The drain-current collapse at high drain voltage has been studied in AlGaN/GaN high-electron-mobility transistors (HEMTs) on both semi-insulating (SI)–SiC and sapphire substrates using small frequency (120 Hz) sinusoidal wave superimposed dc IDS–VDS characteristics. Low drain-current collapses were observed in AlGaN/GaN HEMTs on SI–SiC substrate when compared with the HEMTs on sapphire substrates. Two and three thermally activated deep traps were observed on SiC-based and sapphire-based HEMTs, respectively. The existence of an additional deep trap (ΔE=0.61 eV) could be associated with the material defects/ dislocations responsible for the severe drain current collapse in sapphire-based HEMTs. The white-light illuminated IDS–VDS characteristics support the existence of more number of deep traps in the sapphire-based HEMTs. © 2002 American Institute of Physics.