2.9 Intervalley Scattering

As displayed in Fig. 2.13, the constant energy surfaces for electrons in Si and GaAs consist of several valleys. For Si, the valleys are energetically equivalent and lie along [100] directions near the zone boundary. Two types of intervalley scattering are possible in Si; g-type processes move a carrier from a given valley to one on the opposite side of the same axis (e.g., from a valley along to one along ). The ' f-type processes move a carrier to one of the remaining valleys. Both g- and f-type scattering produce very large changes in momentum, so they require phonons with wave vectors near the zone boundary. Such phonons are termed intervalley phonons and may be either acoustic or optical phonons. Note from Fig. 1.27a that near the zone boundary the energies of both acoustic and optical phonons are comparable and are somewhat less than the longitudinal optical phonon energy, ? ? _o. The specific phonons involved in g- and f-type scattering are listed in Table 2.1 at the end of this chapter.

Fig. 2.13: (a) constant energy surfaces for silicon, and (b) constant energy surfaces for gallium arsenide.

Intervalley scattering in GaAs is somewhat different because the valleys shown in Fig. 2.13b are not energetically equivalent. The central, ?, valley lies about 0.3 eV below the ellipsoidal, L, valleys located along directions. An illustration in energy-momentum space of a ? to L