1) Semiconductors are materials that have electrical conductivity between that of metals and insulators. Their conductivity can be modified by doping. 2) Common semiconductors used in electronics include silicon and germanium which are purified to be intrinsic semiconductors. Their conductivity increases when small amounts of impurities are introduced. 3) A p-n junction is formed by combining p-type and n-type semiconductors. It allows current to pass easily in one direction but blocks it in the other direction, forming the basis of diodes and transistors.