A series of ‘intrinsic’ amorphous silicon (a-Si:H) films produced by photo- and plasma- chemical vapour deposition was studied using scanning tunnelling microscopy (STM). The high resistivity of these films meant that photo-induced enhancement of the carrier concentration in the material was required to enable a stable tunnelling regime to be obtained. This enhancement was achieved using a 3 mW He-Ne laser shone through the back of the sample. Tunnelling I/V spectra showed rectification behaviour typical of metal-insulator-semiconductor tunnelling. In addition, spatially independent photovoltage effects were observed while tunnelling under illumination. A series of I/V data taken as a function of tip/sample separation shows a gradual transition from spectra dominated by weak surface states to a more bulk-like behaviour. The surface roughness of the photo-CVD material (33 Å rms) was found to be an order of magnitude greater than that for the plasma-CVD material (6 Å rms). This difference may be the result of hydrogen radical etching in the former process.