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We have observed that the estimation of two-dimensional electron gas density is dependent on the device geometry. The geometric contribution leads to the anomalous estimation of the GaN based heterostructure properties. The observed discrepancy is found to originate from the anomalous area dependent capacitance of GaN based Schottky diodes, which is an integral part of the high electron mobility transistors. The areal capacitance density is found to increase for smaller radii Schottky diodes, contrary to a constant as expected intuitively. The capacitance is found to follow a second order polynomial on the radius of all the bias voltages and frequencies considered here. In addition to the quadratic dependency corresponding to the areal component, the linear dependency indicates a peripheral component. It is further observed that the peripheral to areal contribution is inversely proportional to the radius confirming the periphery as the location of the additional capacitance. The peripheral component is found to be frequency dependent and tends to saturate to a lower value for measurements at a high frequency. In addition, the peripheral component is found to vanish when the surface is passivated by a combination of N2 and O2 plasma treatments. The cumulative surface state density per unit length of the perimeter of the Schottky diodes as obtained by the integrated response over the distance between the ohmic and Schottky contacts is found to be 2.75 × 1010 cm−1.

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The properties of the oxide/AlGaN heterointerface are investigated from field-dependent off-state degradation and recovery in thermally grown NiO x -, TiO2-, and Al2O3-based metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs). Al- and Ti-oxides form type-I straddling band alignment with positive and negative band offsets, respectively, and Ni-oxide forms type-II staggered band alignment with AlGaN. These oxides show promising results for high-performance HEMTs suitable for switching applications. The oxides are grown by rapid thermal oxidation of a thin film of the respective metals. The properties are quantified from the degradation and recovery of critical device parameters. The oxides largely follow the field-dependent degradation by electron and hole trapping. The critical fields for electron–hole pair generation and dominance of hole trapping over electron trapping have great significances in applied power electronics due to the presence of inherently large voltage transients. In addition to the quality of interfaces, oxide band alignment with AlGaN is important. The degradation and recovery are faster in Al2O3, indicating the presence of shallow traps and type-I band alignment. Even though NiO x shows the highest gate leakage current due to the type-II heterostructure, it shows negligible degradation in most of the parameters. The prestressed TiO2 devices show a performance similar to that of Al2O3 samples. Meanwhile, TiO2 shows the least degradation and endures the largest off-state voltage owing to the better heterointerfacial properties and type-I band alignment with negative band offsets. Both Al2O3 and TiO2 follow the field-dependent degradation due to electron trapping followed by hole trapping, which is concluded from the crossover in the threshold voltage for the formation of two-dimensional electron gas and confirmed by repeating the measurements for various gate-to-drain separated devices. The device breakdown for the applied off-state step stress occurs at 120, 140, and 100 V for NiO x , TiO2, and Al2O3, respectively. The corresponding gate-connected field plate devices show breakdown voltages in an excess of 600 V.

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We have shown that the off-state degradation in GaN based metal-oxide-semiconductor high electron mobility transistors has three field dependent regimes. We have considered Al2O3 and AlOxNy as the gate dielectrics. The degradation is dominated by electron trapping near the drain edge at relatively low electric field; hole trapping followed by structural defects tends to dominate at high electric field. The structural defects may potentially be caused by an inverse piezoelectric effect when the AlGaN/GaN interface may relax partially. The hole trapping, in particular, at moderate to high electric field can lead to an anomalous trend. The off-state drain current bears the signature of the degradation. The transistor performance is found to deteriorate for all the cases independent of the degradation mechanism. The trapping contribution toward the degradation is partially recoverable; however, the structural damage is found to be permanent. It is also observed that both the gate dielectrics qualitatively suffer from the same degradation mechanism with oxynitride high electron mobility transistors showing higher resilience than their oxide counterpart.