Semiconductors are used to make transistors, which are the building blocks of modern electronics. For example, the main processor chip in a personal computer contains a few million transistors. The basic physics of these transistors will be described shortly.
Semiconductors are materials that are neither conductors nor insulators.
Rather they have resistivities intermediate between those of conductors like copper and insulators like plastic. If the impurity adds some extra electrons that are free to move in the material, it is called n-type semiconductor because of the n egative charge of the electrons. The holes are positively charged because an electron is missing from them. Holes can move through the crystal, since a nearby electron can jump into a hole, fillng it up, but leaving a new hole nearby. Such materials are called p-type semiconductors, because it is p ostive charge that can move freely.