Special Purpose Junction Diodes

This section moves on from rectifier circuits to a set of SPECIAL PURPOSE JUNCTION DIODES -- p-n junction diodes deliberately designed and operated to exploit some particular electrical or optical property of the junction, beyond plain rectification. Four such diodes are covered in turn: the ZENER DIODE, which works on the principle of controlled junction (Zener) breakdown and is used mainly as a voltage regulator; and three devices that instead exploit PHOTOSENSITIVITY, a semiconductor junction's ability to respond to incident light -- the PHOTO DIODE, which converts light into a measurable electrical signal; the SOLAR CELL (photovoltaic cell), which converts light directly into usable electrical power; and the LIGHT EMITTING DIODE (LED), which runs the same underlying physics in reverse, converting an electrical current into emitted light. Each of the four is built from the …