A phonon is the vibration
quantum of a
crystalline lattice. Even at
absolute zero (0K), the basis atoms of a
crystal vibrate about their average
equilibrium positions. The concept of a
crystal having a precisely-known, static structure is thus only an approximation.
Solid-state physicists find it very useful to make this approximation, known as the
Born-Oppenheimer Approximation or
adiabatic approximation, when calculating
bandstructure--the electron
eigenstates and
eigenenergies in a solid. The treatment of crystalline vibrations as quantized packets (phonons) allowed physicists to explain why the
specific heat of solids is proportional to T
3 at very low temperatures.
Phonons are important for less academic reasons. The vibrations of a crystal act to scatter electrons (and holes). In a silicon MOSFET (a type of transistor), phonons are the fundamental barrier to current flow. The speed of a digital integrated circuit such as a microprocessor is dependent on how fast MOSFETs can charge and discharge capacitors. Since phonons limit current, they greatly degrade circuit speed. At lower temperatures, crystals vibrate less. For this reason, microprocessors could run many times faster if cooled (by liquid nitrogen for instance).