the vacuum level, is highlighted for a long time as to be one of the best candidates for the electron emitters. We have developed growth techniques of high quality diamond films, and total photoelectron emission yield spectroscopy (TPYS) equipment to be able to investigate NEA on diamond. Now it is possible to determine NEA quantitatively, and characterize electronic properties of surface and bulk of diamond by means of NEA. According to these results, we find that free-exciton derived electron emission plays an important role in electron emission from diamond with the NEA surface, and finally realizes a NEA electron emitter with diamond p-n junction diodes. The diamond semiconductor, that makes it possible to study “excitonics” at room temperature, will be one of the most exciting materials for the next surface science.