Resistance noise measurements are presented of intrinsic hydrogenated amorphous silicon films as a function of voltage and temperature in the dark. The noise displays Gaussian statistics pointing to a large number of independent noise sources. It is demonstrated that the spectral dependence and temperature dependence are the result of a large number of independent thermally activated fluctuators. A distribution of activation energies for the characteristic rates is obtained from the measurements that peak at 0.85 eV with a full width at half maximum of ϳ0.2 eV. The attempt rate amounts to 7ϫ10 12 s Ϫ1 . We discuss general mechanisms for mobility fluctuations and number fluctuations due to generation and recombination of free carriers. A model for generation-recombination noise in a semiconductor with localized electronic states distributed throughout the mobility gap appears consistent with both the variance of the fluctuations and the temperature dependence of the rate constants. We identify the activation energy and attempt rate as the barrier limiting thermal emission of holes from defect states to the valence band. The width of the distribution of activation energies points to inhomogeneous broadening due to band bending. Light-induced metastable changes in the material are observed to affect the variance quite significantly. The present measurements and analysis of the noise under equilibrium conditions pave the way to successfully examine nonthermal equilibrium fluctuations in the photoconductivity and under injection conditions, which is the subject of the accompanying paper in this volume.

We present and discuss the first noise measurements on hydrogenated amorphous silicon (a-Si:H) under nonthermal equilibrium conditions. Under steady-state illumination noise measurements are carried out for temperatures ranging from 100 to 450 K. We conclusively identify generation-recombination noise as the prevailing noise mechanism in intrinsic a-Si:H. We examine the dynamics of holes with noise spectroscopy in n-type devices that include thermal activation from the hole quasi-Fermi level, a clear Meyer-Neldel relation pointing to a temperature dependence of the hole quasi-Fermi level, and diffusion-limited transport to recombination centers at low temperatures. The variance is found to be independent of illumination intensity, indicative of a uniform distribution of defect states throughout the gap. Noise measurements further suggest that metastable defects are created close to midgap in degraded samples. Under conditions of electron injection we find that the noise becomes space-charge suppressed, similar to what has been observed in crystalline semiconductors. ͓S0163-1829͑98͒07131-8͔

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