Bulk scattering mechanisms

Scattering by lattice waves: Scattering by lattice waves includes the absorption or emission of either acoustical or optical phonons. Since the density of phonons in a solid increases with temperature, the scattering time due to this mechanism will decrease with temperature as will the mobility. Theoretical calculations reveal that the mobility in non-polar semiconductors, such as silicon and germanium, is dominated by acoustic phonon interaction, and is expected to be proportional to T ^-3/2, while the mobility due to optical phonon scattering only is expected to be proportional to T ^-1/2.


Scattering by impurities: By impurities we mean foreign atoms in the solid which are efficient scattering centers especially when they have a net charge. Ionized donors and acceptors in a semiconductor are a common example of such impurities. The amount of scattering due to electrostatic forces between the carrier and the ionized impurity depends on the interaction time and the number of impurities. Larger impurity concentrations result in a lower mobility. The dependence on the interaction time helps to explain the temperature dependence. The interaction time is directly linked to the relative velocity of the carrier and the impurity which is related to the thermal velocity of the carriers. This thermal velocity increases with the ambient temperature so that the interaction time increases, the amount of scattering decreases, resulting in a mobility increase with temperature. To first order the mobility due to impurity scattering is proportional to T ^3/2/N_I, where N_I is the density of charged impurities.