In this paper, an extensive study of the photocurrent amplification mechanism of silicon bulk-barrier diodes (BBDs) is presented, based on simulation and experimental results. As a result of this study an analytical model is proposed. The proposed model extends previously published models and includes analytical expressions for all significant quantities of the device optoelectronic behaviour. Such quantities are barrier lowering, photocurrent, quantum efficiency and response speed as functions of the applied voltage, incident light power, light wavelength and modulation frequency, as well as a function of technological parameters. Simulation and experimental results verify the validity of the proposed analytical model, and they show that BBDs are majority carrier photodetectors with high internal photocurrent gain. Compared with common photodetectors, BBDs exhibit a very high sensitivity in the blue region of the visible spectrum.