Shallow and deep centers were studied by means of temperature dependent Hall effect and photoluminescence (PL) measurements in two sets of undopedn‐AlGaN samples grown by organometallic vapor phase epitaxy. The samples of these two series were grown under different conditions and had, as a result, electron concentrations differing by several orders of magnitude. The composition dependence of ionization energies of dominant donors in these two sets of samples is very different indicating that different types of centers are involved, but in both cases they are most probably related to some native defects. These defects behave as hydrogen‐like donors for low Al compositions and become increasingly deeper with increasing Al content. The shallow‐deep transition occurs at aboutx=0.2 in the low conductivity AlxGa1−xN series and at aboutx=0.5 for the high conductivity series. Several PL bands were detected in AlGaN and it is shown that the band at 3.05 eV is due to a radiative transition between deep donors in the upper part of the band gap and holes in the valence band or on shallow acceptors. For the yellow luminescence band at 2.25 eV it is demonstrated that this band consists of two overlapping bands and that the dominant band is due to a transition between the native donors and a carbon‐related deep center. ©1996 American Institute of Physics.