Introduction

Almost every modern electronic gadget -- a cell phone, a smart watch, a computer, even an LED lamp -- shares one common ingredient: semiconductor devices. Before semiconductors were understood and put to use, the electrical behaviour of solids was described using only two categories. CONDUCTORS, typically metals, have very high electrical conductivity; silver, for instance, conducts at about $6.25\times10^7\,S\,m^{-1}$. INSULATORS, such as glass, have very low conductivity, of the order of $10^{-10}\,S\,m^{-1}$ or less. Semiconductors occupy the interesting middle ground: silicon, a typical semiconductor, has a conductivity of about $1.56\times10^{-3}\,S\,m^{-1}$ -- many orders of magnitude below a good conductor, yet many orders of magnitude above a good insulator.

What makes semiconductors genuinely useful, though, is not simply that their conductivity sits between these two extremes, but that this conductivity can be TAILORED. By controlling the material's composition (through doping, covered later in this chapter) and by understanding how its conductivity responds to temperature, a semiconductor's electrical behaviour can be engineered to suit almost any requirement -- which is exactly why semiconductor devices now sit at the heart of nearly every piece of modern electronics.