The large signal characteristics of Cu-gate and Ni/Au-gate AlGaN/GaN high-electron-mobility transistors (HEMTs) were compared. The tested devices were fabricated on two different parts of the same wafer following the same fabrication steps, the only difference being in the Schottky contact material. Comparison of the direct-current (DC) and radio frequency (RF) characteristics points out a critical drain current collapse in the Cu-gate devices, with detrimental effects on the RF performance, whereas the Ni/Au-gate HEMTs performed properly during DC, pulsed, and RF measurements. Investigations on the drain current transients and on the ID–VGS characteristics, obtained by pulsed signals, suggest the presence of an acceptor trap density in the AlGaN barrier, beneath the Cu Schottky gate contact, responsible for the poorer performance of the Cu-gate device; an activation energy of 430 meV was extracted. This defectivity seemed to be due to a copper diffusion event, activated by thermal stress induced in the sample during the plasma-enhanced chemical vapor deposition (PECVD) of SiN. Numerical simulations carried out on the tested structure qualitatively support the presence of a trap density explaining the nature of the observed drain current transients.