# Drift Velocity vs. Saturation Velocity

#### jaydnul

Joined Apr 2, 2015
161
You see typical drift velocities for the charge in wire as fractions of a meter per second in most cases. But the saturation velocity in a MOSFET can be almost a million meters per second. How can both of these values be correct, are the charges in semiconductors really moving that fast?

See the numerical example here for drift velocity: https://en.wikipedia.org/wiki/Drift_velocity
See here for the saturation velocity: https://en.wikipedia.org/wiki/Saturation_velocity

Thanks

#### nsaspook

Joined Aug 27, 2009
11,292
You see typical drift velocities for the charge in wire as fractions of a meter per second in most cases. But the saturation velocity in a MOSFET can be almost a million meters per second. How can both of these values be correct, are the charges in semiconductors really moving that fast?

See the numerical example here for drift velocity: https://en.wikipedia.org/wiki/Drift_velocity
See here for the saturation velocity: https://en.wikipedia.org/wiki/Saturation_velocity

Thanks
Think of the scale of possible movement. Look at the electric field strength at 10-100 kV/cm. This is over a microscopic distance in a semiconductor device near barriers or junctions. The same stable potential across a typical copper conductor wire of 1 cm length would cause it to instantly flash into a plasma. The drift velocity (average velocity) is normally slow in a wire but the actual random charge carrier speed is much higher.
By comparison, the Fermi flow velocity of these electrons (which, at room temperature, can be thought of as their approximate velocity in the absence of electric current) is around 1570 km/s

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