An electron cyclotron resonance plasma-assisted deposition method has been used to prepare polycrystalline GaN films. These GaN films can have dark conductivities as low as 10(-10) S cm(-1) and they exhibit well-defined conductivity activation energies. The sample conductivity is decreased by surface adsorption of gases and it increases again when the adsorbed species is photodesorbed from the surface using UV light. As a result, the photoresponse is very slow, especially the decay process, which may require several days for a sample to recover when it is mounted in a vacuum. This recovery time can be reduced significantly if the sample temperature is increased or the sample is exposed to air. In this respect, these GaN films have a number of features in common with ZnO.