The influence of interfacial states of energy arising from defects in the metal-Microwave Plasma Assisted Chemical Vapor Deposition (MPCVD) diamond Schottky interface has been investigated by electrical DC measurements, in time and temperature. A significant temperature-dependent applied-bias offset for zero current conditions (near 0.3 V at room temperature) is noticed. At room temperature the decay constant is about 0.1 s. The current–voltage behavior is strongly dependent on the environment conditions, air or vacuum. This dependence ceases after the application of a high forward electrical field. The electrical conduction shows activation energies near 0.1 eV with differences with the applied voltage time. Under near UV–visible photoexcitation, the photodiode shows a high current gain (up to three orders of magnitude) and a corresponding relatively high short circuit current. A model involving states of energy inside an interfacial layer acting as traps for the majority carriers is discussed. A potential application in metal–insulator–semiconductor devices is focused.