In a semiconductor, when electrons collide with atoms and other disturbances, their movement change in a random direction. However, the applied voltage or electric field accelerates the electrons in one direction at an average speed. The charge carriers average velocity due to the electric field is called drift velocity.
We know from electromagnetics that charged particles move in response to an applied electric field. This movement is termed drift and the resulting current flow is known as drift current. At low fields, carrier drift velocity is cm/s.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock