If you’ve read our articles on diodes and transistors, you’ll be familiar with the idea of semiconductors. Traditionally, people thought of materials fitting into two neat categories: those that allow electricity to flow through them quite readily (conductors) and those that don’t (insulators). Metals make up most of the conductors, while nonmetals such as plastics, wood, and glass are the insulators.
In fact, things are far more complex than this—especially when it comes to certain elements in the middle of the periodic table (in groups 14 and 15), notably silicon and germanium. Normally insulators, these elements can be made to behave more like conductors if we add small quantities of impurities to them in a process known as doping. If you add phosphorus (or antimony) to silicon, you give it slightly more free electrons than it would normally have—and the power to conduct electricity. Silicon “doped” that way is called n-type. Add boron instead of phosphorus and you remove some of silicon’s free electrons, leaving behind “holes” that work as “negative electrons,” carrying a positive electric current in the opposite way. That kind of silicon is called p-type. Putting areas of n-type and p-type silicon side by side creates junctions where electrons behave in very interesting ways—and that’s how we create electronic, semiconductor-based components like diodes, transistors, and memories.
