Velocity saturation

In semiconductors, when a strong enough electric field is applied, the carrier velocity in the semiconductor reaches a maximum value. 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 by emitting phonons as soon as the carrier energy is large enough to do so.

Design considerations

When designing semiconductor circuits, 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 circuit. 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 emit a constant current.

References