We have developed a Kramers-Kronig consistent analytical expression to fit the dielectric functions (epsilon(1), epsilon(2)) of hydrogenated amorphous silicon (a-Si:H)-based alloys measured using a combination of photoconductivity, transmission and reflection, and ellipsometric spectroscopies. The alloys of interest include amorphous silicon-germanium (a-Si1-xGex:H) and silicon-carbon (a-Si1-xCx:H), with optical bandgaps ranging from similar to 1.30 to 1.95 eV. The fit can be performed simultaneously throughout the following regions: (i) the sub-bandgap (or Urbach tail) region where the absorption coefficient increases exponentially with photon energy, GO the band-to-band onset region where transitions are assumed to occur between parabolic bands with constant dipole matrix element, and (iii) the above-bandgap region where a Lorentz oscillator model is applicable. We describe an approach whereby, from a single accessible measure of the optical bandgap, (epsilon(1), epsilon(2)) can be generated for a sample set consisting of optimum electronic quality a-Si:H-based alloys prepared by plasma-enhanced chemical vapor deposition. (C) 2003 Elsevier B.V. All rights reserved.