[[I'm not entirely happy with this section, so please feel free to suggest improvements....Dan]]
While transistor theory is outside the scope of this study guide, I will attempt to at least give you a basic understanding of how transistors are put together and how they work. Most transistors we use in amateur radio are made of silicon. Silicon is a semiconductor. That is to say, it’s neither a conductor with a very low resistance, like copper, or an insulator with a very high resistance, like plastic or glass.
You can manipulate the electrical characteristics of silicon by adding slight amounts of impurities to a pure silicon crystal. When transistor manufacturers add an impurity that adds free electrons to the silicon crystal, it creates a crystal with a negative charge. We call that type of silicon N-type silicon. N-typeis a semiconductor material that contains excess free electrons. (E6A02) Free electrons are the majority charge carriers in N-type semi-conductor material.
When you add other types of impurities to a pure silicon crystal, you can create a crystal with a positive charge. We call this type of material P-type semiconductor material. In N-type semiconductor material, the majority charge carriers are the free electrons. (E6A16) The majority charge carriers in P-type semiconductor material are called holes. (E6A03) P-type is the type of semiconductor material that contains an excess of holes in the outer shell of electrons. (E6A15)
You can think of holes as spots in the crystal that accepts free electrons. Because of that, the name given to an impurity atom that adds holes to a semiconductor crystal structure is call an acceptor impurity. (E6A04)
Silicon isn’t the only semiconductor material used to make transistors. At microwave frequencies, gallium arsenide used as a semiconductor material in preference to germanium or silicon. (E6A01)