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Experimental results for electron drift velocity and diffusivity in germanium, obtained with the time-of-flight technique in hyperpure material. are presented for temperatures between 8 and 240 K and fields between 1 and 104,V/cm oriented along (111) and (100) directions. An anisotropy of the drift velocity and of the diffusion coefficient is found with the electric field applied along (111) and (100) directions, the latter due to the intervalley diffusion. The effect of the electron-electron interaction in the anisotropy properties has also been investigated. Theoretical Monte Carlo calculations have been performed with a model which includes lower. (111). nonparabolic bands as well as upper bands at the center of the Brillouin zone and along the (100) directions. Acoustic scattering with proper energy relaxation, optical scattering, and intervalley scattering between equivalent and nonequivalent valleys has been taken into account. Besides drift velocities and diffusion coefficients, other quantities such as mean electron energy, electron distribution function, and valley repopulation have been obtained from the Monte Carlo simulation.

Drift velocities for holes in high-purity Si were measured for fields between about 3 and SX 10" V/cmã nd temperatures between 6 and 300'K for the crystallographic directions (100), (110), and (111).The Ohmic mobility is theoretically interpreted on the basis of a two-band model consisting of a spherical parabolic and a spherical nonparabolic band, and the relaxation-time approximation.The low-temperature Ohmic mobility is strongly influenced by the nonparabolicity of the heavy-hole band. The high-field region (E) 10' V/cm) was analyzed using a single warped heavy-hole band model and a Monte Carlo technique.Anisotropy of hot-hole drift velocity is associated with warping of the valence band. Optical-and acousticscattering mechanisms are found to be of comparable strength,