Unlike silicon and traditional III-V semiconductors, the III-nitrides exhibit high spontaneous and piezoelectric polarization charges at epitaxial polar heterojunctions. In the process of investigating scaling properties of gate-stacks consisting atomic-layer deposited Al 2 O 3 /III-Nitride heterojunctions, we find interface charges that appear closely linked to the polarization charges of the underlying nitride substrate. Through capacitance-voltage measurement on a series of samples of varying dielectric thicknesses, we find the presence and propose an origin of benign donor-type interface charges (Q it $6 Â 10 13 cm À2 ) at the AlN/Al 2 O 3 junction. This interface charge is almost equal to the net polarization charge in AlN. The polarization-related dielectric/AlN interface charge and the role of oxygen in the dielectric as a possible modulation dopant potentially offer opportunities for various device applications.GaN high electron mobility transistors (HEMTs) outperform Si devices for high voltage switching by virtue of their large bandgap and additionally possess the potential for very high speed switching. This requires highly scaled low sheetresistance HEMT structures with very thin barriers. However, ultrathin epitaxial barriers (such as AlN or InAlN) result in substantial leakage currents preventing the capability to block high drain voltages, and dielectrics can substantially mitigate this problem. Thus, dielectrics such as SiO (Ref. 4) are being investigated intensively both for composite gate stacks as well as for the suppression of current collapse 5 by passivating surface states in these devices. Atomic layer deposited (ALD) Al 2 O 3 has drawn the attention of the community due to its large bandgap and outstanding dielectric 6 and passivation 7 properties. The superior quality (in terms of uniformity) of ALD over sputtering and electron-beam deposition, coupled with high band gap ($6.5 eV), 8 high dielectric constant ($9.1), high break down field ($10MV/cm), high thermal (amorphous $1000 C), and chemical stability of ALD-grown Al 2 O 3 makes it a natural choice as a gate insulator for AlN/ GaN HEMTs (Ref. 9) and its variants. The study of the ALD Al 2 O 3 /III-nitride interface is of prime importance for device characteristics of AlN/GaN HEMTs. In this work, we present a comprehensive characterization of AlN/GaN MOS-HEMT gate stacks with ALD Al 2 O 3 of various thicknesses. Through capacitance-voltage (C-V) measurement, we find the presence and propose an origin of benign donor-type positive interface charge (Q it ) at the AlN/Al 2 O 3 junction and relate its presence to the polarization charges in AlN. The presence of Q it explains the trend of pinch-off voltage and twodimensional electron gas (2DEG) density with ALD thicknesses both qualitatively and quantitatively. Recent report 10 (appeared after this submission) on ALD/GaN structure also invokes positive interface charge to explain the trend of pinch-off voltage with ALD thicknesses. AlN/GaN HEMT structures were grown in a Veeco Gen 930 molec...

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