  We report on the interfacial electronic properties of HfO2 gate dielectrics
both, with GaN towards normally-OFF recessed HEMT architectures and the AlGaN
barrier for normally-ON AlGaN/GaN MISHEMTs for GaN device platforms on Si. A
conduction band offset of 1.9 eV is extracted for HfO2/GaN along with a very
low density of fixed bulk and interfacial charges. Conductance measurements on
HfO2/GaN MOSCAPs reveal an interface trap state continuum with a density of
9.37x1012 eV-1cm-2 centered at 0.48 eV below EC. The forward and reverse
current densities are shown to be governed by Fowler-Nordheim tunneling and
Poole-Frenkel emission respectively. Normally-ON HfO2/AlGaN/GaN MISHEMTs
exhibit negligible shifts in threshold voltage, transconductances of 110mS/mm
for 3 {\mu}m gate length devices, and three-terminal OFF-state gate leakage
currents of 20 nA/mm at a VD of 100 V. Dynamic capacitance dispersion
measurements show two peaks at the AlGaN/GaN interface corresponding to slow
and fast interface traps with a peak Dit of 5.5x1013 eV-1cm-2 and 1.5x1013
eV-1cm-2 at trap levels 0.55 eV and 0.46 eV below EC respectively. The
HfO2/AlGaN interface exhibits a peak Dit of 4.4x1013 eV-1cm- 2 at 0.45 eV below
EC.
