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We report on photoluminescence (PL) experiments performed on a n-type modulation doped GaN/Al0.15Ga0.85N heterojunction grown by low pressure metal organic chemical vapor deposition (MOCVD) technique. In the temperature range 7.4-80 K, we have investigated (i) the photoluminescence spectra from the degenerate 2D electron gas confined near the GaN/AlGaN interface, (ii) the impurity and band edge photoluminescence of the 3D undoped region of the GaN layer. The experiments were performed in the following manner: we excited the sample at 333.6 nm in two ways, on the AlGaN barrier (2D channel side) and on the substrate side. The comparison between the corresponding spectra and the temperature dependence of lines-intensity allowed us to identify without ambiguity the 2D and 3D character of the different lines. We find a very broad line which could come from the first E-1 subband transitions and at a higher energy two lines with which intensities increase with temperature. We attributed those lines to excitonic transitions associated to the second subband. Between E-1 and these 2D excitonic lines we observed a very intense line with which intensity decreased rapidly when temperature increased. The intensity of this line behave typically like the one of a transition at the Fermi energy of the 2D degenerate electron gas (Fermi edge singularity). (C) 1997 Elsevier Science S.A.