Photoconductive detectors were fabricated on autodoped n-GaN films, with resistivity varying from 10 Ohm-cm to 107Ohm-cm, by molecular beam epitaxy. The mobility-lifetime product, determined from the measurement of photoconductive gain, was found to decrease monotonically from 10−2 cm2/V to 10−7 cm2/V as the dark resistivity was increased. This variation in the ντ products is attributed to changes in photocarrier lifetimes. In order to understand the recombination mechanisms responsible for this photoconductive behavior, the dependence of photoconductivity on excitation intensity (δμσ fλ) was investigated. The exponent λ was found to vary from 0.5 to 1.0, as the dark resistivity of the films increased. These results indicate the presence of exponential band tails extending from the conduction band edge. Furthermore, the dependence of photoconductivity on dark resistivity indicates that the photoconductive response is governed primarily by the location of the dark fermi level. A model accounting for these observations is presented.