In semiconductors, when a strong enough electric field is applied, the carrier velocity in the semiconductor reaches a maximum value, saturation velocity.[1] When this happens, the semiconductor is said to be in a state of velocity saturation. As the applied electric field increases from that point, the carrier velocity no longer increases because the carriers lose energy through increased levels of interaction with the lattice, by emitting phonons and even photons as soon as the carrier energy is large enough to do so.[2]
When designing semiconductor devices, especially on a sub-micrometre scale as used in modern microprocessors, velocity saturation is an important design characteristic. Velocity saturation greatly affects the voltage transfer characteristics of a field-effect transistor, which is the basic device used in most integrated circuits designed and produced in the world. If a semiconductor device enters velocity saturation, an increase in voltage applied to the device will not cause a linear increase in current as would be expected by Ohm's law. Instead, the current may only increase by a small amount, or not at all. It is possible to take advantage of this result when trying to design a device that will pass a constant current regardless of the voltage applied, a current limiter in effect.