Photoionization spectroscopy of traps in doped and undoped AlGaN/GaN HEMTs

Deep-level defects and surface states are supposed to be responsible for the limitation of AlGaN/GaN high electron mobility transistor (HEMT) performance. In order to investigate the influence of these traps, photoionization spectroscopy was used to study doped and undoped HEMTs grown on sapphire in different metalorganic vapour-phase epitaxy reactors. This measurement technique is based on the optical reversion of the current collapse and it allows one to determine photoionization cross-sections of the participating traps. For doped and undoped HEMTs nearly the same two defect levels with excitation energies of 3.2 eV and 2.9 eV were determined. By varying the source–gate voltage it was found that the photoionization cross-section is reduced for positive gate bias, i.e. the virtual gate on the gate–drain access region is partially neutralized due to the removal of trapped electrons from surface states.