The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/pssb.202100312.

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Epitaxially grown GdScO3 was integrated in a GaN-based metal-insulator-semiconductor high electron mobility transistor as a high-κ gate passivation layer. Microstructural investigations using transmission electron microscopy and x-ray diffraction confirm the epitaxial growth of GdScO3 on GaN deposited by pulsed laser deposition on the AlGaN-GaN heterostructure. The metal-insulator-semiconductor high electron mobility transistor was compared to unpassivated and to Al2O3 passivated high electron mobility transistors. A layer of 20 nm GdScO3 reduces the gate leakage current below the level of the Al2O3 passivated transistors and below the off-current of the high electron mobility transistor without any gate dielectric. Time-dependent measurements show a strong dependence of the drain leakage current in the off-state on light illumination, which indicates slow trapping effects in GdScO3 or at the GdScO3–GaN interface.

The controlled growth of surface‐modifying polymer films by electrodeposition often fails because of the lack of redox activity of these compounds. Here, electroactive complexants help to electrodeposit non‐electroactive polymers. Hence, we investigate the counterion‐induced electrodeposition of polyelectrolytes: three quaternized poly(N,N‐dialkylaminoethyl methacrylate)s (qPDAAEMA), in particular their methyl, ethyl, and isopropyl derivatives (i. e. qPDMAEMA, qPDEAEMA, and qPDPAEMA), provide transparent solutions in the presence of hexacyanoferrate(II) (ferrocyanide) at specific concentration windows of the KCl supporting electrolyte. Below a certain KCl concentration, insolubility dominates irrespective of the hexacyanoferrate valency, whilst above an upper threshold, full solubility is observed. Between these limits, oxidation reversibly electrodeposits polymer/hexacyanoferrate(III) (ferricyanide) complexes. Hydrodynamic voltammetry (and data analysis using in‐house software) provides access to the deposition efficiency (DE). qPDEAEMA with ethyl substituents shows highest DEs; larger or smaller substituents fall short because of a balance between “hydrophobicity” and charge separation, shifting the window toward smaller salt concentrations with increasing alkyl size. We always observe a DE maximum close to the minimum salt concentration, whilst electrochemical quartz crystal microbalance (EQCM) measurements indicate a change in film water content close to the maximum. These effects, being also discussed in terms of polymer conformation, can direct the future engineering of electroassisted coatings.

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