where formula_64 is the Debye temperature. At lower temperatures there are fewer phonons available for drag, and at higher temperatures they tend to lose momentum in phonon-phonon scattering instead of phonon-electron scattering. At lower temperatures, material boundaries also play an increasing role as the phonons can travel significant distances. Practically speaking, phonon drag is an important effect in semiconductors near room temperature (even though well above formula_65), that is comparable in magnitude to the carrier-diffusion effect described in the previous section.