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
p-silicon has aluminium atoms sustituted occasionally for silicon
vacancies, or holes, which are mobile. n-Silicon has occasional phosphorus
atoms that contribute extra electrons.