5.3 High Field Transport

In the case of small electric fields the scattering events are elastic. The drift velocity is linearly proportional to the electric field.

The scattering efficiency，however，is reduced already at moderate fields. Then，the electron temperature becomes larger than the lattice temperature. If the carrier energy is large enough it can transfer energy to the lattice by the emission of an optical phonon. This mechanism is very efficient and limits the maximum drift velocity. The limiting value for the drift velocity is termed the drift-saturation velocity.

For Ge the drift-saturation velocity at room temperature is 6×106cm/s，for Si it is 1×107 cm/s ［Fig.5.6（a）］. In GaAs there is a maximum drift velocity of about 2×107cm/s and a following reduction in velocity with increasing field（1.2×107cm/s at 10 kV/cm，0.6×107cm/s at 200kV/cm）. This regime is called negative differential resistivity（NDR）.