Variations of the channel resistance of a AlGaN/GaN high electron mobilities transistor caused by subsequently incident photons with sub-bandgap energies after ultraviolet light-induced changes became stable were studied. Temperature-dependent measurements yielded a 0.342 eV thermal activation energy and wavelength-varying measurements yielded a 0.8 eV cut-off photon energy. The ratio between the two values is very close to the theoretical value of the ratio between the valence and conduction band discontinuities. A qualitative description about the transports of excess carriers through the band discontinuities, is also proposed and consistent with the experimental results.