A better way of increasing electrical conductivity in these semiconductors is to purify them as much as possible, then "dope" them with small amounts of Al or P atoms to replace some of the Si atoms. (The replacements for Ge in the fourth row would be Ga and As.) Every alurninum atom substituted introduces a one-elertron deficiency into the diamond framework, and every phosphorus atom brings with it an extra electron. Both of these controlled impurities make silicon a better conductor of electricity. With phosphorus, the extra electron fits into none of the covalent bonds of the diamond lattice, and can serve as the carrier of current. With aluminum, the cascading of electrons, tumbling domino-fashion into the electron vacancies, creates what physicists describe as an electron "hole," which migrates against the electric field rather than with it, but which also can carry a net electric current. Phosphorus-doped silicon is n-silicon (for negative electrons), and aluminum-doped silicon is p-silicon (for positive holes). When the two types are brought together, the result is a p-n junction. Electric current can be transmitted only one way across this p-n junction, as shown in the diagram above. The junction is a rectifier, capable of turning alternating current into pulses of direct current. Other semiconductor devices using p- and n-silicon have been designed, which can replace most of the old vacuum tubes with tiny and rugged transistors that operate on only a fraction of the power formerly required.