A method to compensate for fabrication tolerances and to fine-tune individual photonic circuit components is inevitable for wafer-scale photonic systems even with most-advanced CMOS-fabrication tools. We report a cost-effective and highly accurate method for the permanent trimming of hydrogenated amorphous silicon photonic devices by UV-irradiation. Microring resonators and Mach-Zehnder-interferometers were utilized as photonic test devices. The MZIs were tuned forth and back over their complete free spectral range of 5.5 nm by locally trimming the two MZI-arms. The trimming range exceeds 8 nm for compact ring resonators with trimming accuracies of 20 pm. Trimming speeds of ≥ 10 GHz/s were achieved. The components did not show any substantial device degradation.