Semiconductor device operation depends on the speed of carriers, electrons and holes, which is determined by the mobility. The mobility μ is expressed by e〈Τ〉/m*, where e, 〈Τ〉, and m* are the electronic charge, average of the relaxation time, and the effective mass. The relaxation time or scattering time is limited by various scatterings of carriers. Among them the phonon scattering plays the most important role. In this chapter we begin with the analysis of the lattice vibrations and the derivation of Boltzmann transport equation. Then collision time, relaxation time and mobility are defined. The transition probabilities and transition matrix elements for the scattering due to various modes of phonons, impurity, electron–electron interaction and so on are evaluated using quantum mechanical approach. These results are used to evaluate scattering rates and relaxation times, and finally respective carrier mobility is obtained. Also a method to evaluate deformation potentials for phonon scattering is given, where the lattice vibrations and the calculated energy band structures are employed.