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1-9.Saturation Velocity

1-9.Saturation Velocity:

Saturation velocity is the maximum velocity a charge carrier in a semiconductor, generally an electron, attains in the presence of very high electric fields[1]. Charge carriersnormally move at an average drift speed proportional to the electric field strength they experience temporally. The proportionality constant is known as mobility of the carrier, which is a material property. A good conductor would have a high mobility value for its charge carrier, which means higher velocity, and consequently higher current values for a given electric field strength. There is a limit though to this process and at some high field value, a charge carrier can not move any faster, having reached its saturation velocity, due to mechanisms that eventually limit the movement of the carriers in the material.

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.