This lecture covers the operation and characteristics of photodiodes, explaining how photons generate electron-hole pairs in the depletion zone, the importance of electric fields in separating carriers, and the different technologies used in photodiodes. It also discusses the materials, advantages, and defects of photoconductors, as well as the equivalent electric model of a photodiode and the circuit. The lecture delves into topics such as load resistance, shunt resistance, junction capacitance, and the current produced by light. Various parameters affecting the performance of photodiodes, including dark current, sensitivity, and response time, are explored in detail.