In the classic model for electrical conduction in metals, the outer valence electrons of the
atoms of the metal are assumed to be free to move between the positive-ion cores (atoms
without their valence electrons) of the metal lattice. In the presence of an applied electric
potential the free electrons attain a directed drift velocity. The movement of electrons and
their associated electric charge in a metal constitute an electric current. By convention, electric current is considered to be positive charge flow, which is in the opposite direction
to electron flow.
In the energy-band model for electrical conduction in metals, the valence electrons
of the metal atoms interact and interpenetrate each other to form energy bands. Since
the energy bands of the valence electrons of metal atoms overlap, producing partially
filled composite energy bands, very little energy is required to excite the highest-energy
electrons so that they become free to be conductive. In insulators the valence electrons
are tightly bound to their atoms by ionic and covalent bonding and are not free to conduct
electricity unless highly energized. The energy-band model for an insulator consists
of a lower filled valence band and a higher empty conduction band. The valence
band is separated from the conduction band by a large energy gap (about 6 to 7 eV, for
example). Thus, for insulators to be conductive, a large amount of energy must be
applied to cause the valence electrons to “jump” the gap. Intrinsic semiconductors have
a relatively small energy gap (i.e., about 0.7 to 1.1 eV) between their valence and conduction
bands. By doping the intrinsic semiconductors with impurity atoms to make
them extrinsic, the amount of energy required to cause semiconductors to be conductive
is greatly reduced.
Extrinsic semiconductors can be n-type or p-type. The n-type (negative) semiconductors
have electrons for their majority carriers. The p-type (positive) semiconductors
have holes (missing electrons) for their majority charge carriers. By fabricating pn junctions
in a single crystal of a semiconductor such as silicon, various types of semiconducting
devices can be made. For example, pn junction diodes and npn transistors can
be produced by using these junctions. Modern microelectronic technology has developed
to such an extent that thousands of transistors can be placed on a “chip” of semiconducting
silicon less than about 0.5 cm square and about 0.2 mm thick. Complex microelectronic
technology has made possible highly sophisticated microprocessors and computer
memories.
Ceramic materials are usually good electrical and thermal insulators due to the
absence of conduction electrons, and thus many ceramics are used for electrical insulation
and refractories. Some ceramic materials can be highly polarized with electric charge and
are used for dielectric materials for capacitors. Permanent polarization of some ceramic
materials produces piezoelectric properties that permit these materials to be used as
electromechanical transducers. Other ceramic materials, for example, Fe304 are semiconductors and find application for thermistors for temperature measurement.
Nanotechnology research is making progress toward manufacturing electronic devices
with nanometer dimensions. Quantum corrals are envisioned to deliver currents in nano
devices where electrical wiring is impossible.
