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The optical absorption data of Hydrogenated Amorphous Silicon was analyzed using a Dunstan model of optical absorption in amorphous semiconductors. This model introduces disorder into the bandband absorption through a linear exponential distribution of local energy gaps, and it accounts for both the Urbach and Tauc regions of the optical absorption edge.Compared to other models of similar bases, such as the O'Leary and Guerra models, it is simpler to understand mathematically and has a physical meaning. The optical absorption data of Jackson et al and Maurer et al were successfully interpreted using Dunstan's model. Useful physical parameters are extracted especially the band to the band energy gap 𝐸 °, which is the energy gap in the absence of disorder, that can be interpreted as the mobility gap of the material.

The optical absorption edge in both Urbach and Tauc regions are analyzed for hydrogenated amorphous silicon optical data of Cody et al and Viturro and Weiser . Dunstan’s model, introduces disorder into the band-band absorption through a linear exponential distribution of local energy gaps. This model has a simple mathematical form and has a clear physical meaning in comparison to the somewhat similar models of O’Leary and Guerra, yet Dunstan’s model is unjustly neglected in literature. The most important physical parameter of the model is the energy gap parameter- Eo, which represents the separation between the valence and conduction band edges, it can be considered as the mobility gap of the material. Dunstan’s model is found successful in capturing the optical data of Cody et al and, Viturro and Weiser. A plot of the energy parameters of the Dunstan and Cody models, Eo against Eopt respectively, was proposed here to separate the masking contribution of tail-related absorption from the real change of the band gap, and was found fruitful for this purpose. This is because Eo is a function only of the separation between valence and conduction band edges, while the meaning of the optical energy Eopt is ambiguous because it is also a function of absorption due to tail-related absorption that results from disorder. This simple plot of Eo vs. Eopt , through the slope ΔEo/ΔEopt of the fitting straight line , was capable of distinguishing the different relative contributions of disorder between Cody et al and Viturro and Weiser optical data, Eo value was found less sensitive to hydrogen content variation for Viturro and Weiser data which is interpreted due to their better control of the thin film deposition process. Keywords: optical energy gap, absorption coefficient, amorphous semiconductor, mobility gap