Trap behaviors in AlGaN-GaN heterostructures were characterized by the means of mercury-probe capacitance-voltage (C-V) technique for samples grown on sapphire and SiC substrates. The effective trap density was determined by a simplified model to give a schematic explanation for the capacitance dispersion. Fluorine plasma treatment was carried out with appropriate time and power for further identification. The V-Ga-related defects in (Al)GaN bulk layers were responsible for the frequency-dependent capacitance dispersion in pinch-off region, while the surface status influenced the capacitance dispersion in pinch-on region directly. (C) 2009 Elsevier Ltd. All rights reserved.