Cathodoluminescence in-depth spectroscopy (CLIS) study was made on GaN and AlGaN/GaN materials. The experimental CLIS spectra were compared with analysis on computer. The calculation reproduces experimentally observed CLIS spectrum shapes remarkably well. High density U-shaped surface states with particular charge neutrality levels should exist on the surface of the GaN layer to account for observed finite onset voltage and stretching of CLIS spectra. AlGaN layer seems to act like a surface passivation layer for GaN leading to band edge emission with a nearly ideal CLIS spectrum shape with negligible recombination at heterointerface. Yellow luminescence is due to deep acceptor levels uniformly distributed in the GaN bulk where radiative transition from conduction band edge to deep acceptors gives luminescence. (C) 2003 Elsevier Science B.V. All rights reserved.