High-quality hydrogenated amorphous silicon (a-Si: H) thin films were grown by hot-wire chemical vapor deposition on glass (Corning 7059) using silane with relatively high hydrogen albeit avoiding the formation of microcrystalline hydrogenated silicon. They were grown as a function of substrate temperature (T-S) ranging from 50 to 515 degrees C resulting in the corresponding hydrogen concentration [C-H] variation from 20.0 to 0.2 at. %. They are optically examined ex situ using spectroscopic phase modulated ellipsometry from near IR to near UV (i.e., 1.5-5.0 eV) obtaining pseudo-dielectric function (,) for investigating the role of hydrogen in network disorder. The raw ellipsometry data were modeled using Bruggeman effective medium theory and the dispersion relations for the amorphous semiconductors. A two-layer model consisting of a top surface roughness layer (d(S)) containing an effective medium mix of 50% a-Si:H and 50% voids and a single "bulk" layer (d(B)) of 100% a-Si:H was used to simulate the data reasonably well. We performed these simulations by nonlinear least-square regression analysis and it was possible to estimate the true dielectric function, energy band gap (E-g), film thickness (d(SE)), bulk void fraction, surface roughness layer (d(S)), and confidence limits (chi(2)). Moreover, it is shown that the Tauc-Lorentz model fits the ellipsometry data reasonably well and helps elucidating the layered structure of a-Si: H thin films. We also compared the optical band gap determined using ellipsometry modeling and the Tauc gap. We discuss the variation of the deduced parameters in terms of role of T-S (T role) or of hydrogen (H role) yielding possible physical meaning and found an agreement with the excitation dependent Raman spectroscopy results reported earlier [S. Gupta, R. S. Katiyar, G. Morell, S. Z. Weisz, and J. Balberg, Appl. Phys. Lett. 75, 2803 (1999)]. Atomic force microscopy was also used to validate the simulations. These analyses led to a correlation between the films' microstructure (or network disorder) and their electronic properties for electronic device applications, in general and for photovoltaic applications, in particular. (c) 2005 American Vacuum Society.