A photodiode is a semiconductor device that converts light into electrical current. When light photons hit the photodiode, they generate electron-hole pairs, allowing current to flow. Photodiodes are commonly used in applications like optical communication, light detection, and imaging systems. These devices can operate in different modes, such as photovoltaic and photoconductive. In the photovoltaic mode, they generate a voltage when exposed to light, while in the photoconductive mode, they change their resistance based on light intensity. Photodiodes are essential components in technologies like fiber optics and digital cameras.