The whole electronics industry is built on three devices: the diode, the BJT, and the FET. Master them and the rest of electronics is variations on a theme. Master them deeply and you will understand why CPUs leak power, why chargers fail in winter, and why Rowhammer was inevitable.
In the physics chapter we built up the toolkit: charge, fields, currents, band theory, doping. Now we put it to work. We will make actual devices — pieces of silicon that do something useful. The diode is first because it is simplest and because every other device is, in some sense, two diodes in disguise. Then transistors — first the bipolar (BJT), then the field-effect (FET) — and finally we wire them up into amplifiers and analyze them with small-signal models.
This chapter is going to feel slow. That is on purpose. These three devices show up everywhere, in every other chapter of this curriculum, and you will use them every time you debug a circuit. Take your time.