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The impact of states at the insulator/AlGaN interface on the capacitance-voltage ͑C-V͒ characteristics of a metal/insulator/AlGaN/GaN heterostructure ͑MISH͒ capacitor was examined using a numerical solver of a Poisson equation and taking into account the electron emission rate from the interface states. A parallel shift of the theoretical C-V curves, instead of the typical change in their slope, was found for a MISH device with a 25-nm-thick AlGaN layer when the SiN x / AlGaN interface state density D it ͑E͒ was increased. We attribute this behavior to the position of the Fermi level at the SiN x / AlGaN interface below the AlGaN valence band maximum when the gate bias is near the threshold voltage and to the insensitivity of the deep interface traps to the gate voltage due to a low emission rate. A typical stretch out of the theoretical C-V curve was obtained only for a MISH structure with a very thin AlGaN layer at 300°C. We analyzed the experimental C-V characteristics from a SiN x / Al 2 O 3 / AlGaN/ GaN structure measured at room temperature and 300°C, and extracted a part of D it ͑E͒. The relatively low D it ͑ϳ10 11 eV −1 cm −2 ͒ in the upper bandgap indicates that the SiN x / Al 2 O 3 bilayer is applicable as a gate insulator and as an AlGaN surface passivant in high-temperature, high-power AlGaN/GaN-based devices.

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