The conductivity of silicon can be controlled and made to vary over several orders of magnitude by adding small amounts of impurities. Sil- icon belongs to group four in the periodic table of elements. It has four valence electrons in its outer shell. A silicon atom in a silicon crystal has four nearest neighbors. Silicon forms covalent bonds where each atom shares its valence electrons with its four nearest neighbors. Each atom has its four original valence electrons plus the four belonging to its neighbors. That gives it eight valence electrons. The eight valence electrons complete the shell producing a stable state for the silicon atom. Electrical conductivity requires current consisting of moving electrons. The valence electrons are attached to an atom and are not free to move far from it. Some valence electrons will receive enough thermal energy to free themselves from the silicon atom. These electrons move to energy levels in a band of energy called the conduction band. Conduction band electrons are not attached to a particular atom and are free to move about the crystal.