HOT-ELECTRON SCATTERING BY NEUTRAL HYDROGENIC DONORS

Accurate rates computed for electron-hydrogen scattering are applied to intravalley scattering of electrons by neutral hydrogenic donors in moderately doped semiconductors (at low compensation) held at cryogenic temperatures. The differential rates for both elastic and impact-excitation scattering are included explicitly. In contrast to the traditional assumption of exclusively isotropic elastic scattering, we show that the elastic scattering strongly favors the foward directions for energies well below impurity ionization. Near the impurity ionization energy, the inelastic impact excitation ignored in traditional treatments accounts for about 10% of the total impurity scattering rate and dominates impurity scattering for higher energies. Inelastic scattering strongly favors the forward directions for all energies. The consequences of both anisotropic and inelastic scattering for hot-electron transport are demonstrated in a sample calculation for a Si:As layer subject to an electric field. In particular, the threshold for non-Ohmic transport is found at much lower field strengths in calculations employing our proposed neutral-impurity scattering rates than in calculations employing an extrapolation of the traditional isotropic elastic rate.