Impact of composition and thickness of step-graded AlGaN barrier in AlGaN/GaN heterostructures

Abstract

We propose an optimization method to change the Al component and the thickness of step-graded AlGaN barrier to achieve better electrical performance of the device. Based on the polarization effect, we find that a second potential well occurs while increasing the Al fraction and thickness of the upper AlGaN layer, which contributes to better drain current performance in the simulations. The wafers with different AlGaN barriers were grown to compare their surface morphology and electrical properties. The surface roughness of the wafer is related to the lattice constant of the upper AlGaN layer. According to the Hall and TLM measurements, the electron sheet density and electron mobility of the 20 nm-Al0.35Ga0.65N/10 nm-Al0.25Ga0.75N barrier sample are 1593 cm2/Vs and 1.17 × 1013 cm−2 respectively, resulting in a high spacing current of 61.45 mA. Moreover, the maximum drain current density of this barrier device is 589.02 mA/mm under the I–V characteristics, 1.37 times as much as that of the 20 nm-Al0.25Ga0.75N barrier. The higher gate barrier height of Al0.35Ga0.65N reduces the gate leakage current.