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The effect of an initially grown high-temperature AlN buffer (HT-AlN) layer's thickness on the quality of an AlN epilayer grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD) in a two-step growth process is investigated. The characteristics of AlN epilayers are analyzed by using triple-axis crystal X-ray diffraction (XRD) and atomic force microscopy (AFM). It is shown that the crystal quality of the AlN epilayer is closely related to its correlation length. The correlation length is determined by the thickness of the initially grown HT-AlN buffer layer. We find that the optimal HT-AlN buffer thickness for obtaining a high-quality AlN epilayer grown on sapphire substrate is about 20 nm. [ABSTRACT FROM AUTHOR]

An Ni/Au Schottky contact on an AlGaN/GaN heterostructure has been prepared. By using the peak-conductance model, the threshold voltage and the series resistance of the AlGaN/GaN diode are simultaneously extracted from the conductance--voltage (G-V) curve and found to be in good agreement with the ones obtained by using the capacitance--voltage (C-V) curve integration and the plot of dV/d(ln I) versus current I. Thus, a method of directly and simultaneously extracting both the threshold voltage and the series resistance from the conductance--voltage curve for the AlGaN/GaN Schottky diode is developed. [ABSTRACT FROM AUTHOR]

Using measured capacitance--voltage curves and current--voltage characteristics for the AlGaN/AlN/GaN heterostructure field-effect transistors with different gate lengths and drain-to-source distances, the influence of drain bias on the electron mobility is investigated. It is found that below the knee voltage the longitudinal optical (LO) phonon scattering and interface roughness scattering are dominant for the sample with a large ratio of gate length to drain-to-source distance (here 4/5), and the polarization Coulomb field scattering is dominant for the sample with a small ratio (here 1/5). However, the above polarization Coulomb field scattering is weakened in the sample with a small drain-to-source distance (here 20 µm) compared with the one with a large distance (here 100 µm). This is due to the induced strain in the AlGaN layer caused by the drain bias. [ABSTRACT FROM AUTHOR]

In this study rectangular AlGaN/AlN/GaN heterostructure field-effect transistors (HFETs) with 22-nm and 12-nm AlGaN barrier layers are fabricated, respectively. Using the measured capacitance–voltage and current–voltage characteristics of the prepared devices with different Schottky areas, it is found that after processing the device, the polarization Coulomb field (PCF) scattering is induced and has an important influence on the two-dimensional electron gas electron mobility. Moreover, the influence of PCF scattering on the electron mobility is enhanced by reducing the AlGaN barrier thickness. This leads to the quite different variation of the electron mobility with gate bias when compared with the AlGaN barrier thickness. This mainly happens because the thinner AlGaN barrier layer suffers from a much stronger electrical field when applying a gate bias, which gives rise to a stronger converse piezoelectric effect. [ABSTRACT FROM AUTHOR]

We report on the temperature-dependent dc performance of AlGaN/GaN polarization doped field effect transistors (PolFETs). The rough decrements of drain current and transconductance with the operation temperature are observed. Compared with the conventional HFETs, the drain current drop of the PolFET is smaller. The transconductance drop of PolFETs at different gate biases shows different temperature dependences. From the aspect of the unique carrier behaviors of graded AlGaN/GaN heterostructure, we propose a quasi-multi-channel model to investigate the physics behind the temperature-dependent performance of AlGaN/GaN PolFETs. [ABSTRACT FROM AUTHOR]