Tensile strain and Fermi level alignment in thermally grown TiO2 and Al2O3 based AlGaN/GaN MOS-HEMTs

Abstract

This work reports on the origin of performance improvement for thermally grown TiO2 and Al2O3 based AlGaN/GaN metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs). The oxides have been used as gate dielectrics and passivation layer. High resolution X-ray diffraction, X-ray photoelectron spectroscopy, and transistor characteristics are analysed to investigate the improvements in the two dimensional electron gas (2DEG) concentration. The HRXRD analysis reveals that in-plane tensile stress of AlGaN layer is increased by 23% (12%) for TiO2 (Al2O3) sample as compared to that of an as-grown sample. The induced tensile stress in the AlGaN barrier layer enhances the piezoelectric polarization charges which effectively improve the carrier confinement and mobility at the interface. The improvement in the DC characteristics is observed as a reduction in the gate leakage current without deteriorating gate control and transconductance. The output characteristics of TiO2 (Al2O3) based MOS-HEMTs have shown a 60% (40%) increment in the maximum saturation drain current and 50% (40%) increment in the transconductance as compared to that of a control sample. The RF characteristics also show similar order of improvements.