Investigation of photoelectrical properties of iron modulation-doped (MD) and continuously doped (CD) GaN layers has been carried out by transient photo-Hall, photo-conductivity, and time-resolved picosecond four-wave-mixing (FWM) techniques. The MD semi-insulating (SI) layers exhibited prolonged photocurrent relaxation time and the presence of deep defects with thermal activation energy of 217 meV. Low electrical activity of threading dislocations (TD) in the upper part of the MD layers as well as high carrier mobility at low temperature was confirmed by FWM measurements and pointed out to vanishing dislocation-related heterogeneous barriers due to Fe doping. In contrast, shorter carrier lifetimes and low mobility in CD-layers were attributed to Fe-related defects, more "detrimental" centers than dislocations. (c) 2006 Elsevier B.V. All rights reserved.