Undoped ZnO films of 100 nm thickness have been deposited on p-type Si (100) substrates at room temperature by radio-frequency magnetron sputtering and subsequently annealed in a rapid thermal annealing apparatus under a nitrogen ambient by varying annealing temperature (T(A)) from 973 to 1273 K in steps of 100 K for 3 min. Photoconductivity spectra in the UV and visible ranges are measured as a function of T(A) in the range of measurement temperatures (T) from 80 to 350 K under an applied bias of 1 V. For the samples annealed at T(A) = 973 and 1073 K, negative photoconductivity is observed below T = 300 K, and its absolute magnitude increases as TA increases. In contrast, the photoconductivity of the samples at T(A) >= 1173 K is always positive, irrespective of T, and its magnitude increases as T(A) increases. These PC behaviors are discussed based on the defect states at the grain boundaries of the ZnO films. (C) 2009 Elsevier B.V. All rights reserved.