We report on charge capture and emission in Metal-Oxide-Semiconductor AlGaN/AlN/GaN High-Electron Mobility Transistors (MOS-HEMT) foreseen as power amplifiers in mm-wave user-equipment operating at RF frequencies. These devices target Enhancement mode operation, necessitating the incorporation of high-permittivity (high-?) dielectrics to mitigate barrier thinning induced leakage. It is shown that defects in the bulk of HfO2 or Al2O3 dielectrics result in significant Vt instabilities, with dielectric-dependent charge emission kinetics: band alignment between high-? shallow defect levels and AlGaN conduction band enables full (Al2O3) or partial (HfO2) charge de-trapping, while a deeper level in HfO2 enables trapping even at threshold conditions. The significance of this work lies in revealing the degradation modes present under DC which can also contribute at RF operating conditions.