Chapter 14: Semiconductors & the Transistor Effect

The discussion establishes the fundamental carriers of charge: electrons and holes. When sufficient thermal energy promotes an electron from the full valence band to the empty conduction band, it results in a mobile, negatively charged electron and a positively charged mobile vacancy, or hole, in the lattice structure. The minimum energy required to create such a pair is defined by the energy gap. The thermal probability of creating these carriers is exponentially dependent on temperature, governed by the Boltzmann factor. The complexity of real-world materials is introduced through impure semiconductors, explaining the critical process of doping. Adding elements like arsenic, which create donor sites, results in n-type material, where electrons are the majority charge carriers, while adding elements like aluminum, which create acceptor sites, results in p-type material, where holes are the majority carriers. The quantitative relationships governing carrier densities and conductivity are explored, linking current flow to carrier mobility and drift velocity. A key experimental tool, the Hall effect, is explained as a method for experimentally identifying the sign and concentration of the primary charge carriers within a semiconductor. The text then transitions to semiconductor devices, detailing the p-n junction, which is formed when p-type and n-type materials meet, resulting in the diffusion of carriers and the creation of an internal electric field and potential barrier. This junction is foundational to devices like solar cells, where absorbed photons generate voltage, and diodes, which exhibit distinct current characteristics. Finally, the principles are extended to the transistor (specifically the p−n−p configuration). This three-terminal device—comprising an emitter, a thin base, and a collector—demonstrates how small voltage variations applied to the base can control a significantly larger current flowing through the device, thereby achieving amplification.