As the applied electric field, E, across the GaAs material is increased, the charge carriers, that is electrons in this case, gain energy from the applied field. At the same time, through collisions (that is, optical phonon scattering), with the lattice, the electrons also lose a small portion of this energy. So long as the resultant balance is positive, the energy and drift velocity of the charge carriers increases with an increase in the applied field. However, at some point, the energy gained from the field becomes equal to the energy lost as the result of collisions. This result in the drift velocity approaching a limiting value referred to as the saturation velocity, vsat
Since gallium arsenide is a multi-valley semiconductor, when the energy of lower valley electrons rises sufficiently, that is at electric fields greater than approximately 3500V/cm, electrons become hot. There is a region in the electron velocity-field characteristics where some of the ‘hot’ electrons populate an upper conduction band that is characterized by larger electron effective mass. The resultant effect is a reduction in the number of high mobility electrons and hence the drift velocity.
In this region the drift velocity is no longer proportional to the electric field, but instead passes through a maximum of about 2*107 cm/sec with increasing field, and decreases to an electric field independent saturation value of about 1.4*107 cm/sec.
The velocity-field characteristics illustrating the three regions of interest are shown in the figure below.For convenience of comparison, characteristics for silicon are also illustrated. From the figure it can readily be noted that in low electric field regions, silicon has a much lower mobility than gallium arsenide. This increases monotonically until the drift velocity saturates at a value of about 1*107 cm/sec.