High quality low bandgap a-(Ge,Si):H alloys were prepared using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping. The charge transport and optical properties of these low bandgap materials as a function of alloy composition have been investigated by employing the microwave photomixing technique and optical absorption spectroscopy. From the measurements of electric field dependence of the drift mobility, we have found strong evidence for the existence of long-range potential fluctuations in a-(Ge,Si):H alloys, and determined the depth and range of the potential fluctuations, and subsequently the charged defect density, as a function of alloy composition. It was found that at ∼30% Si in Ge, the photoresponse begins to decrease rapidly with increasing Si content due to the decreases in the mobility and lifetime, and meanwhile, both the charged defect density and the Urbach energy increase significantly. The latter indicates an increase in the compositional disorder. It is the potential fluctuations whose effect can be also enhanced by incorporating Si to the alloy system that result in the deterioration of the electronic properties of a-(Ge,Si):H alloys, similar to the case of the incorporation of Ge at the Si end. This enhanced effect accompanies with an increase in the depth, and a decrease in the range of potential fluctuations, leading to a decrease in the mobility, and consequently in the photoconductivity. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Si to the alloys play an important role in the potential fluctuations.