We perform a comprehensive electrical transport and physical characterization of metal oxide semiconductor heterostructure field effect transistors with ZrO2 gate dielectrics, having partially (referred here as MOS-HFET) and fully (here called true-MOS-FET) recessed GaN/AlGaN/GaN barrier, giving normally-on and normally-off behavior, respectively. The mobility of the MOS-HFETs decreases with the proximity of the Coulomb scattering centers, situated at the ZrO2/AlGaN interface. The effect of the etching procedure and ZrO2 deposition on the formation of the interfacial charges, N-int, is evaluated by X-ray Photoelectron Spectroscopy and by fitting the threshold voltage values to numerical model. For the both device types, the extracted value of N-int lies within 15% around 2.8 x 10(13) cm(-2), which is of the order of polarization charge, showing that our low-damage three step etching procedure does not introduce extra interface states. (C) 2016 Elsevier Ltd. All rights reserved.