GaN material had attracted much attention in the high-voltage, high-power and device-switching applications because of their material characteristics of wide band gap and high breakdown voltage. Additionally, high electron concentration is found at the AlGaN/GaN heterointerface, which is caused by spontaneous polarization and piezoelectric polarization. This high electron concentration is called two dimensional electron gas(2DEG) with high mobility. This study focuses on improve the threshold voltage to reach normally-off operation.
In this thesis, the normally-off recessed gate AlGaN/GaN HEMTs with suitable threshold voltages by modulating the mole fraction of Al in the various AlGaN layer thickness under the gate region are proposed using the Synopsys technology computer aided design (TCAD) simulator. It is found that the slope of threshold voltage versus Al mole fraction curves is decreased with decreasing the thickness of AlGaN layer due to the decrease of piezoelectric effect on 2DEG concentration to improve threshold voltage.