and germanium are the cornerstones of transistor technology and
the mini-electronics industry. Pure silicon and germanium are poor
conductors of electricity because their outer electrons are tied
up in the covalent bonds of the diamondlike framework. Diamond is
an insulator because it is not easy to supply enough energy
to pull the covalent-bond electrons loose and permit them to conduct
electricity through the crystal. Less energy is required to free
the electrons in silicon, and even less is required in germanium.
These atoms are larger and hold their electrons less tightly. They
are not conductors in the metallic sense of the word, but are semiconductors.
At low voltages they are insulators, but they begin to conduct electricity
if the applied voltage is high enough. Electrical conductivity decreases
as temperature increases in metals because the vibrations of the
atoms make passage of electrons more difficult. This effect is overshadowed
in semiconductors by the greater number of electrons that are "shaken
loose" from bonds at higher temperatures; thus semiconductors become
better conductors as the temperature increases.