diodes

From a single PN junction to the four-diode bridge in every wall adapter. The IV curve, rectification, Zener voltage references, fast Schottky diodes for switching supplies, LEDs and photodiodes turning current to light and back, and the five failure modes that bite real designs.

What makes silicon work where diamond won't, why doping turns an insulator into a tunable conductor, and how slapping p-type silicon next to n-type creates the depletion region that becomes a diode. The physics every chip is built on.