The historical journey in Chap. 1 revealed that a fil rouge run over about four decades from the initial concept of Lilienfeld in 1926 to the tangible CMOS circuitry of Wanlass in 1963 to conceive a reliable, tiny, light, and low power solid-state device equivalent to the more unreliable, large, heavy, and power hungry vacuum electronic triode. The efforts landed on the invention of the transistor. The historical review pointed out that, over the years, the ultra-pure, monocrystalline silicon has become the semiconductor of choice and the silicon dioxide, a material of paramount importance for the fabrication of reliable transistors. This picture remains accurate even though heterostructured devices based on compound semiconductors such as GaAs, GaN, and InP have seized their own market niches. The present chapter introduces fundamental concepts of solid-state physics such as conduction and valence bands, doping, and charge transport. The core of the chapter addresses the fundamental physics of three main silicon semiconductor devices: the pn junction, the junction transistor, and the Metal-Oxide-Semiconductor (MOS) field effect transistor. Eventually, the chapter delivers concepts as bias point and small-signal modeling, which are fundamental for understanding the use of the transistor as an amplifier.