In a conductor, the atoms are held together by what we call electrovalent… — Page 249, Lesson 268

I want to walk you through the fundamental difference between conductors, insulators, and semiconductors — this is the bedrock of all aircraft electrical and electronic systems. Let's start with conductors. In a conductor, the atoms are held together by what we call electrovalent bonds. Because of this type of bonding, there are large numbers of electrons that are free to move from one atom to the next. These are called free electrons, and they form the basis for current flow within the material. Since there are so many free electrons, current flows easily through the material. Another way to say that is: the material has high conductivity and therefore low resistivity. Good examples of conductors are gold, silver, and copper. Now, insulators are the opposite. In an insulator, the atoms are held together by covalent bonds. These materials possess very few free electrons. That means current flow is difficult — the material has low conductivity, which is the same as saying it has high resistivity. A good example of an insulator is mica. Then we have semiconductors. As the name implies, they fall somewhere between a conductor and an insulator. The two examples given are silicon and germanium. Both of these materials are formed by atoms with covalent bonds, just like insulators. At normal temperatures, they do possess some free electrons, but they are closer to being insulators than conductors. So if you apply an EMF — an electromotive force, or voltage — across a semiconductor, you get an intermediate current flow. It's higher than what you'd get in an insulator, but less than what you'd get in a conductor. Here's the key point for practical use: we can improve the conductivity of a semiconductor by the controlled addition of impurities into the silicon or germanium material. That process is called doping. This is what allows us to create the precise electrical properties we need for diodes, transistors, and other electronic components found throughout modern aircraft systems.