Al0.3Ga0.7As:Si/GaAs modulation-doped field-effect transistor-type heterostructures were grown using two different growth temperatures (500 and 620 degrees C) and three doping modes (delta-doping, pulse-doping, and uniform-doping). Deep level transient spectroscopy (DLTS) measurements were performed on these structures using a new Fourier-analysis method. Up to four DLTS peaks, related to the different possible configurations of the nearest Al and Ga neighbors around each DX site, were observed. Both the growth temperature and the doping-mode are found to affect the DLTS spectra, in particular the number of observed peaks and their width. These results are interpreted in terms of the different mobilities of the Si doping atoms on the surface during growth.