Threshold Voltage Engineering in Al2O3/AlGaN/GaN MISHEMTs with Thin Barrier Layer: MIS-gate Charge Control and High Threshold Voltage Achievement

Abstract

In this work, thin barrier layer (TBL) recessed-gate Al<inf>2</inf>O<inf>3</inf>/AlGaN/GaN MISHEMT devices with high threshold voltage (V<inf>th</inf>) and large output current have been fabricated. We found that the Al<inf>2</inf>O<inf>3</inf> dielectric in gate area would introduce strain and enhance the polarization effect at AlGaN/GaN heterostructure interface based on V<inf>th</inf> characteristic analysis of the devices with different TBL thickness and material quality evaluation. Meanwhile, high concentration of positive charge locates at Al<inf>2</inf>O<inf>3</inf>/AlGaN MIS interface. These increased polarization charge and MIS interface charge lead to negative V<inf>th</inf>. Post-gate annealing can effectively not only release stress in the gate area and reduce the polarization charge, but also improve the quality of MIS interface and reduce MIS interface charge, so that V<inf>th</inf> has large positive shift and E-mode operation can be realized. Through the gate bias stress, further positive shift of V<inf>th</inf> was obtained by charge trapping due to the traps with deep level in the MIS gate region were occupied stably by electrons. The 5 nm TBL device exhibits a high V<inf>th</inf> of 6.5 V and large output current of 435 mA/mm.