This document discusses the operational principle of diodes. It begins by introducing diodes and their use to control electric circuit current. Diodes are composed of an anode and cathode and can be modeled as a simple electronic switch. They are made by doping silicon with impurities to create either a p-type or n-type semiconductor, which form a p-n junction when joined. A diode acts as a closed switch under forward bias when positive is applied to the anode, and as an open switch under reverse bias. In the depletion region of a p-n junction, an insulating region is formed. Diodes have applications in rectification circuits and as light sources in LEDs.